Some of the key reasons to consider getting a GPS antenna include:

1) For use in a car or any other vehicle where the GPS device cannot or will not be placed near a window

2) For use when trekking or hiking in challenging geographical locations like jungles or canyons

3) For use in highly built-up areas like urban city centers

4) For use in any place where the GPS device does not have good line of sight to the sky

5) For use in a vehicle in motion to minimize temporary signal loss

6) For enhancing the best possible GPS signal accuracy by having a lock on the most number of GPS satellites

In fact, specific to piloting here a number of advantages in using the GPS antenna:

1) Reduces flight time

With a GPS antenna, it is possible for a pilot to reduce the time spent on aircraft turning from around five minutes to as low as one minute.

2) Increase efficiency

As the antenna is constantly maintaining a constant phase lock with GPS stations, no flight time is wasted while waiting to reacquire the lock on lost GPS signals. In addition, it also reduces the overall flight costs due to the reduction in turning time. Without a doubt, a GPS antenna helps to increase efficiency.

3) Minimum investment required

Since the technology is based on commercially available components, only a minimal investment is required. A low-cost and stabilized GPS antenna can be added to any existing vehicle or aircraft.

Generally, a GPS antenna can handle many different types of situations regardless of the outside environment as the antennas are designed mainly for stationary applications. The GPS antenna is a high-quality solution for adding GPS RF signals to marine GPS navigation systems. One particular model of GPS antenna is the Bullet III which is an active antenna with 35-dB preamp and dual band pass filter.

In fact, this brand has been in use for many years as it has proven its strength, durability and reliability. However, regardless of which brand of GPS antenna system you choose to purchase, all that matters is that you does what you need it to do at the right price and with the expected levels of reliability and durability.

Get The Correct GPS Mount To Secure Your GPS Device

It is important to get the proper GPS mount in order to secure your GPS device so that it is less likely to get damaged. In fact, it is quite tough to find the perfect GPS mount subsequent to your initial purchase of the GPS device, since your retailer may longer hold inventory of GPS mounts suited to older GPS equipment. You may need to spend some time and effort trawling through the Internet's various auction sites to find a GPS mount for an older device. For this particular reason alone, after you have decided on the GPS device you plan to buy, you should also consider buying the GPS mount at the same time since your retailer might be able to recommend one that fits your device and its intended use.

Get as much research regarding GPS mounts done on the Internet. Many GPS device manufacturers may suggest suitable GPS mounts in their online literature describing the devices' features and functions. There is a wide variety of GPS mounts designed for various purposes. For those who are not familiar with their GPS devices, some mistakes may be made in the process of purchasing GPS mounts.

Some examples of different mounts available include:

1) Aviation mounts

This type of mount gives the pilot an option of positioning the mount over or under the yoke depending on their requirements. Some pilots may even bring along their own portable GPS mount and fix it to an area where they prefer.

2) Marine mounts

Marine GPS mounts are generally used by boaters to fix it onto their marine craft so that they are able to use the GPS device with ease. Like aviation units, most of the marine units are easily removable for safe-keeping and convenience.

3) Laptop mounts

There are people who use laptop GPS mounts for fleet vehicles. Therefore the dealer of the GPS mount has to ensure that the mount has the same specifications to fit all the vehicles in the fleet.

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If used within the automobile, the GPS cellular phones work with a receiver. By subscribing to a navigation service and inserting the right software which is in conjunction with your phone, the system allows you to receive driving directions. If you wish to use the tracking system outside the automobile, then an integrated GPS cell phone is required.

People who are constantly on the move will find the GPS cellular phones a handy device as their movements can be tracked. It will allow you to call for assistance from anywhere and your location will be tracked. For instance, if your car has an accident in a remote area, emergency help can easily be reached to you with the help of this tracking system. Even for the average hiker in the mountains or the woods, this device has become indispensable as it means being able to call for help instantly if the person is injured or in any danger.

Cell phones are continuously transmitting and receiving signals to and from the operator tower which is nearest to its location. Wireless communication has made it possible that the cell phone is active even in the standby mode. GPS cellular phones work on the same premises as the navigation tool which is generally used for remote trips. Cell phones are thus equally easy to trace and both GSM as well as CDMA phones which are GPS integrated, can be tracked using the system.

The mechanism of the GPS devices is that the signals received from these cell phones are identified by the service provider by identifying the operator/operators nearest to the location. These numbers are then compared with the relative strength of the signals received and the strongest signal is then identified. This procedure enables the cellular phone to be tracked within 100 meters of its exact location. This is very useful if the person is lost or in danger or unfortunately, as has become more common in today`s times, to trace a potential terrorist or miscreant.

Another add on to this invention is the child GPS locater device which if integrated in your phone is going to give you immense peace of mind as a parent. You will know exactly where the child is all the time by just looking at your cellular phone and you can check on the child silently without having to call him up 20 times! Of course, parents, please be aware that, morally, this does not mean a license to snoop but is meant for keeping a genuine look-out for your child when he is away from home. And even when the child or children are with you, the GPS cellular phones are an amazing comfort in over-crowded places like amusement parks or shopping malls where children are likely to get lost.

Of course, with any advanced technology the issue of privacy always raises its ugly head. But, that is the price one may have to pay for convenience`s sake and regular check-up of the device for malicious software, updating cell phone features with the service giver and having a predetermined tacking system with family and friends will help you make the optimum use of this technology.

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• Faux-leather carrying case protects Garmin Nuvi GPS device
• Expanding elastic sides for a snug and fully accessible fit
• Outer dimensions: 4.92 x 3.07 x 0.78 inches (125mm x 7mm x 20mm)
• Distributed by Amazon.com; backed by one-year Amazon Basics warranty
• Ships in Certified Frustration-Free Packaging

Product Description
AmazonBasics products are quality electronics accessories offered at a great value.Amazon.com Product Description
The AmazonBasics Carrying Case for Portable GPS Units is a durable, faux-leather protector for your portable GPS device. Designed to shield your GPS from bumps and scratches, this soft-lined case features expandable sides and a pocket for small notes and other items.
Carrying Case for Portable GPS
At a Glance Durable, faux... More >>

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Most GPS models come with internal map capabilities. Manufacturer provides maps with details up to street level in the hard drive. The menu option enables you to access the desired data. Comprehensive street level maps can help you identify the coordinates of the destination on the map by using just the name, or locate the destination on map by giving the coordinates. For some models you will have to subscribe to a plan to update the maps on an annual basis. Check out some models with internal map capabilities at: http://www.reviewgist.com/gps-devices-reviews?mapCapabilities=Internal

Some models provide map information in memory cards having detailed information of a particular region. Thus you will need extra cards if you have to store information for more coverage. Map cartridges are more flexible but can prove to be expensive. Check the few models at: http://www.reviewgist.com/gps-devices-reviews?mapCapabilities=Map+cartridges+%2F+Data+cards

Models which enable one to download the latest maps are also available. Check such models at:

http://www.reviewgist.com/gps-devices-reviews?mapCapabilities=Download+maps

Some brands provide map information in CD-ROM, DVD-ROM discs. Check out one such model at: http://www.reviewgist.com/gps-devices-reviews?mapCapabilities=DVD

Receiver type

There are 24 GPS satellites in six orbits around the earth. GPS receivers track these satellites to compute the information of your location, the bearing, distance and time left to reach destination and other route options. Since there are 12 satellites at a time in view, each of them is tracked continuously to generate the information. Latest GPS receivers marketed today are commonly 12 parallel channel receivers. You can find some such models at: http://www.reviewgist.com/gps-devices-reviews?receiverType=Parallel-Channel+%2812%29

The associated advantages of tracking more satellites are:

faster cold start

regular initialization

better reception

do not require an external antenna if you are in an open vehicle

Other receiver types are:

20 parallel channel receivers http://www.reviewgist.com/gps-devices-reviews?receiverType=Parallel-Channel+%2820%29

16 parallel channel receivers http://www.reviewgist.com/gps-devices-reviews?receiverType=Parallel-Channel+%2816%29

14 parallel channel receivers http://www.reviewgist.com/gps-devices-reviews?receiverType=Parallel-Channel+%2814%29

Display size

Large displays are a visual treat when you consider buying TVs but not the same when it comes to buying a GPS. The map on the display gives you information regarding:

current location

where next turn is

name of street you are turning onto

estimated time of arrival

While driving, casting your glance away a second longer can prove to be risky, hence a device with a smaller screen will be advantageous. The display should be easy to read irrespective of surroundings. The navigation aids come in various sizes. Some units with screen size as big as 7in. (http://www.reviewgist.com/gps-devices-reviews?displaySize=7+in.) enable you to get connected to external video devices. Some hand held ones are as small as 1.5in.X 0.9in. (http://www.reviewgist.com/gps-devices-reviews?displaySize=1.5+in.+x+0.9+in.) Some of the convenient, easy to read GPS are:

3.5 in. (http://www.reviewgist.com/gps-devices-reviews?displaySize=3.5+in.)

2.8 in. x 2.1 in. (http://www.reviewgist.com/gps-devices-reviews?displaySize=2.8+in.+x+2.1+in.)

Resolution

Screen resolution decides how clearly you can see and read in broad daylight. Color, glare free LCDs are soothing to the eyes. High end models have color screens which gives information about weather and terrain. Some GPS units with different resolutions are:

320x240 http://www.reviewgist.com/gps-devices-reviews?resolutions=320+X+240

480x272 http://www.reviewgist.com/gps-devices-reviews?resolutions=480+x+272

PC Interface

The portable models have a USB or serial ports which gets you connected to a PC. It enables you to download latest maps and system software. GPS units with USB and serial ports can be found at:

USB: http://www.reviewgist.com/gps-devices-reviews?pcInterface=USB

Serial port: http://www.reviewgist.com/gps-devices-reviews?pcInterface=Serial+RS-232

Bluetooth GPS that communicates with mobile device wirelessly are also available. Some can be found at: http://www.reviewgist.com/gps-devices-reviews?pcInterface=Bluetooth+Wireless+Technology

Built-in memory

With larger memory space one can store more route information and waypoints. Units with software that can transfer data from the unit to a PC can be used to gather more data than it can hold. So depending on how much map data and information you wish to store, you can select the units with the required memory space. You get various models with wide range of built-in memory space.

1 to 30MB http://www.reviewgist.com/gps-devices-reviews?builtInmemory=1-to-30

30 to 512MB http://www.reviewgist.com/gps-devices-reviews?builtInmemory=30-to-512

512 to 2500MB http://www.reviewgist.com/gps-devices-reviews?builtInmemory=512-to-2500

2500 to 40000MB http://www.reviewgist.com/gps-devices-reviews?builtInmemory=2500-to-40000

Antennas

External antennas are used when stronger signals are needed. If in a vehicle the system cannot be placed near the window, external Antenna hookups can prove to be ideal solution. Choice of external hookups is most practical for hiking under heavy tree cover, under tall buildings or in any place which doesn't have a good view of sky. GPS devices with external antenna hookup can be found at: http://www.reviewgist.com/gps-devices-reviews?antenna=External+Antenna+Hookup

Built-in antennas are less prone to breakage and can communicate with more satellites. Some such devices are available at:

http://www.reviewgist.com/gps-devices-reviews?antenna=Built-in

Quadrifilar Helix- This is also an external antenna useful for situations where open sky is limited. Check the models at:

http://www.reviewgist.com/gps-devices-reviews?antenna=Quadrifilar+Helix

Flip up antennas too give strong performance. Some such models are : http://www.reviewgist.com/gps-devices-reviews?antenna=Flip-up

Price

This is one factor not to be compromised on, if you don't wish yourself or your loved ones to get lost in the maze of roads. The technology in this field has developed rapidly and the old models which are cheaper may have crowded maps, or confusing menu options. Latest models high on price have more detailed maps. Of course, lesser the features lesser will be the cost. If you don't need features like live traffic data or weather data, don't invest money in models offering these features. Features like speech recognition for hands free operation, 3D maps, and information on fuel prices all come for additional price.

Under $200- http://www.reviewgist.com/gps-devices-reviews?Price=59.5-to-199.99.You get good light weight portable navigation units in this range. Maps look great with reasonably timely route recalculations. Good value for money! Those of you looking for more memory space may check out models at higher range.

Between $200 to $312- You get better performance and features like anti glare, touch screen with good resolution. You also get good in-built memory space and battery life. http://www.reviewgist.com/gps-devices-reviews?Price=199.99-to-311.99

Between $312 to $416- Good screen settings, voice guided directions and overall good performance. http://www.reviewgist.com/gps-devices-reviews?Price=311.99-to-415.45

Between $416 to $1172- Accurate route guidance, easy to use and compatibility with Bluetooth cell phones are some attractive features of devices in the price range. http://www.reviewgist.com/gps-devices-reviews?Price=415.45-to-1171.78

Brands

There are various manufacturers manufacturing GPS units with different features to suit a wide range of customers. The major brands available today are:

Garmin has many models of handheld and mounted types. Check its models at: http://www.reviewgist.com/gps-devices-reviews?BrandName=Garmin

Magellan: http://www.reviewgist.com/gps-devices-reviews?BrandName=Magellan

Mitac: http://www.reviewgist.com/gps-devices-reviews?BrandName=Mitac

Lowrance: http://www.reviewgist.com/gps-devices-reviews?BrandName=Lowrance

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• Bright, 4.3-inch diagonal color WQVGA TFT touchscreen with 480 x 272 pixels and white backlight
• Preloaded with City Navigator North America NT
• Bluetooth wireless technology for hands-free calling with compatible devices
• High-sensitivity GPS receiver for improved performance and reception
• NOTE: Model number on the box is 765T because the traffic receiver is included; however, the model number on the device itself is 765 as the "T" in 765T refers to the additional component

Product Description
Have you lost your sense of direction? Let Garmin nuvi 765t 4.3" GPS be your guiding system to take you hither and yonder. It opens United States, Canada, and Puerto Rico for visual exploration. You'll get the knowledge of a native where and when ever you choose to go. It's the surefire tool for any traveler even for those with no sense of direction. The Garmin nuvi 765t is fully equipped to help you get where you're going. The nuvi 765T offers full coverage mapping... More >>

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An advanced sports gps watch can accurately measure:

Speed

Distance

Elevation above sea-level

What’s more, many watches with gps can plot 3D maps over a traveled route that can later be downloaded to a PC or Mac and then reviewed and analyzed in a custom software package.

Combining this with advanced sports watch features, like memory storage/recall, interval timers, heart rate zones and speed, pace and distance alerts and you get a complete wrist-watch training computer, capable of taking you to the next level!

While having all these features can be exciting, it can also be overwhelming when you first start shopping around for the right watch. As most gps watches are quite sophisticated, this makes it difficult to know which features suit your needs and which are nice, but most likely not ones you'll use in day-to-day use.

Our guide will take you through the process and will familiarize you with the key questions you need to ask yourself when you are comparing watches. With, the most common questions clear in your mind, selecting the right model will likely be a breeze.

Step 1: Decide how you will be using your watch There are different types of GPS watches for different purposes; Your buying decision hinges on how you will use your GPS watch, so this is an important thing to be clear on.

This may seem obvious, but knowing which of these categories you fall into can save you a lot of time and effort.

Some watches will cater to fitness and sport, among other things, these models will have a lot of features that will suit runners, cyclists, triathletes and other athletes, like memory storage and recall, interval timers, heart rate monitors and pace and distance alerts. Other models will have navigational features allowing you to mark locations, plot courses, then navigate between waypoints in your area. Some watches will even support both.

GPS Sport Watch Models The watches below utilize GPS for fitness and performance.

Garmin Forerunner 101, 201, 301, 205, 305, 405

Timex Ironman Bodylink Heart Rate Monitor Watch T5F011

Timex Ironman Trail Runner Bodylink Heart Rate Monitor Watch T5J985

Timex Women's Ironman Trail Runner Bodylink Heart Rate Monitor Watch T5G311

Timex Ironman Triathlon T5E701

Timex Ironman T5E691 Men's 100-Lap Speed + Distance Watch

Polar Training Computer GPS Watch FT60M

Polar RS800G3 Multisport GPS Computer Heart Rate Monitor Watch

Suunto T3c Multisport Pack

Suunto T4c Multisport Pack

Suunto T6c Multisport Pack<

GlobalSat Personal GPS Sport Watch With Heart Monitor

Navigation GPS Watches The watches below utilize GPS for navigation.

Garmin Forerunner 101, 201, 301, 205, 305

Suunto X10 Sports Computer Wristop Altimeter Watch

Suunto X9Mi Wrist-Top GPS Computer Watch

Suunto X9i Wrist-Top GPS Computer Watch

GlobalSat Personal GPS Sport Watch With Heart Monitor

Step 2: Question - Decide if you need real-time signal reliability? A common complaint about GPS systems is the signal dropping out without warning.

This is especially common GPS watch models with older chipsets. This problem can be somewhat resolved by ensuring you only use your GPS watch in places where there is a clear, unobstructed view of the sky – i.e. not around tall buildings or trees.

The latest GPS watches use the SirFStar III chipset, which is more sensitive and reliable than older chipsets. This is not to say the older watches are not good choices, some have dropped considerably in price making them great value for money, but deciding if high reliablility is important will impact your choice.

If you only wish to pinpoint your location infrequently, then high reliability is not such a big deal.

For instance, say you are out kayaking on the Amazon river, you come to a fork and need to decide whether to go right or left… chances are you probably don’t care too much if your GPS signal drops occasionally, so long as you can quickly pick it up again when you need it.

On the other hand you are running your 5th London Marathon and need to know your exact kilometer pace at the 10km mark so as to know if you are running too fast or too slow - then a signal drop out is highly undesirable.

An alternative to gps that may be worth considering is a foot pod device that measures your leg turn-over or cadence and calculates speed and distance and all the other good stats based on your leg-speed alone. When correctly calibrated a foot pod can be quite accurate and does not suffer from signal obstruction like a gps unit does.

GPS Sport Watch Models with Older Chipsets These GPS watches utilize older chipsets. They are generally slower to link to the satellites and may lose the signal more frequently.

Garmin Forerunner 101, 201, 301, 205, 305, 405

Suunto X9Mi Wrist-Top GPS Computer Watch

Suunto X9i Wrist-Top GPS Computer Watch

GPS Sport Watch Models with Modern Chipsets These GPS watches utilize a modern chipset like the SiRFstar-III or equivalent.

Garmin Forerunner 205, 305, 405

Timex Ironman Bodylink Heart Rate Monitor Watch T5F011

Timex Ironman Trail Runner Bodylink Heart Rate Monitor Watch T5J985

Timex Women's Ironman Trail Runner Bodylink Heart Rate Monitor Watch T5G311

Timex Ironman Triathlon T5E701

Timex Ironman T5E691 Men's 100-Lap Speed + Distance Watch

Polar Training Computer GPS Watch FT60M

Polar RS800G3 Multisport GPS Computer Heart Rate Monitor Watch

Suunto T3c Multisport Pack

Suunto T4c Multisport Pack

Suunto T6c Multisport Pack

Suunto X10 Sports Computer Wristop Altimeter Watch

GlobalSat Personal GPS Sport Watch With Heart Monitor

Step 3: Consider comfort and style and "Coolness" While tons of features are probably main reason for buying a GPS watch in the first place, if you like to wear your watch in a casual setting as well as for sport then it goes without saying you should find one you like the look of. But I’m sure you don’t need us to tell you.

The latest watch from Garmin, the Forerunner 405, has a few less features than it’s predecessor, the 305, but looks pretty darn cool, whereas the Forerunner 305 is more like a brick on your wrist! The Suunto T series watches models that support GPS look as natural in town as on the track.

Also consider comfortably the watch fits your wrist. If you are more comfortable with an external gps device attached to you, then you can enjoy a smaller wristwatch, whereas a single gps-watch unit is a lot bulkier.

Step 4: Other criteria Often the most heavily promoted features of popular products, are not the ones you are really interested in. This is an unfortunate part of marketing, but luckily we’ve dug deep and come up with a list of some less obvious features that might be just what you're looking for.

Sunrise & Sunset Time: Forerunner 405, 205/305

Computer Fitness Opponent: Forerunner 405, 305/205 (Virtual Partner Feature)

Goal Tracking: Forerunner 405

Touch Sensitive Bezel: Forerunner 405

Wireless Data Transfer: Forerunner 405

Compete Against Historical Sessions: Forerunner 405, 205/305 (courses feature)

Distance Traveled While Paused: Forerunner 205/305

4 field customizable display: Forerunner 205/305

Vertical Speed: Suunto X9/i, X9M/i, Timex Bodylink Trail Runner T5J985

Altitude Alarm: Suunto X9/i, X9M/i, Timex Bodylink Trail Runner T5J985

Thermometer: Suunto X9/i, X9M/i

Barometer: Suunto X9/i, X9M/i

Barometric Altimeter: Suunto X9/i, X9M/i, Suunto T6C Multisport Pack

Weather Alarm (Barometric Pressure): Suunto X9/i, X9M/i

Altitude Stats: Suunto X9/i, X9M/i, Suunto T6C Multisport Pack, Polar RS800G3

Compass Feature: Forerunners 101, 201, 301, 205, 305, 405, Suunto X9/i, X9M/i, Timex Bodylink Trail Runner T5J985, Globalsat GH-615B/615M

6400 MILS Bearing Display: Suunto X9/i, X9M/i

Beat to beat variation in heart beat intervals: Polar RS800G3

EPOC: Suunto T6C Multisport Pack

Training Effect: Suunto T3C and T6C Multisport Packs

A note about power Let’s make one thing clear. GPS watches suck up a LOT of juice! Fortunately, many models use rechargeable batteries.

There are generally two types of GPS sports watch – those that you wear on your wrist and those that require you to attach an external device to your body. The wristwatch type are almost always rechargeable, whereas external gps watches generally use AA or AAA batteries.

Most provide around 8-14 hours of continuous gps tracking and allow the gps to be switched off when not needed, so as to save power.

For you ultra distance runners, even 15 hours might not be enough. Fortunately popping a spare battery into an external gps is a trivial task, even on the run. For everyone else, I’m guessing 8 hours of gps time is more than adequate.

A Note about Water Usage Like fire and cats, gps watches don’t mix too well with water. Most units can survive a few splashes, or the accidental dropping in the kitchen sink, but using your watch for swimming or diving is definitely not recommended and the signal reception can not be expected to be accurate under these circumstances.

This is unfortunate for Triathletes for who a gps watch would be a valuable tool for recording and assessing their performances over multi-disciplines

By now you should (hopefully) have a better understanding about which gps sports watch unit is right for you. You may even know what you’re looking for. If you are pretty sure what you want, but want more information head to our website, where we have more information about gps and other types of sports watches.

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What is GPS? It is the Global Positioning System which was developed by Department of Defense, US. They developed this service in order provide correct directions to the US military in time. Later on, it became public and now it is within the reach of an average citizen.

The working of GPS largely depends on three things: satellites; receivers; Earth monitoring stations. The signals from satellites are processed in the receivers, that is, cell phone, or any other device). This processing is done in order to make the user understand the location in best possible way as a user can not directly receive a signal anyway.

G.P.S. Cell phones in future are expected to have the feature of tracking any cell phone user. This service will be beneficial in case of emergency to track the number of the user who is in trouble like in case of an accident or any other emergency. The G.P.S. cell phones do not have the same quality as that of the GPS devices used by drivers, mariners and hikers. The devices go in detail of the location whereas the cell phones enabled don`t provide the locations with every corner explained. G.P.S. cell phone has the facility of Global Positioning System while it serves as a normal phone used for calling or sending text.

Locating people using G.P.S. cell phones is a great advantage of the GPS service. You don`t have to fear about your kids and family by keeping a track of their locations. In spite of its benefits, there are some issues concerning the personal privacy. The personal privacy may get violated if one uses the G.S.P. cell phone to track some one. There have been some laws passed to guarantee the privacy of the cellular phone user. Every object in this universe has its pros and cons and so does G.P.S. cellular phone.

Of late, different brands of cell phone are introducing G.P.S. services with their cellular phone models. Nokia 5140 comes with an accessory that enables the user to access the global positioning system services. The G.P.S. module LAM-1 also works with Nokia`s 9210 and 9210i mobile phone models. getGoing is a GPS service available with Verizon. Other mobile phones that support the global positioning system services are Audiovox CDM8600 and CDM9500, Motorola T720 and T730, and Samsung's SCH-a530 and SCH-a530s. GPS service like Go2Directory is available with Kyocera 3035e, VX4400, and VX4400B, Verizon Z-800, Audiovox CMD8600 and CMD9500, LG brand VX10, and Motorola T720 and T730.

In past few years there has been tremendous advancement in cell phone technology. GPS technology is a convenient method to locate a place or a person, though it has some issues but the government will surely see to it and conclude a satisfying result as the need for G.P.S. cellular phone is being greatly felt amongst the officials as well as the civilians.

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GPS Navigation Systems have come a long way

Gps systems have come a long way since they were first designed for the US millitary. The first signal from NAVSTAR 1 was received on Feb. 22, 1978. NAVSTAR 1 was launched from Vandenberg Air Force Base in California and was the first of 24 satellites that make up the Global Positioning System (GPS). The first generation of satellites that make up the Global Positioning Systems 24 satellites were launched between Feb. 22, 1978 and Oct. 9, 1985. Since it First became operational the Global Positioning Systems has revolutionized the way America goes to war and provides a GPS system in which the world relies on for precise navigation.

The Global Positioning System's constellation of orbiting satellites is managed by the United States Air Force 50th Space Wing. The unit is the host wing at Schriever Air Force Base, located in east Colorado Springs, Colorado. They are responsible for tracking and maintaining the command and control, warning, navigational, and communications satellites for Air Force Space Command as well as the Global Positioning System satellites. Full Operational Capability was declared by NAVSTAR in April 1995 (NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging, and is the official U.S. Government name given to the GPS satellite system).

The US government granted the Global Positioning System available for civilian use in the late 1980s and with no subscription fees or setup charges to use the GPS system the civilian GPS market exploded, especially in the last decade. A new generation of sophisticated GPS satellites are replacing the older satellites and there are now 34 GPS satellites in orbit that provide combat capability for military applications and aircraft navigation aids. Civilian applications include ATM's, bank and stock market transactions as well as power grid management. Currently 31 of the 34 GPS satellites in orbit transmit navigation and timing signals to civilian and military users around the world.

Gps for civilian use

The civilian GPS system wasn't always as it is today and the US military is still keeping the most accurate Global Positioning Systems available classified for national security. But GPS upgrades for two new civilian signals to enhanced user accuracy and reliability, particularly with respect to aviation safety were planned in 1998. Then on May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded GPS signal globally, and in 2004 QUALCOMM announced the successful tests of assisted GPS for mobile phones that led the way for the GPS aided cell phones that are widely used today.

In 2005, the first third generation GPS satellite was launched and began transmitting a second civilian signal for enhanced user performance. Then in September of 2007 the Air Force completed a four-phase transition of the Global Positioning Systems ground segment to it's new Architecture Evolution Plan. The ground segment's provide command and control of the satellites and generates the navigation message for satellites to broadcast to the users GPS device to calculate their earth's position. The new Gps satellites include new high-powered, anti-jam military-code, along with other accuracy, reliability, and data integrity improvements for both civil and military users. This modernized version of the world’s greatest free utility was designed to ensure the US has the most precise and secure positioning, navigation and timing capability through 2030.

Europe and Russia develop their own GPS

In 2004 the United States signed an agreement with the European Community establishing cooperation with Europe's planned Galileo system. Galileo is a global navigation satellite system (GNSS) that is currently being built by the European Union and is separate from but complimentary to the United States Global Positioning System. The European Union's Galileo system should be operational by 2013. The European Community's political aim is to provide an independent GPS system that the European nations can rely on in times of war or political disagreement, because both Russia or the USA could disable use of their national systems by others (through encryption).

The Russian GPS system GLONASS is a radio-based satellite navigation system that was developed by the former Soviet Union and now operated for the Russian government by the Russian Space Forces. Like the European GPS system the Russian GPS system also functions separate from but is complimentary to the United States Global Positioning System. Russia began launching satellites for their GPS system into space on October 12, 1982 and was completed in 1995. The system rapidly fell into disrepair fallowing the collapse of the Russian economy but in 2001 the Russian government began restoring the system with hopes of restoring global coverage by the end of 2009.

GPS Systems today

Depending on the GPS unit a GPS receiver only needs a signal from 3-4 satellites to calculate the units position and will work in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or set up fees like with a cell phone to operate GPS receivers and although some GPS receivers have extra features like real time traffic updates that will have a monthly charge, some GPS receivers like Garmins Nuvi "T" series come with free live traffic for the life of the unit. Today's GPS is extremely accurate thanks to their parallel multi-channel design. Garmin's 12 parallel channel receivers are very quick to lock onto satellites when first turned on and they maintain a strong lock even in dense foliage or city's with tall buildings because they continuously track signals from up to 12 satellites at any given time. Even if a 12 parallel channel GPS receiver loses signals from 8 satellites at once it will still function properly.

WAAS (Wide Area Augmentation System) was developed by the Federal Aviation Administration to augment the Global Positioning System to improve its accuracy, integrity, and availability. WAAS was originally intended to enable aircraft to rely on GPS for all phases of flight, including the precision approach to airport's within its coverage area. All though originally intended for aviation most GPS receivers today are WAAS-enabled including automotive, boating chartplotters and hand-held units. WAAS uses a network of ground based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals. Measurements from these reference stations are routed to master stations and then they send correction messages to geostationary WAAS satellites. Those satellites then broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the correction data while computing their positions to improve accuracy. WAAS enabled GPS receivers are accurate to within 3 meters and that make them the most accurate GPS receivers for civilian use on the market today. In fact a WAAS-enabled GPS receiver can even give you directions right down to the lane your car is traveling in (as long as the mapping program supports "lane assist" directions) and With no additional equipment or fees required to take advantage of WAAS-enabled GPS receivers they are becoming as common as cell phone's.

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1. Purchase a GPS enabled Cell Phone

Most of cellular phones out there today come with GPS tracking capabilities. Before purchasing a phone, you will need to know what kind of GPS capabilities it has. Some cell phones use the communication network to transmit their location, other phones use the actual GPS network. For this scenario, you will want to get a phone that uses the GPS network. If you want to track a phone that is already owned by someone else, you must make sure it also uses the GPS network.

2. Install Required Software For Tracking

In order for the cell phone that you want to track to show up in the location tracking system, some software will need to be installed into the cell phone so it can communicate directly with the tracking system. This feature allows the phone to be turned on remotely so it can be tracked. You will also need to be sure that the cell phone is compatible with the software you will be installing. You should also verify the software compatibility of the phone and tracking service when you check to see if the phone can also uses the GPS system.

3. Sign Up for the GPS Tracking Service

The tracking service is the system that keeps track of the location of the cell phone and sends alerts. It can either be software that can be downloaded onto a PC, or it can be a system that is online and you are assigned a login ID to access the service. The tracking service comes with the software that you need to install on the cell phone to be able to track it, so you don't typically need to make sure the service is compatible as well because it will automatically be compatible through the software that needs to be installed on the phone. The tracking service usually comes with monthly subscription fees. The cost and capabilities of these tracking services can vary greatly.

GPS cell phone tracking today is a relatively easy and affordable process due to the advances in technology. Most cell phones manufactured today have some form of GPS capability already. Software needs to be installed on the phone that allows it to send tracking data to the tracking service.

There are a host a privacy issues that now come into play with this technology. Jealous ex's can use this to unknowingly track their estranged significant others if they can get a hold of their cell phone long enough to install the tracking software. Another privacy busting scenario is that a jealous lover can just get a new cell phone and attach it to the car of the person they want to track. The auto kit option allows the phone to be directly connected to the car battery for uninterrupted function. However, these issues better discussed in another article.

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The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed/direction, and time.

Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr. John Walsh, a key decision maker when it came to the budget for the GPS program[1]). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year,[2] including the replacement of aging satellites, and research and development. Despite these costs, GPS is free for civilian use as a public good.

GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Simplified method of operation

A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS microwave signal gives the distance to each satellite, since the signal travels at a known speed - the speed of light. These signals also carry information about the satellites' location and general system health (known as almanac and ephemeris data). By determining the position of, and distance to, at least three satellites, the receiver can compute its position using trilateration.[3] Receivers typically do not have perfectly accurate clocks and therefore track one or more additional satellites, using their atomic clocks to correct the receiver's own clock error.

[edit] Technical description

Unlaunched GPS satellite on display at the San Diego Aerospace museum

Unlaunched GPS satellite on display at the San Diego Aerospace museum

[edit] System segmentation

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).[4]

[edit] Space segment

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design calls for 24 SVs to be distributed equally among six circular orbital planes.[5] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.[6] The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).[2]

Orbiting at an altitude of approximately 20,200 kilometers (12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day, so it passes over the same location on Earth once each day. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.[7]

As of September 2007, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.[8]

[edit] Control segment

The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA).[9] The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2d Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within one microsecond and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.[10]

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).

[edit] User segment

The user's GPS receiver is the user segment (US) of the GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2006, receivers typically have between twelve and twenty channels.

A typical OEM GPS receiver module, based on the SiRF Star III chipset, measuring 15×17 mm, and used in many products.

A typical OEM GPS receiver module, based on the SiRF Star III chipset, measuring 15×17 mm, and used in many products.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bit/s speed. Data are actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000[11] is a newer and less widely adopted protocol. Both are proprietary and controlled by the US-based National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

[edit] Navigation signals

Main article: GPS signals

GPS broadcast signal

GPS broadcast signal

Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-of-day, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part of the message). The ephemeris data gives the satellite's own precise orbit and is output over 18 seconds, repeating every 30 seconds. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for 6 hour time-outs. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to the low data transmission rate. The almanac consists of coarse orbit and status information for each satellite in the constellation and takes 12 seconds for each satellite present, with information for a new satellite being transmitted every 30 seconds (15.5 minutes for 31 satellites). The purpose of the data is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. An important thing to note about navigation data is that each satellite transmits only its own ephemeris, but transmits an almanac for all satellites.

Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise (P) code, which is usually encrypted and reserved for military applications. The C/A code is a 1,023 chip pseudo-random (PRN) code at 1.023 million chips/sec so that it repeats every millisecond. Each satellite has its own C/A code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency. The P-code is a 10.23 megachip/sec PRN code that repeats only every week. When the "anti-spoofing" mode is on, as it is in normal operation, the P code is encrypted by the Y-code to produce the P(Y) code, which can only be decrypted by units with a valid decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user. Frequencies used by GPS include

* L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.

* L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.

* L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.

* L4 (1379.913 MHz): Being studied for additional ionospheric correction.

* L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2008.

[edit] Calculating positions

[edit] Using the C/A code

To start off, the receiver picks which C/A codes to listen for by PRN number, based on the almanac information it has previously acquired. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern, then measures the time delay for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number as the satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange[12].

Overlapping pseudoranges, represented as curves, are modified to yield the probable position

Overlapping pseudoranges, represented as curves, are modified to yield the probable position

Next, the orbital position data, or ephemeris, from the Navigation Message is then downloaded to calculate the satellite's precise position. A more-sensitive receiver will potentially acquire the ephemeris data quicker than a less-sensitive receiver, especially in a noisy environment.[13] Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. Receivers can substitute altitude for one satellite, which the GPS receiver translates to a pseudorange measured from the center of the earth.

Locations are calculated not in three-dimensional space, but in four-dimensional spacetime, meaning a measure of the precise time-of-day is very important. The measured pseudoranges from four satellites have already been determined with the receiver's internal clock, and thus have an unknown amount of clock error. (The clock error or actual time does not matter in the initial pseudorange calculation, because that is based on how much time has passed between reception of each of the signals.[clarify][citation needed]) The four-dimensional point that is equidistant from the pseudoranges is calculated as a guess as to the receiver's location, and the factor used to adjust those pseudoranges to intersect at that four-dimensional point gives a guess as to the receiver's clock offset. With each guess, a geometric dilution of precision (GDOP) vector is calculated, based on the relative sky positions of the satellites used. As more satellites are picked up, pseudoranges from more combinations of four satellites can be processed to add more guesses to the location and clock offset. The receiver then determines which combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock offsets. After the final location and time are calculated, the location is expressed in a specific coordinate system, e.g. latitude/longitude, using the WGS 84 geodetic datum or a local system specific to a country.

[edit] Using the P(Y) code

Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism: if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite.[citation needed] In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators. RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective.

[edit] Accuracy and error sources

The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about 1% of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only the GPS C/A signal.

Position accuracy can be improved by using the higher-chiprate P(Y) signal. Assuming the same 1% bit time accuracy, the high frequency P(Y) signal results in an accuracy of about 30 centimeters.

Electronics errors are one of several accuracy-degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy.

Sources of User Equivalent Range Errors (UERE) Source Effect

Ionospheric effects ± 5 meter

Ephemeris errors ± 2.5 meter

Satellite clock errors ± 2 meter

Multipath distortion ± 1 meter

Tropospheric effects ± 0.5 meter

Numerical errors ± 1 meter

[edit] Atmospheric effects

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere and ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the signal is affected for a longer time. Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.

Because ionospheric delay affects the speed of microwave signals differently based on frequency—a characteristic known as dispersion—both frequency bands can be used to help reduce this error. Some military and expensive survey-grade civilian receivers compare the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.

The effects of the ionosphere generally change slowly, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1 only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems such as WAAS, which transmits it on the GPS frequency using a special pseudo-random number (PRN), so only one antenna and receiver are required.

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect is both more localized and changes more quickly than ionospheric effects and is not frequency dependent. These traits making precise measurement and compensation of humidity errors more difficult than ionospheric effects.

Changes in altitude also change the amount of delay due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct.

[edit] Multipath effects

GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas may be used to reduce the signal power as received by the antenna. Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.

Multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.

[edit] Ephemeris and clock errors

The navigation message from a satellite is sent out only every 30 seconds. In reality, the data contained in these messages tend to be "out of date" by an even larger amount. Consider the case when a GPS satellite is boosted back into a proper orbit; for some time following the maneuver, the receiver's calculation of the satellite's position will be incorrect until it receives another ephemeris update. The onboard clocks are extremely accurate, but they do suffer from some clock drift. This problem tends to be very small, but may add up to 2 meters (6 ft) of inaccuracy.

This class of error is more "stable" than ionospheric problems and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending out a more accurate almanac on a separate channel.

[edit] Selective availability

The GPS includes a feature called Selective Availability (SA) that introduces intentional, slowly changing random errors of up to a hundred meters (328 ft) into the publicly available navigation signals to confound, for example, guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.

SA typically added signal errors of up to about 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. To improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy.

During the Gulf War, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in a decision to disable Selective Availability. This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation, while at the same time denying it to the enemy. But since SA was also denying the same accuracy to thousands of friendly troops, turning it off or setting it to an error of zero meters (effectively the same thing) presented a clear benefit.

In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The military resisted for most of the 1990s, and it ultimately took an executive order to have SA removed from the GPS signal. The amount of error added was "set to zero"[14] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,[15] have stated that it is not intended to be reintroduced.

The US military has developed the ability to locally deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.[14]

One interesting side effect of the Selective Availability hardware is the capability to correct the frequency of the GPS caesium and rubidium atomic clocks to an accuracy of approximately 2 × 10-13 (one in five trillion). This represented a significant improvement over the raw accuracy of the clocks.[citation needed]

On 19 September 2007, the United States Department of Defense announced that they would not procure any more satellites capable of implementing SA. [16]

[edit] Relativity

According to the theory of relativity, due to their constant movement and height relative to the Earth-centered inertial reference frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45,900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7,200 ns per day. When combined, the discrepancy is 38 microseconds per day; a difference of 4.465 parts in 1010.[17]. To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.[18]

GPS observation processing must also compensate for another relativistic effect, the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system, a system in which simultaneity is not uniquely defined. The Lorentz transformation between the two systems modifies the signal run time, a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position.[19]

The atomic clocks on board the GPS satellites are precisely tuned, making the system a practical engineering application of the scientific theory of relativity in a real-world environment.

[edit] GPS interference and jamming

Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.

Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun. GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earth's magnetic field.[20] Another source of problems is the metal embedded in some car windscreens to prevent icing, degrading reception just inside the car.

Man-made interference can also disrupt, or jam, GPS signals. In one well documented case, an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning TV antenna preamplifier.[21] Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in the online magazine Phrack.[22]

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb during the Iraq War.[23] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The UK Ministry of Defence tested a jamming system in the UK's West Country on 7 and 8 June 2007. [24]

Some countries allow the use of GPS repeaters to allow for the reception of GPS signals indoors and in obscured locations, however, under EU and UK laws, the use of these is prohibited as the signals can cause interference to other GPS receivers that may receive data from both GPS satellites and the repeater.

Due to the potential for both natural and man-made noise, numerous techniques continue to be developed to deal with the interference. The first is to not rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a fallback plan in case of a GPS malfunction".[25] Receiver Autonomous Integrity Monitoring (RAIM) is a feature now included in some receivers, which is designed to provide a warning to the user if jamming or another problem is detected. The U.S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). In demonstration videos, the DAGR is able to detect jamming and maintain its lock on the encrypted GPS signals during interference which causes civilian receivers to lose lock.[26]

[edit] Techniques to improve accuracy

[edit] Augmentation

Main article: GNSS Augmentation

Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation process.

Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.

[edit] Precise monitoring

The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.

After SA, which has been turned off, the largest error in GPS is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.

Receivers with decryption keys can decode the P(Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and "authorized" agencies and are not available to the public. Without keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. However, this technique is slow, so it is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization, below). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a period 1000 times smaller than that of the C/A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.

Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to an accuracy of less than 10 centimeters (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).

[edit] GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset (19 seconds) with International Atomic Time (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC, which as of 2006 is 14 seconds. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, at the current rate of change of the Earth's rotation, is sufficient to last until the year 2330.

As opposed to the year, month, and day format of the Julian calendar, the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980 and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation messages use a 13-bit field, which only repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137.

[edit] GPS modernization

Main article: GPS modernization

Having reached the program's requirements for Full Operational Capability (FOC) on July 17, 1995,[27] the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS system. Announcements from the Vice President and the White House in 1998 initiated these changes, and in 2000 the U.S. Congress authorized the effort, referring to it as GPS III.

The project aims to improve the accuracy and availability for all users and involves new ground stations, new satellites, and four additional navigation signals. New civilian signals are called L2C, L5 and L1C; the new military code is called M-Code. Initial Operational Capability (IOC) of the L2C code is expected in 2008.[28] A goal of 2013 has been established for the entire program, with incentives offered to the contractors if they can complete it by 2011.

[edit] Applications

The Global Positioning System, while originally a military project, is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry.

[edit] Military

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The military use GPS for the following purposes:

[edit] Navigation

GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies.

[edit] Target tracking

Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile. These weapons systems pass GPS co-ordinates of targets to precision-guided munitions to allow them to engage the targets accurately.

Military aircraft, particularly those used in air-to-ground roles use GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be looked up in Google Earth).

[edit] Missile and projectile guidance

GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions.

Artillery projectiles with embedded GPS receivers able to withstand forces of 12,000G have been developed for use in 155 mm howitzers.[29]

[edit] Search and Rescue

Downed pilots can be located faster if they have a GPS receiver.

[edit] Reconnaissance and Map Creation

The military use GPS extensively to aid mapping and reconnaissance.

[edit] Other

The GPS satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System.[30]

[edit] Civilian

See also: GPS applications

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.

Many civilian applications benefit from GPS signals, using one or more of three basic components of the GPS; absolute location, relative movement, time transfer.

The ability to determine the receiver's absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation. The capacity to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. Being able to synchronize clocks to exacting standards enables time transfer, which is critical in large communication and observation systems. An example is CDMA digital cellular. Each base station has a GPS timing receiver to synchronize its spreading codes with other base stations to facilitate inter-cell hand off and support hybrid GPS/CDMA positioning of mobiles for emergency calls and other applications.

Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes.

To help prevent civilian GPS guidance from being used in an enemy's military or improvised weaponry, the US Government controls the export of civilian receivers. A US-based manufacturer cannot generally export a GPS receiver unless the receiver contains limits restricting it from functioning when it is simultaneously (1) at an altitude above 18 kilometers (60,000 ft) and (2) traveling at over 515 m/s (1,000 knots).[31]

[edit] History

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The design of GPS is based partly on the similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS system came when the Soviet Union launched the first Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.

The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology the GPS system relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first world-wide radio navigation system.

The first experimental Block-I GPS satellite was launched in February 1978.[28] The GPS satellites were initially manufactured by Rockwell International and are now manufactured by Lockheed Martin.

[edit] Timeline

* In 1972, the US Air Force Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental fight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.

* In 1978 the first experimental Block-I GPS satellite was launched.

* In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board, U.S. President Ronald Reagan announced that the GPS system would be made available for civilian uses once it was completed.

* By 1985, ten more experimental Block-I satellites had been launched to validate the concept.

* On February 14, 1989, the first modern Block-II satellite was launched.

* In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.

* By December 1993 the GPS system achieved initial operational capability[32]

* By January 17, 1994 a complete constellation of 24 satellites was in orbit.

* Full Operational Capability was declared by NAVSTAR in April 1995.

* In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[33] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.

* In 1998, U.S. Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety.

* On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.

* In 2004, the United States Government signed a historic agreement with the European Community establishing cooperation related to GPS and Europe's planned Galileo system.

* In 2004, U.S. President George W. Bush updated the national policy, replacing the executive board with the National Space-Based Positioning, Navigation, and Timing Executive Committee.

* November 2004, QUALCOMM announced successful tests of Assisted-GPS system for mobile phones.[3]

* In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.

* The most recent launch was on 17 November 2006. The oldest GPS satellite still in operation was launched in August 1991.

* On September 14, 2007, the aging mainframe-based Ground Segment Control System was transitioned to the new Architecture Evolution Plan. [4]

[edit] Satellite numbers

Name Launch Period No of satellites launched, inc. launch failures Currently in service

Block I 1978-1985 11 0

Block II 1985-1990 9 0

Block IIA 1990-1997 19 15+11

Block IIR 1997-2004 12 12

Block IIR-M 2005- 3 3

Total 54 (plus one not launched) 30+1

1One test satellite

[edit] Awards

Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:

* Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).

* Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force.

One GPS developer, Roger L. Easton, received the National Medal of Technology on February 13, 2006 at the White House.[34]

On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious aviation award in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."

[edit] Other systems

Main article: Global Navigation Satellite System

Other satellite navigation systems in use or various states of development include:

* Beidou — China's regional system that China has proposed to expand into a global system named COMPASS.

* Galileo — a proposed global system being developed by the European Union, joined by China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine planned to be operational by 2011–12.

* GLONASS — Russia's global system which is being restored to full availability in partnership with India.

* Indian Regional Navigational Satellite System (IRNSS) — India's proposed regional system.

* QZSS - Japanese proposed regional system, adding better coverage to the Japanese islands.

[edit] See also

Satellite navigation systems Portal

Nautical Portal

* RAIM

* SIGI

* radio navigation

* High Sensitivity GPS

* Degree Confluence Project Use GPS to visit integral degrees of latitude and longitude.

* Exif, GPS data transfer.

* Geotagging

* Geocaching

* NaviTraveler.com, - a GPS point sharing community.

* GPS Drawing Digital mapping and drawing with GPS tracks.

* GPS tracking

* GPS/INS

* Assisted GPS

* GPX (XML schema for interchange of waypoints)

* ID Sniper rifle

* OpenStreetMap, free content maps and street pictures (GFDL)

* Telematics: Many telematics devices use GPS to determine the location of mobile equipment.

* The American Practical Navigator—Chapter 11 "Satellite Navigation"

* Point of Interest

* Automotive navigation system

* NextGen

[edit] Notes

1. ^ Parkinson, B.W. (1996), Global Positioning System: Theory and Applications, chap. 1: Introduction and Heritage of NAVSTAR, the Global Positioning System. pp. 3-28, American Institute of Aeronautics and Astronautics, Washington, D.C.

2. ^ a b GPS Overview from the NAVSTAR Joint Program Office. Accessed December 15, 2006.

3. ^ HowStuffWorks. How GPS Receivers Work. Accessed May 14, 2006.

4. ^ globalsecurity.org [1].

5. ^ Dana, Peter H. GPS Orbital Planes. August 8, 1996.

6. ^ What the Global Positioning System Tells Us about Relativity. Accessed January 2, 2007.

7. ^ USCG Navcen: GPS Frequently Asked Questions. Accessed January 3, 2007.

8. ^ Massatt, Paul and Brady, Wayne. "Optimizing performance through constellation management", Crosslink, Summer 2002, pages 17-21.

9. ^ US Coast Guard General GPS News 9-9-05

10. ^ USNO. NAVSTAR Global Positioning System. Accessed May 14, 2006.

11. ^ NMEA NMEA 2000

12. ^ http://gge.unb.ca/Resources/HowDoesGPSWork.html

13. ^ AN02 Network Assistance (HTML). Retrieved on 2007-09-10.

14. ^ a b Office of Science and Technology Policy. Presidential statement to stop degrading GPS. May 1, 2000.

15. ^ FAA, Selective Availability. Retrieved Jan. 6, 2007.

16. ^ http://www.defenselink.mil/releases/release.aspx?releaseid=11335

17. ^ Rizos, Chris. University of New South Wales. GPS Satellite Signals. 1999.

18. ^ The Global Positioning System by Robert A. Nelson Via Satellite, November 1999

19. ^ Ashby, Neil Relativity and GPS. Physics Today, May 2002.

20. ^ Space Environment Center. SEC Navigation Systems GPS Page. August 26, 1996.

21. ^ The hunt for an unintentional GPS jammer. GPS World. January 1, 2003.

22. ^ Low Cost and Portable GPS Jammer. Phrack issue 0x3c (60), article 13]. Published December 28, 2002.

23. ^ American Forces Press Service. CENTCOM charts progress. March 25, 2003.

24. ^ [2]

25. ^ Ruley, John. AVweb. GPS jamming. February 12, 2003.

26. ^ Commercial GPS Receivers: Facts for the Warfighter. Hosted at the Joint Chiefs website, linked by the USAF's GPS Wing DAGR program website. Accessed on 10 April, 2007

27. ^ US Coast Guard news release. Global Positioning System Fully Operational

28. ^ a b Hydrographic Society Journal. Developments in Global Navigation Satellite Systems. Issue #104, April 2002. Accessed April 5, 2007.

29. ^ XM982 Excalibur Precision Guided Extended Range Artillery Projectile. GlobalSecurity.org (2007-05-29). Retrieved on 2007-09-26.

30. ^ Sandia National Laboratory's Nonproliferation programs and arms control technology.

31. ^ Arms Control Association. Missile Technology Control Regime. Accessed May 17, 2006.

32. ^ United States Department of Defense. Announcement of Initial Operational Capability. December 8, 1993.

33. ^ National Archives and Records Administration. U.S. GLOBAL POSITIONING SYSTEM POLICY. March 29, 1996.

34. ^ United States Naval Research Laboratory. National Medal of Technology for GPS. November 21, 2005

[edit] External links

Wikimedia Commons has media related to:

Global Positioning System

Government links

* GPS.gov—General public education website created by the U.S. Government

* National Space-Based PNT Executive Committee—Established in 2004 to oversee management of GPS and GPS augmentations at a national level.

* USCG Navigation Center—Status of the GPS constellation, government policy, and links to other references. Also includes satellite almanac data.

* The GPS Joint Program Office (GPS JPO)—Responsible for designing and acquiring the system on behalf of the US Government.

* U.S. Naval Observatory's GPS constellation status

* U.S. Army Corps of Engineers manual: NAVSTAR HTML and PDF (22.6 MB, 328 pages)

* PNT Selective Availability Announcements

* GPS SPS Signal Specification, 2nd Edition—The official Standard Positioning Signal specification.

* Federal Aviation Administration's GPS FAQ

Introductory / tutorial links

* How does GPS work? TomTom explains GPS, navigation, and digital maps

* GPS Academy Garmin interactive video web site explaing what exactly GPS is and what it can do for you

* HowStuffWorks' Simplified explanation of GPS and video about how GPS works.

* Trimble's Online GPS Tutorial Tutorial designed to introduce you to the principles behind GPS

* GPS and GLONASS Simulation(Java applet) Simulation and graphical depiction of space vehicle motion including computation of dilution of precision (DOP)

Technical, historical, and ancillary topics links

* Dana, Peter H. "Global Positioning System Overview"

* Satellite Navigation: GPS & Galileo (PDF)—16-page paper about the history and working of GPS, touching on the upcoming Galileo

* History of GPS, including information about each satellite's configuration and launch.

* Chadha, Kanwar. "The Global Positioning System: Challenges in Bringing GPS to Mainstream Consumers" Technical Article (1998)

* GPS Weapon Guidance Techniques

* RAND history of the GPS system (PDF)

* GPS Anti-Jam Protection Techniques

* Crosslink Summer 2002 issue by The Aerospace Corporation on satellite navigation.

* Improved weather predictions from COSMIC GPS satellite signal occultation data.

* David L. Wilson's GPS Accuracy Web Page A thorough analysis of the accuracy of GPS.

* Innovation: Spacecraft Navigator, Autonomous GPS Positioning at High Earth Orbits Example of GPS receiver designed for high altitude spaceflight.

* The Navigator GPS Receiver GSFC's Navigator spaceflight receiver.

* Neil Ashby's Relativity in the Global Positioning System

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Systems Biological system · Complex system · Complex adaptive system · Conceptual system · Cultural system · Dynamical system · Economic system · Ecosystem · Formal system · Global Positioning System · Human organ systems · Information systems · Legal system · Metric system · Nervous system · Non-linear system · Operating system · Physical system · Political system · Sensory system · Social system · Solar System · System · Systems of measurement

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Systems scientists Russell L. Ackoff · William Ross Ashby · Gregory Bateson · Ludwig von Bertalanffy · Kenneth E. Boulding · Peter Checkland · C. West Churchman · Heinz von Foerster · Charles François · Jay Wright Forrester · Ralph W. Gerard · Debora Hammond · George Klir · Niklas Luhmann · Humberto Maturana · Donella Meadows · Mihajlo D. Mesarovic · Howard T. Odum · Talcott Parsons · Ilya Prigogine · Anatol Rapoport · Francisco Varela · John N. Warfield · Norbert Wiener

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The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed/direction, and time.

Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr. John Walsh, a key decision maker when it came to the budget for the GPS program[1]). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year,[2] including the replacement of aging satellites, and research and development. Despite these costs, GPS is free for civilian use as a public good.

GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Simplified method of operation

A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS microwave signal gives the distance to each satellite, since the signal travels at a known speed - the speed of light. These signals also carry information about the satellites' location and general system health (known as almanac and ephemeris data). By determining the position of, and distance to, at least three satellites, the receiver can compute its position using trilateration.[3] Receivers typically do not have perfectly accurate clocks and therefore track one or more additional satellites, using their atomic clocks to correct the receiver's own clock error.

[edit] Technical description

Unlaunched GPS satellite on display at the San Diego Aerospace museum

Unlaunched GPS satellite on display at the San Diego Aerospace museum

[edit] System segmentation

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).[4]

[edit] Space segment

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design calls for 24 SVs to be distributed equally among six circular orbital planes.[5] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.[6] The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).[2]

Orbiting at an altitude of approximately 20,200 kilometers (12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day, so it passes over the same location on Earth once each day. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.[7]

As of September 2007, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.[8]

[edit] Control segment

The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA).[9] The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2d Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within one microsecond and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.[10]

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).

[edit] User segment

The user's GPS receiver is the user segment (US) of the GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2006, receivers typically have between twelve and twenty channels.

A typical OEM GPS receiver module, based on the SiRF Star III chipset, measuring 15×17 mm, and used in many products.

A typical OEM GPS receiver module, based on the SiRF Star III chipset, measuring 15×17 mm, and used in many products.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bit/s speed. Data are actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000[11] is a newer and less widely adopted protocol. Both are proprietary and controlled by the US-based National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

[edit] Navigation signals

Main article: GPS signals

GPS broadcast signal

GPS broadcast signal

Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-of-day, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part of the message). The ephemeris data gives the satellite's own precise orbit and is output over 18 seconds, repeating every 30 seconds. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for 6 hour time-outs. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to the low data transmission rate. The almanac consists of coarse orbit and status information for each satellite in the constellation and takes 12 seconds for each satellite present, with information for a new satellite being transmitted every 30 seconds (15.5 minutes for 31 satellites). The purpose of the data is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. An important thing to note about navigation data is that each satellite transmits only its own ephemeris, but transmits an almanac for all satellites.

Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise (P) code, which is usually encrypted and reserved for military applications. The C/A code is a 1,023 chip pseudo-random (PRN) code at 1.023 million chips/sec so that it repeats every millisecond. Each satellite has its own C/A code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency. The P-code is a 10.23 megachip/sec PRN code that repeats only every week. When the "anti-spoofing" mode is on, as it is in normal operation, the P code is encrypted by the Y-code to produce the P(Y) code, which can only be decrypted by units with a valid decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user. Frequencies used by GPS include

* L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.

* L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.

* L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.

* L4 (1379.913 MHz): Being studied for additional ionospheric correction.

* L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2008.

[edit] Calculating positions

[edit] Using the C/A code

To start off, the receiver picks which C/A codes to listen for by PRN number, based on the almanac information it has previously acquired. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern, then measures the time delay for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number as the satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange[12].

Overlapping pseudoranges, represented as curves, are modified to yield the probable position

Overlapping pseudoranges, represented as curves, are modified to yield the probable position

Next, the orbital position data, or ephemeris, from the Navigation Message is then downloaded to calculate the satellite's precise position. A more-sensitive receiver will potentially acquire the ephemeris data quicker than a less-sensitive receiver, especially in a noisy environment.[13] Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. Receivers can substitute altitude for one satellite, which the GPS receiver translates to a pseudorange measured from the center of the earth.

Locations are calculated not in three-dimensional space, but in four-dimensional spacetime, meaning a measure of the precise time-of-day is very important. The measured pseudoranges from four satellites have already been determined with the receiver's internal clock, and thus have an unknown amount of clock error. (The clock error or actual time does not matter in the initial pseudorange calculation, because that is based on how much time has passed between reception of each of the signals.[clarify][citation needed]) The four-dimensional point that is equidistant from the pseudoranges is calculated as a guess as to the receiver's location, and the factor used to adjust those pseudoranges to intersect at that four-dimensional point gives a guess as to the receiver's clock offset. With each guess, a geometric dilution of precision (GDOP) vector is calculated, based on the relative sky positions of the satellites used. As more satellites are picked up, pseudoranges from more combinations of four satellites can be processed to add more guesses to the location and clock offset. The receiver then determines which combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock offsets. After the final location and time are calculated, the location is expressed in a specific coordinate system, e.g. latitude/longitude, using the WGS 84 geodetic datum or a local system specific to a country.

[edit] Using the P(Y) code

Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism: if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite.[citation needed] In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators. RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective.

[edit] Accuracy and error sources

The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about 1% of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only the GPS C/A signal.

Position accuracy can be improved by using the higher-chiprate P(Y) signal. Assuming the same 1% bit time accuracy, the high frequency P(Y) signal results in an accuracy of about 30 centimeters.

Electronics errors are one of several accuracy-degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy.

Sources of User Equivalent Range Errors (UERE) Source Effect

Ionospheric effects ± 5 meter

Ephemeris errors ± 2.5 meter

Satellite clock errors ± 2 meter

Multipath distortion ± 1 meter

Tropospheric effects ± 0.5 meter

Numerical errors ± 1 meter

[edit] Atmospheric effects

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere and ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the signal is affected for a longer time. Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.

Because ionospheric delay affects the speed of microwave signals differently based on frequency—a characteristic known as dispersion—both frequency bands can be used to help reduce this error. Some military and expensive survey-grade civilian receivers compare the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.

The effects of the ionosphere generally change slowly, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1 only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems such as WAAS, which transmits it on the GPS frequency using a special pseudo-random number (PRN), so only one antenna and receiver are required.

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect is both more localized and changes more quickly than ionospheric effects and is not frequency dependent. These traits making precise measurement and compensation of humidity errors more difficult than ionospheric effects.

Changes in altitude also change the amount of delay due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct.

[edit] Multipath effects

GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas may be used to reduce the signal power as received by the antenna. Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.

Multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.

[edit] Ephemeris and clock errors

The navigation message from a satellite is sent out only every 30 seconds. In reality, the data contained in these messages tend to be "out of date" by an even larger amount. Consider the case when a GPS satellite is boosted back into a proper orbit; for some time following the maneuver, the receiver's calculation of the satellite's position will be incorrect until it receives another ephemeris update. The onboard clocks are extremely accurate, but they do suffer from some clock drift. This problem tends to be very small, but may add up to 2 meters (6 ft) of inaccuracy.

This class of error is more "stable" than ionospheric problems and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending out a more accurate almanac on a separate channel.

[edit] Selective availability

The GPS includes a feature called Selective Availability (SA) that introduces intentional, slowly changing random errors of up to a hundred meters (328 ft) into the publicly available navigation signals to confound, for example, guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.

SA typically added signal errors of up to about 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. To improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy.

During the Gulf War, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in a decision to disable Selective Availability. This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation, while at the same time denying it to the enemy. But since SA was also denying the same accuracy to thousands of friendly troops, turning it off or setting it to an error of zero meters (effectively the same thing) presented a clear benefit.

In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The military resisted for most of the 1990s, and it ultimately took an executive order to have SA removed from the GPS signal. The amount of error added was "set to zero"[14] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,[15] have stated that it is not intended to be reintroduced.

The US military has developed the ability to locally deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.[14]

One interesting side effect of the Selective Availability hardware is the capability to correct the frequency of the GPS caesium and rubidium atomic clocks to an accuracy of approximately 2 × 10-13 (one in five trillion). This represented a significant improvement over the raw accuracy of the clocks.[citation needed]

On 19 September 2007, the United States Department of Defense announced that they would not procure any more satellites capable of implementing SA. [16]

[edit] Relativity

According to the theory of relativity, due to their constant movement and height relative to the Earth-centered inertial reference frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45,900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7,200 ns per day. When combined, the discrepancy is 38 microseconds per day; a difference of 4.465 parts in 1010.[17]. To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.[18]

GPS observation processing must also compensate for another relativistic effect, the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system, a system in which simultaneity is not uniquely defined. The Lorentz transformation between the two systems modifies the signal run time, a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position.[19]

The atomic clocks on board the GPS satellites are precisely tuned, making the system a practical engineering application of the scientific theory of relativity in a real-world environment.

[edit] GPS interference and jamming

Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.

Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun. GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earth's magnetic field.[20] Another source of problems is the metal embedded in some car windscreens to prevent icing, degrading reception just inside the car.

Man-made interference can also disrupt, or jam, GPS signals. In one well documented case, an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning TV antenna preamplifier.[21] Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in the online magazine Phrack.[22]

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb during the Iraq War.[23] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The UK Ministry of Defence tested a jamming system in the UK's West Country on 7 and 8 June 2007. [24]

Some countries allow the use of GPS repeaters to allow for the reception of GPS signals indoors and in obscured locations, however, under EU and UK laws, the use of these is prohibited as the signals can cause interference to other GPS receivers that may receive data from both GPS satellites and the repeater.

Due to the potential for both natural and man-made noise, numerous techniques continue to be developed to deal with the interference. The first is to not rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a fallback plan in case of a GPS malfunction".[25] Receiver Autonomous Integrity Monitoring (RAIM) is a feature now included in some receivers, which is designed to provide a warning to the user if jamming or another problem is detected. The U.S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). In demonstration videos, the DAGR is able to detect jamming and maintain its lock on the encrypted GPS signals during interference which causes civilian receivers to lose lock.[26]

[edit] Techniques to improve accuracy

[edit] Augmentation

Main article: GNSS Augmentation

Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation process.

Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.

[edit] Precise monitoring

The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.

After SA, which has been turned off, the largest error in GPS is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency.

Receivers with decryption keys can decode the P(Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and "authorized" agencies and are not available to the public. Without keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. However, this technique is slow, so it is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization, below). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a period 1000 times smaller than that of the C/A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.

Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to an accuracy of less than 10 centimeters (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).

[edit] GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset (19 seconds) with International Atomic Time (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC, which as of 2006 is 14 seconds. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, at the current rate of change of the Earth's rotation, is sufficient to last until the year 2330.

As opposed to the year, month, and day format of the Julian calendar, the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980 and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation messages use a 13-bit field, which only repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137.

[edit] GPS modernization

Main article: GPS modernization

Having reached the program's requirements for Full Operational Capability (FOC) on July 17, 1995,[27] the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS system. Announcements from the Vice President and the White House in 1998 initiated these changes, and in 2000 the U.S. Congress authorized the effort, referring to it as GPS III.

The project aims to improve the accuracy and availability for all users and involves new ground stations, new satellites, and four additional navigation signals. New civilian signals are called L2C, L5 and L1C; the new military code is called M-Code. Initial Operational Capability (IOC) of the L2C code is expected in 2008.[28] A goal of 2013 has been established for the entire program, with incentives offered to the contractors if they can complete it by 2011.

[edit] Applications

The Global Positioning System, while originally a military project, is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry.

[edit] Military

Please help improve this article by expanding this section.

See talk page for details. Please remove this message once the section has been expanded.

The military use GPS for the following purposes:

[edit] Navigation

GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies.

[edit] Target tracking

Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile. These weapons systems pass GPS co-ordinates of targets to precision-guided munitions to allow them to engage the targets accurately.

Military aircraft, particularly those used in air-to-ground roles use GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show GPS co-ordinates that can be looked up in Google Earth).

[edit] Missile and projectile guidance

GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions.

Artillery projectiles with embedded GPS receivers able to withstand forces of 12,000G have been developed for use in 155 mm howitzers.[29]

[edit] Search and Rescue

Downed pilots can be located faster if they have a GPS receiver.

[edit] Reconnaissance and Map Creation

The military use GPS extensively to aid mapping and reconnaissance.

[edit] Other

The GPS satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System.[30]

[edit] Civilian

See also: GPS applications

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.

This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.

Many civilian applications benefit from GPS signals, using one or more of three basic components of the GPS; absolute location, relative movement, time transfer.

The ability to determine the receiver's absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation. The capacity to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. Being able to synchronize clocks to exacting standards enables time transfer, which is critical in large communication and observation systems. An example is CDMA digital cellular. Each base station has a GPS timing receiver to synchronize its spreading codes with other base stations to facilitate inter-cell hand off and support hybrid GPS/CDMA positioning of mobiles for emergency calls and other applications.

Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes.

To help prevent civilian GPS guidance from being used in an enemy's military or improvised weaponry, the US Government controls the export of civilian receivers. A US-based manufacturer cannot generally export a GPS receiver unless the receiver contains limits restricting it from functioning when it is simultaneously (1) at an altitude above 18 kilometers (60,000 ft) and (2) traveling at over 515 m/s (1,000 knots).[31]

[edit] History

Please help improve this article by expanding this section.

See talk page for details. Please remove this message once the section has been expanded.

The design of GPS is based partly on the similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS system came when the Soviet Union launched the first Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.

The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology the GPS system relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first world-wide radio navigation system.

The first experimental Block-I GPS satellite was launched in February 1978.[28] The GPS satellites were initially manufactured by Rockwell International and are now manufactured by Lockheed Martin.

[edit] Timeline

* In 1972, the US Air Force Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental fight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.

* In 1978 the first experimental Block-I GPS satellite was launched.

* In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board, U.S. President Ronald Reagan announced that the GPS system would be made available for civilian uses once it was completed.

* By 1985, ten more experimental Block-I satellites had been launched to validate the concept.

* On February 14, 1989, the first modern Block-II satellite was launched.

* In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.

* By December 1993 the GPS system achieved initial operational capability[32]

* By January 17, 1994 a complete constellation of 24 satellites was in orbit.

* Full Operational Capability was declared by NAVSTAR in April 1995.

* In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[33] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.

* In 1998, U.S. Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety.

* On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.

* In 2004, the United States Government signed a historic agreement with the European Community establishing cooperation related to GPS and Europe's planned Galileo system.

* In 2004, U.S. President George W. Bush updated the national policy, replacing the executive board with the National Space-Based Positioning, Navigation, and Timing Executive Committee.

* November 2004, QUALCOMM announced successful tests of Assisted-GPS system for mobile phones.[3]

* In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.

* The most recent launch was on 17 November 2006. The oldest GPS satellite still in operation was launched in August 1991.

* On September 14, 2007, the aging mainframe-based Ground Segment Control System was transitioned to the new Architecture Evolution Plan. [4]

[edit] Satellite numbers

Name Launch Period No of satellites launched, inc. launch failures Currently in service

Block I 1978-1985 11 0

Block II 1985-1990 9 0

Block IIA 1990-1997 19 15+11

Block IIR 1997-2004 12 12

Block IIR-M 2005- 3 3

Total 54 (plus one not launched) 30+1

1One test satellite

[edit] Awards

Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:

* Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).

* Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force.

One GPS developer, Roger L. Easton, received the National Medal of Technology on February 13, 2006 at the White House.[34]

On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious aviation award in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."

[edit] Other systems

Main article: Global Navigation Satellite System

Other satellite navigation systems in use or various states of development include:

* Beidou — China's regional system that China has proposed to expand into a global system named COMPASS.

* Galileo — a proposed global system being developed by the European Union, joined by China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine planned to be operational by 2011–12.

* GLONASS — Russia's global system which is being restored to full availability in partnership with India.

* Indian Regional Navigational Satellite System (IRNSS) — India's proposed regional system.

* QZSS - Japanese proposed regional system, adding better coverage to the Japanese islands.

[edit] See also

Satellite navigation systems Portal

Nautical Portal

* RAIM

* SIGI

* radio navigation

* High Sensitivity GPS

* Degree Confluence Project Use GPS to visit integral degrees of latitude and longitude.

* Exif, GPS data transfer.

* Geotagging

* Geocaching

* NaviTraveler.com, - a GPS point sharing community.

* GPS Drawing Digital mapping and drawing with GPS tracks.

* GPS tracking

* GPS/INS

* Assisted GPS

* GPX (XML schema for interchange of waypoints)

* ID Sniper rifle

* OpenStreetMap, free content maps and street pictures (GFDL)

* Telematics: Many telematics devices use GPS to determine the location of mobile equipment.

* The American Practical Navigator—Chapter 11 "Satellite Navigation"

* Point of Interest

* Automotive navigation system

* NextGen

[edit] Notes

1. ^ Parkinson, B.W. (1996), Global Positioning System: Theory and Applications, chap. 1: Introduction and Heritage of NAVSTAR, the Global Positioning System. pp. 3-28, American Institute of Aeronautics and Astronautics, Washington, D.C.

2. ^ a b GPS Overview from the NAVSTAR Joint Program Office. Accessed December 15, 2006.

3. ^ HowStuffWorks. How GPS Receivers Work. Accessed May 14, 2006.

4. ^ globalsecurity.org [1].

5. ^ Dana, Peter H. GPS Orbital Planes. August 8, 1996.

6. ^ What the Global Positioning System Tells Us about Relativity. Accessed January 2, 2007.

7. ^ USCG Navcen: GPS Frequently Asked Questions. Accessed January 3, 2007.

8. ^ Massatt, Paul and Brady, Wayne. "Optimizing performance through constellation management", Crosslink, Summer 2002, pages 17-21.

9. ^ US Coast Guard General GPS News 9-9-05

10. ^ USNO. NAVSTAR Global Positioning System. Accessed May 14, 2006.

11. ^ NMEA NMEA 2000

12. ^ http://gge.unb.ca/Resources/HowDoesGPSWork.html

13. ^ AN02 Network Assistance (HTML). Retrieved on 2007-09-10.

14. ^ a b Office of Science and Technology Policy. Presidential statement to stop degrading GPS. May 1, 2000.

15. ^ FAA, Selective Availability. Retrieved Jan. 6, 2007.

16. ^ http://www.defenselink.mil/releases/release.aspx?releaseid=11335

17. ^ Rizos, Chris. University of New South Wales. GPS Satellite Signals. 1999.

18. ^ The Global Positioning System by Robert A. Nelson Via Satellite, November 1999

19. ^ Ashby, Neil Relativity and GPS. Physics Today, May 2002.

20. ^ Space Environment Center. SEC Navigation Systems GPS Page. August 26, 1996.

21. ^ The hunt for an unintentional GPS jammer. GPS World. January 1, 2003.

22. ^ Low Cost and Portable GPS Jammer. Phrack issue 0x3c (60), article 13]. Published December 28, 2002.

23. ^ American Forces Press Service. CENTCOM charts progress. March 25, 2003.

24. ^ [2]

25. ^ Ruley, John. AVweb. GPS jamming. February 12, 2003.

26. ^ Commercial GPS Receivers: Facts for the Warfighter. Hosted at the Joint Chiefs website, linked by the USAF's GPS Wing DAGR program website. Accessed on 10 April, 2007

27. ^ US Coast Guard news release. Global Positioning System Fully Operational

28. ^ a b Hydrographic Society Journal. Developments in Global Navigation Satellite Systems. Issue #104, April 2002. Accessed April 5, 2007.

29. ^ XM982 Excalibur Precision Guided Extended Range Artillery Projectile. GlobalSecurity.org (2007-05-29). Retrieved on 2007-09-26.

30. ^ Sandia National Laboratory's Nonproliferation programs and arms control technology.

31. ^ Arms Control Association. Missile Technology Control Regime. Accessed May 17, 2006.

32. ^ United States Department of Defense. Announcement of Initial Operational Capability. December 8, 1993.

33. ^ National Archives and Records Administration. U.S. GLOBAL POSITIONING SYSTEM POLICY. March 29, 1996.

34. ^ United States Naval Research Laboratory. National Medal of Technology for GPS. November 21, 2005

[edit] External links

Wikimedia Commons has media related to:

Global Positioning System

Government links

* GPS.gov—General public education website created by the U.S. Government

* National Space-Based PNT Executive Committee—Established in 2004 to oversee management of GPS and GPS augmentations at a national level.

* USCG Navigation Center—Status of the GPS constellation, government policy, and links to other references. Also includes satellite almanac data.

* The GPS Joint Program Office (GPS JPO)—Responsible for designing and acquiring the system on behalf of the US Government.

* U.S. Naval Observatory's GPS constellation status

* U.S. Army Corps of Engineers manual: NAVSTAR HTML and PDF (22.6 MB, 328 pages)

* PNT Selective Availability Announcements

* GPS SPS Signal Specification, 2nd Edition—The official Standard Positioning Signal specification.

* Federal Aviation Administration's GPS FAQ

Introductory / tutorial links

* How does GPS work? TomTom explains GPS, navigation, and digital maps

* GPS Academy Garmin interactive video web site explaing what exactly GPS is and what it can do for you

* HowStuffWorks' Simplified explanation of GPS and video about how GPS works.

* Trimble's Online GPS Tutorial Tutorial designed to introduce you to the principles behind GPS

* GPS and GLONASS Simulation(Java applet) Simulation and graphical depiction of space vehicle motion including computation of dilution of precision (DOP)

Technical, historical, and ancillary topics links

* Dana, Peter H. "Global Positioning System Overview"

* Satellite Navigation: GPS & Galileo (PDF)—16-page paper about the history and working of GPS, touching on the upcoming Galileo

* History of GPS, including information about each satellite's configuration and launch.

* Chadha, Kanwar. "The Global Positioning System: Challenges in Bringing GPS to Mainstream Consumers" Technical Article (1998)

* GPS Weapon Guidance Techniques

* RAND history of the GPS system (PDF)

* GPS Anti-Jam Protection Techniques

* Crosslink Summer 2002 issue by The Aerospace Corporation on satellite navigation.

* Improved weather predictions from COSMIC GPS satellite signal occultation data.

* David L. Wilson's GPS Accuracy Web Page A thorough analysis of the accuracy of GPS.

* Innovation: Spacecraft Navigator, Autonomous GPS Positioning at High Earth Orbits Example of GPS receiver designed for high altitude spaceflight.

* The Navigator GPS Receiver GSFC's Navigator spaceflight receiver.

* Neil Ashby's Relativity in the Global Positioning System

[show]

v • d • e

Satellite navigation systems

Historical Flag of the United States Transit

Operational Flag of the Soviet Union / Flag of Russia GLONASS · Flag of the United States GPS

Developmental Flag of the People's Republic of China Beidou/COMPASS · Flag of Europe Galileo · Flag of India IRNSS · Flag of Japan QZSS

Related topics EGNOS · GAGAN · GPS·C · LAAS · MSAS · WAAS

[show]

v • d • e

Time signal stations

Longwave DCF77 · HBG · JJY · MSF · TDF · WWVB

Shortwave BPM · CHU · RWM · WWV · WWVH · YVTO

GNSS time transfer Beidou · Galileo · GLONASS · GPS · IRNSS

Defunct time stations OMA · VNG

[show]

v • d • e

Global structure in Systems, Systems sciences and Systems scientists

Categories Category:Conceptual systems · Category:Physical systems · Category:Social systems · Category:Systems · Category:Systems science · Category:Systems scientists · Category:Systems theory

Systems Biological system · Complex system · Complex adaptive system · Conceptual system · Cultural system · Dynamical system · Economic system · Ecosystem · Formal system · Global Positioning System · Human organ systems · Information systems · Legal system · Metric system · Nervous system · Non-linear system · Operating system · Physical system · Political system · Sensory system · Social system · Solar System · System · Systems of measurement

Fields of theory Chaos theory · Complex systems · Control theory · Cybernetics · Holism in science · Sociotechnical systems theory · Systems biology · System dynamics · Systems ecology · Systems engineering · Systems theory · Systems science

Systems scientists Russell L. Ackoff · William Ross Ashby · Gregory Bateson · Ludwig von Bertalanffy · Kenneth E. Boulding · Peter Checkland · C. West Churchman · Heinz von Foerster · Charles François · Jay Wright Forrester · Ralph W. Gerard · Debora Hammond · George Klir · Niklas Luhmann · Humberto Maturana · Donella Meadows · Mihajlo D. Mesarovic · Howard T. Odum · Talcott Parsons · Ilya Prigogine · Anatol Rapoport · Francisco Varela · John N. Warfield · Norbert Wiener

Retrieved from "http://en.wikipedia.org/wiki/Global_Positioning_System"

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Tackling emergency situations - The attack on the Twin Towers in September/11/2001 caused research to evolve emergency calling capabilities. It was made compulsory for all cell phones to be able to track calls within a distance of a 100 meters. This made it necessary to integrate GPS technology in cell phones. This was the easiest and most innovative way of achieving the desired results. However, the average GPS cell phone is a bit different from the GPS receivers that you take hiking or use inside vehicles. There is no direct access to GPS data and a wireless network can only give you the accurate location. Further, GPS data can be transmitted only in case of grievous emergency or something akin to the 9/11 situation. A 911 emergency call can make this happen. So in order that you can track somebody, you need particular cell phones, network and service.

You cannot get lost - People who constantly tour places can take advantage of the GPS cell phone. It helps to track their locations and movements. You can easily call for assistance if you get into trouble on the way. Help will come to you as your position can easily be tracked. Hence, this proves to be a very handy device if you suffer an accident or are lost in the woods. The tracking system can instantly bring help to you. That is why hikers find the GPS cell phone a very useful device.

How it can be tracked - There are operator towers everywhere in the vicinity and the nearest one will receive and transmit signals to the owner of a GPS cell phone. You do not have to fear losing touch with the tower even when you are in the standby mode. The GPS cell phone works along the lines of the navigation tool which is the ideal one for remote trips. Thus, cell phones which are CDMA enabled or have GSM functionality can be tracked provided they are GPS integrated.

The functioning - It is important to understand the functioning of the GPS cell phone. The service provider identifies the location of the cell phone by zeroing in on the tower closest to the signals being transmitted. The strongest of these signals is identified which leads to tracking the position of the GPS cell phone within 100 meters of its location. Besides helping out people in times of distress, this innovative device also helps to track miscreants and terrorists.

Best to track your child - Another ingenuous feature of the GPS cell phone is its ability to function as a child GPS locater device. You can easily keep tabs on your child instead of ringing him up at frequent intervals. This feature is particularly useful in crowded malls and parks where your child may give you the slip. However, this provides a genuine opportunity to locate your young kids and not a means to interfere in their privacy. This facility is useful when your kids are very small and there are great chances of them getting lost. Thus, GPS cell phone is a boon in many ways if you discount the ugly ways in which it can transgress into your personal life.

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I really would like to understand the difference between Car GPS and Trecking GPS. Are they similar?

I mean here from the point of syncing with satellites. I saw some trecking GPS that sync with 12 ( 12 channels). Does that mean that they are more sensitive than Car GPS?

Is it possible to use car GPS for trecking

Or are there GPS that can be used for both applications at the same time, and may be also for sailing, let's say

If they are identical hardware-wise, does that mean one can use maps files for cars on a trecking GPS, or vice-versa?

Thank you in advance.

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This left one of the best funded and best equipped Formula One teams in the field in limbo while the management team attempted to find a buyer for the team and save 700 plus worker’s jobs at the teams Brackley HQ. Between December 2008 and February 2009 a number of possible buyers’ names were being thrown about by the media and team boss Nick Fry claimed the management was negotiating with a dozen possible buyers. The media reported that the team was close to being saved a number of times by parties including Prodrive, Mexican billionaire Carlos Slim and a ‘consortium of Brazilian investors’ however none of these came to fruition and it was soon looking likely that the only realistic option was a management buyout.

In Feburary 2009 it came to light that the Honda management were in negotiations with Richard Branson’s Virgin Group and they looked set to buy the team however speculation ended when Richard Branson revealed he was interested in F1 but only once certain conditions were met, mainly that costs were reduced and that the sport was championing green technology.

On the 6th March 2009 the Honda Motor Company and the former Honda Racing F1 Team management announced that the team had been saved buy a management buyout from technical director Ross Brawn and the new team Brawn GP was born. The team went on to test its car for the first time in an official test at Circuit de Cataluña, Barcelona in March where they were they performed very well, setting an unofficial lap record in the process. The team was accused of running the car underweight in an attempt to attract sponsors to help fund the fledgling team however these claims were denied by the team. Others accused the team along with rivals Williams and Toyota of having an illegal rear diffuser under the new 2009 regulations, claims which were also denied by all three teams. Whatever accusations were thrown at the team, nobody could deny that it looks like Brawn GP had a very competitive car that could turn out to be one of the fastest come the season opening race in Australia.

Roll on the last weekend in March and it was time for the start of the 2009 Formula One season, throughout the weekend Brawn GP showed that it’s pace in pre-season testing was no bluff and in-fact they had one of the best cars on the grid. They proved this by first locking out the front row in qualifying and then going on to secure a historic 1-2 finish in the race, which was only the third time in history that a brand new team had managed it.

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GPS cell phones of today are well-featured such that the desired destination can be keyed into. This technology is amazing and the practical use has increased the value of every GPS cellular phone. The device guides the user in a pleasing voice. These phones can be considered to be mini computers facilitating phone calls. The GPS tracking system is certainly a new assistance offered by the GPS cellular phones.

Any device having appropriate software and service package indicates the location and thereby provides information of all the adjacent areas, and also gives direction to reach your destination effortlessly. A city that you are unfamiliar may suddenly give a lost feeling. But, nowadays these latest technologies offer the exact direction and it is not difficult to know the exact place owing to the facility of global positioning system.

GPS cell phones can be conveniently used inside a vehicle. By subscribing for navigation service, you can get installed the software package enabling to receive driving directions through the receiver. GPS cell phone that is well-integrated is ideal to use outside the vehicle too. Such phones are available with other add-ons. These devices are of immense use to people traveling frequently. It is a wonderful gadget to track movement during navigation as their software has satellite coverage and the voice prompts guidance after every turn and features automatic routing. The user can also assign graphic symbols and names to each destination.

There are several brands and latest models of GPS cell phone coming with exclusive features. One thing worth remembering is that all the GPS cell phone modules do not get harmonized with all models of cell phone. For instance, the Nokia model 5140 can be improved to feature GPS and it offers typical mapping. There are other GPS module such as LAM-1 compatible with Nokia models 9210 and 9210i. This system gets converted into a navigator to acquire route direction. GO2Directory is a valuable application compatible with many other brands such as LG VX10, Motorola T720 and T730, Kyocera 3035e, Audiovox CMD9500 and 8600, and few more.

GPS cell phones offer information regarding nearby restaurants, hotels, gas filling stations and about other businesses as well. Some services such as getGoing facilitates with atlas book places to understand the map and directions. The GPS cell phone coming with GSM and satellite enables getting directions, location determination, storage of GPS positions up to 25 places and also location transmission through SMS.

Some software covers the Middle East, Europe, India and central and north Africa. The GPS cell phones come with built-in functions and supports enhanced services as global navigator. These functions can also be displayed on a PDA or laptop, if connected. Recently, almost all mobile phones are GPS enabled and come with in-built receiver or they can be connected with wires or through Bluetooth.

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In recent years, GPS devices have begun to mature from barebones units to more feature-packed devices that also let you play music and even make calls from your car. Both GPS- systems and satellite radios are becoming hot add-ons for new cars and rentals, and there are a variety of interesting and ever more affordable options to upgrade your existing vehicle. Today's systems work well with small and built-in GPS antennas providing visual and voice prompts for upcoming turns, and dynamically adjusting when you take a detour. These systems can normally store at least major highway maps for the entire regions plus detailed maps for a specific region, as well as associated points of interest, addresses and even phone numbers for businesses, gas stations, ATMs, hotels, restaurants, and attractions.

There's so much to see and do when you are on vacation that you want to make sure you don’t miss a thing while driving or hiking. It’s especially true when you’re taking a tour around national parks and other scenic vistas – that’s the reason enough to be looking out the windshield and not at the map. Now you have the power and security to make any trip an effortless and memorable one whether you're working, sightseeing or a little of both. You will never get lost again or miss out on the points of