In the first part of this series of 10 articles I explained the differences between GPS Navigation Systems and GPS Tracking Systems, and how they are two completely different of implementations of Location Based Services. In this second article I will develop some additional concepts related to the elements that constitute a GPS Tracking System. There are 3 main parts to a GPS Tracking System:

- A GPS device or GPS Tracker, which receives the location information and then delivers it to a software application.

- A data transmission system, which takes the information provided by the GPS Tracker, and delivers it to the software application.

- A Software Application, which presents to its users the data recollected by the GPS Tracker in several formats including maps and reports.

This article will present the main components of a GPS Tracking Solution, and will start expanding on the first component: GPS Trackers. It will take me this article and the next one to go in detail over GPS devices.

GPS Devices

==========

In this section I will define what a GPS Device, and how they work; after that I will present the types of GPS Trackers, including most of the features that these devices offer, and my opinion in regards to each type.

At its simplest definition, the GPS device, or better the GPS tracker, is the component in charge of receiving the information about the location of the vehicle, and providing this data to the GPS Tracking Application through the Data Transmission System (which will be explained in coming articles). GPS Trackers are usually small boxes (metal or plastic) that can be the size of a man's wallet, or a little bigger. All of them need at least one antenna (GPS antenna), and most of them need an additional antenna to enable the data transmission module. So this leads to a first classification of GPS Trackers:

-Full satellite trackers. This type of trackers will use satellites to receive and transmit data.

-Hybrid trackers. This type of trackers will use satellites to acquire location (we will see some variations here later in this article), and another method to transmit the data (a data modem, a data port to download the data, etc).

On the next article I will expand on these concepts.

So, the main task of a GPS Tracker is to provide information of the location of a vehicle or an asset, or a person. The location of the units is usually acquired from the GPS satellites, but there is another method based on triangulation with cell towers to calculate locations. Getting the location from satellites is the most accurate mechanism, providing a minimal margin of error most of the times (from 2 up to 50 feet). GPS location can be acquired anywhere in the world. The only down side for this location technology is that the GPS antenna has to have a view to the sky. For example, if the vehicle gets into a garage, most probably there will not be GPS locations available.

Getting the location based on a triangulation with the cell towers (those that are also used by our cell phones to transmit voice and data) has a bigger margin of error (up to a few hundred feet), making it a not very accurate location mechanism. This type of location also requires the presence of the named cell towers to work. The upside of this mechanism is that it will perfectly work within buildings, which is not the case for GPS satellite location. Some GPS Trackers are designed to work with both location mechanisms, creating a new concept called Assisted GPS (AGPS).

There are mainly three types of GPS Trackers: Passive Trackers, PING Trackers, and Live Trackers. Also, there are some GPS Trackers that have more features than other - not just a location of the vehicle.

In this article I have dissected a GPS Tracking solution into three main components: a GPS Tracker, a data transmission system, and a GPS Tracking application. I have also started exposing the details of GPS Trackers, specifically the two main ways to locate a vehicle. Finally, I introduced two more elements to consider in GPS Trackers: types of trackers and advanced features of a GPS device.



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GPS antennas are an important part of any and all wireless systems. The GPS antenna works by combining a planar antenna and a frequency converter, which converts the high-frequency phase-modulated spread spectrum signal of the GPS system to an intermediate frequency. Nowadays, you can find many different types and brands of GPS antenna available in the market. You can select the one that is most suitable for your own personal or business needs. You can look up the details on the Internet of the various models available and select one that fits your needs and your budget.

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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What is GPS? The acronym stands for Global Positioning System. It is the new "Polaris" that aids in navigating, positioning and tracking with the use of a satellite-controlled system that broadcasts signals to the equipment on the ground. With receivers hand carried by users, GPS determines the exact location of a vehicle, person or assets and other things useful and valuable to which it is attached and records the position at regular intervals. It is a powerful tracking system that has provided the world with diverse applications for the military and civilian users.

What is GPS? The GPS is a space-based radionavigation system controlled and funded by the U.S. Department of Defense and operated by the U.S. Military. This is the GPS Operational Constellation. The tracking system has space segments consisting of GPS satellites sending signals coming from space. There can be at least 24 operational satellites orbiting in 12 hours that recapitulate the same ground track as the earth turns underneath them. The GPS satellites then transmit the data in a very precise time reference plied by what is called "atomic clocks" onboard the satellite. These atomic clocks then passively transmit the navigation messages in specially coded signals, enabling the equipment on the ground called "receivers" to compute position, time, direction and velocity in three-dimensional locations known as latitude, longitude and altitude.

What is GPS? Your Most Reliable and Most Precise Assistant

The GPS was primarily for the use of the military. However, after the Korean Flight 007 in 1983 tragedy, which would have been prevented had its crew only had access to better navigational tools, President Ronald Reagan issued a directive which would guarantee that GPS signals be made available to the world for free or without restrictions. Though it was intended for civilians as well as military applications at first, from its design, civilian users would not be getting the same accuracy that the military could.

Going public, what is GPS serving the commercial market? GPS became the new powerful tool that improved efficient routing of vessels at sea. It has saved a ship's navigator hours of celestial inference and calculation. GPS enhanced safety at sea made it possible to report precise position to rescuers in case of disasters.

What is GPS navigating the land? GPS also improved efficiency on land: delivery trucks can now receive GPS signals and easily transmit their position back to a central dispatcher; police and fire departments use GPS to efficiently dispatch their vehicles and reduced response time; GPS keeps motorists from getting lost by showing their position and intended route on dashboard displays; railroads now use GPS technology in replacing older maintenance-intensive mechanical signals.

What is GPS navigating the air? Long before the GPS, aircrafts typically fly from one waypoint to another and pilots on long-distance flights relied on navigational beacons situated across the country. The dawning of GPS supplemented existing navigational techniques for aircrafts inexpensively. With GPS navigating the air, airplanes can now fly a direct route to a destination that save significant amount in consumption of fuel and time, the methods of guiding planes to a safe landing in a poor weather or visibility has been improved and simplified and aided pilots with a precise position data to keep the plane on course.

What is GPS surveying, mapping the earth, managing the land and agriculture? GPS is used by surveyors and map makers for precision positioning; map locations of such facilities as telephone poles, sewer lines, and fire hydrants; map construction sites and property lines in minutes. In mapping the earth, GPS points have assigned codes in order to identify roads, streams, or other objects during data collection for analyses and comparison through a computer program called "Geographic Information Systems (GIS)."

GPS can be used in forestry, for mineral exploration, and wildlife habitat management to define positions of important assets precisely and identify their changes. In agriculture, a farm equipment with GPS receivers can provide precise position information and it also gives farmers great accuracy in the application of fertilizers and harvesting crops.

Agricultural GPS systems can be used to create crop yield maps during harvesting, making it easy for farmers to plan exactly how the fields should be used and fertilized for future crops.

So, what is GPS? It is a powerful navigational tool acting as your most reliable and most precise assistant that is transforming the way nations operate in space.



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Jeff Sanders

Garmin GPS Navigation Systems

www.GpsFrontier.com

04/05/09



WAAS-Enabled GPS System

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. The WAAS system typically provides better than 1.0 meters laterally and 1.5 meters vertically throughout most of the contiguous United States and large parts of Canada and Alaska. This accuracy is capable of provideing aircraft with the precision needed for safe approaches and inflight navigation for all weather conditions. Integrity of the WAAS information is no more than 3 seconds of bad data per year allowing the system to be considerd safe by the FAA for instrument flight rules.

Although orignally developed for aviation, WAAS is not just limited to the aviation industry, any GPS receiver that is capable of receiving the WAAS signal will be able to benefit from it's correction data, making the GPS positioning more accurate. 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 maping program supports "lane assist" directions). Because the Wide Area Augmentation System is quickly becoming standard in the GPS industry, most new GPS receivers today are WAAS-enabled. Just like with the conventinal GPS, the WAAS system doesn't come with any extra cost or fees to use. All that is required is that the GPS receiver be WAAS-enabled so it can receive and decode the data then be able to apply corrections to it's position. Currently the WAAS service is limited to the U.S.A., Canada, Alaska and Hawaii. Although independant from WAAS, Europe and Asia are working on their own supplemental GPS correction systems. Europe has the "Euro Geostationary Navigation Overlay Service" (EGNOS) and Japan is working on their "Multi-Functional Satellite Augmentation System" (MSAS) .



How The WAAS System Works

WAAS uses a network of approximately 25 ground based Wide-area Reference Stations (WRS) in North America and Hawaii, to measure small variations in GPS satellite signals in the western hemisphere. These precisely surveyed ground stations monitor and collect information on the GPS signals and send their data to the three Wide-area Master Stations (WMS). The WMS's generate two different sets of corrections: fast and slow. The fast corrections are for errors that are changing rapidly and are a primary concern to the GPS satellites instantaneous positions and clock errors. These corrections are user position independent, which means they can be applied instantly by any receiver in the WAAS broadcasting area. The slow corrections are for long-term ephemeric and clock error estimates and ionospheric delay information.

Once these corrections are generated, the Master Stations sends them to two pairs of Ground Uplink Stations (GUS) that transmit the correction messages to a series of geostationary satellites that broadcast their correction data back to earth. Then WAAS-enabled GPS receivers use this information to make corrections to the original GPS signial, giving WAAS-enabled GPS receiver a more accurate position. GPS receiver's use the information broadcast from each GPS satellite to determine their location and the current time. Depending on the GPS device, a GPS receiver only needs to receive a signal from 3-4 satellites (out of the 31 satellites currently transmiting a signal for civilan users) to be able to calculate it's position. In addition to the GPS signal, a WAAS-enabled GPS receiver can also receive the geostationary WAAS satellite signal.

The two different types of correction messages from the WAAS system (fast and slow) are used by the GPS receiver in different ways. The fast type of correction data includes the corrected satellite position and clock data to determine its current location using normal GPS calculations. Once an approximate position fix is obtained the GPS receiver begins to use the slow corrections to improve its accuracy. Slow correction data Includes the ionospheric delay. When the GPS signal travels from the satellite to the receiver, it passes through the ionosphere. The receiver calculates the location where the signal pierced the ionosphere and, if it has received an ionospheric delay value for that location, it corrects for the error that the ionosphere created. Unlike the fast data, the slow data doesn't need to be updated frequently because the ionosphere conditions don't change rapidly. While the slow data can be updated every minute if necessary, they are only updated every two minutes and are considered valid for up to six minutes.



Limitations of the WAAS System

(1) The WAAS system is currently only available to United States and large parts of Canada and Alaska but there plans to expand the system to other countries and continents. (2) Because the WAAS broadcasting satellites are geostationary causes them to be less than 10° above the horizon for locations north of 71.4° latitude. This means aircraft in areas of Alaska or northern Canada may have difficulty maintaining a lock on the WAAS signal. (3) In order to calculate an ionospheric grid point's delay, that point must be located between a satellite and a reference station. The low number of satellites and ground stations limit the number of points which can be calculated. (4) Aircraft conducting WAAS approaches must possess certified GPS receivers.

Jeff Sanders

Garmin GPS Navigation Systems

www.GpsFrontier.com

04/05/09



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The crying needs for successful and enjoyable holiday travel – getting to the destination, and providing some entertaining distraction along the way – are now answered with the help of heaven-sent, handy GPS-based navigation systems. Whether you are traveling cross-country or taking a quick weekend trip, having a GPS device will make your journey easier, faster and much more fun.

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 interest thanks to GPS. Whether in your own car, a rental car or RV Rental, integrating the in-car entertainment like an MP3 player and tour guide content will allow you to travel at your own pace; visiting the places you want to see. With multilingual turn-by-turn visual guidance, voice prompt navigation and easy to read graphics, traveling from place to place has never been easier and so visitor-friendly. It also gives you suggested directions, so you don't get lost, for the most popular driving routes as well and invite you to get off the beaten path to take the roads less traveled. You will get to see more, hear more, laugh more and experience more than you ever could have on your own.

When vacationing in unfamiliar cities or countries GPS also comes handy and helps you enjoy your stay and remain focused on the business at hand. Now you can navigate to different places with your GPS specific for the regions you are traveling in. GPS can also help you avoid being scammed by local taxi drivers. With a GPS you know if a "short cut" is really what it is or if the taxi driver is just trying to rip you off by taking you on a longer route. Your visit will be more fun with a GPS since it allows you to enjoy your experience without worrying so much about how to get around.

GPS-based navigation systems are real indispensable additions to your travel gear. Just grab a GPS and enjoy your trip!



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The Global positioning System (GPS) is often used by computer equipment, such as NTP Server systems, to provide an accurate timing reference for time critical applications. This article provides an overview of GPS for timing applications and describes the equipment used to install a GPS antenna in a static location.

Overview - Using GPS for Accurate Time

The Global Positioning System is a US military system for worldwide navigation. The system consists of 24 orbiting satellites, each satellite has a highly accurate atomic clock on-board synchronised to UTC time. The satellites continuously broadcast time and position information. The time and position information can be obtained worldwide with a GPS receiver and antenna. GPS works continuously in any whether conditions, anywhere in the world. Additionally, there is no set up fee or subscription charges to utilise the GPS systems. Many computer timing systems and NTP Server systems utilise GPS as an accurate external timing reference.

The Accuracy of GPS Timing Systems

GPS receivers provide highly accurate position and timing information. Typically, a GPS receiver can provide positioning information to an accuracy of 15m. NTP Server systems can obtain timing information from GPS to a resolution of a few nanoseconds.

The GPS Signal

The transmitted GPS signal is very weak low-power radio signal, designated L1 and L2. L1 is the civilian GPS frequency transmitted at 1575.42 MHz. The signals travel by line of sight and can pass through clouds, glass and plastics but are blocked by objects such as metal and brickwork. Therefore, the ideal location for a GPS antenna is on rooftop with a full 360-degree view of the sky. However, often installation on the side of a building or in a window can provide adequate results. As a rule of thumb, the better the view of the sky, the greater the likelihood of a good consistent signal lock.

GPS Timing Antenna Types

The GPS antenna acts as an amplifier to boost the GPS signal for transmission along a cable, usually coax, to the GPS receiver. GPS Timing antenna’s provided with NTP server systems utilise a pole-mounting system. The antenna screws to a threaded pole for installation on rooftops. This arrangement provides the GPS antenna with a rigid mount easily able to withstand high winds without damage. Typically the GPS antenna is fairly small in size, measuring less than 90cm in diameter. Low-cost patch type antennas are also available, but these are generally better suited to vehicle applications.

GPS Antenna Cable Types and Cabling Distances

The cable distance that can be utilised by a GPS antenna depends mainly on the amplification of the GPS antenna and the quality of coax used in the installation. A typical GPS timing antenna may have a gain of 35 db. Relatively low-quality coax such as RG58 has an attenuation of 0.64 db/m at 1575 MHz. Therefore, a cable run of 55m can be obtained using RG58 cable. With very high quality coax cable, such as LMR400, an unaided cable run of 200m can be achieved. However, very high quality coax can be expensive. A good price-performance compromise is LMR200 cable, which can be run unaided to 80m.

Extending Cabling Distance with In-Line GPS Amplifiers

In-line GPS amplifiers provide further amplification of the GPS signal to increase the cable distance between the GPS antenna and receiver. GPS amplifiers are fitted in-line with the antenna cable and obtain power from the receiver via the coax cable. No external power-supplies are required. Typically, a GPS amplifier may add a further 20 dB of gain, adding 30m of low-quality RG58 coax, 40m of LMR200 coax or 100m of high quality LMR400 coax. Additionally, multiple in-line amplifiers may be utilised to further increase cable distance.

Sharing a Single Antenna Between Multiple Receivers - GPS Splitters

GPS splitters allow a single GPS antenna to be utilised by two or more NTP server systems. The GPS splitter splits the signal received from the GPS antenna into multiple outputs for synchronizing multiple NTP servers. GPS splitters are generally available with 2, 4 or 8 outputs.

Protecting GPS Systems - GPS Surge Suppressors

Surge suppressors protect expensive NTP server equipment from electro-static discharges, such as lightning, that may be picked up by an externally mounted GPS antenna. Surge suppressors are installed in-line on the coax cable between the antenna and receiver, ideally where the cable enters the building. Surge suppressors require a low-impedance ground, to discharge any received surge. The surge suppressor requires no power-supply or additional cabling.



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In Taiwan, GPS suppliers besides taking over most international companies' orders, also successfully develop GPS watches for hiking, golfing and cycling.  GPS receiver navigation devices have developed rapidly in recent years and are now much more than talking maps. Auto electronics, consumer electronics, home appliances, information and communications technology (ICT), and telecoms companies have worked together to give them a much more diverse range of functions including use in those of MP3 players, digital photo albums, movie players, and real-time road-condition receivers. They now provide information on navigation, recreation, and entertainment, all in a single unit.

Taiwan's GPS industry started to take off in 2007. After MiTAC bought Navman, they've become the second biggest GPS company in Europe. Also, their sales of 800 million of PND has made them become the third largest GPS supplier in the world. At the same time, TomTom and Garmin have expended their ordering to Taiwan OEM's from components to fabrication products. The global leader in satellite navigation, Garmin Ltd. and its subsidiaries have designed, manufactured, marketed and sold navigation, communication and information devices and applications since 1989 - most of which are enabled byGPS technology. One of their principal subsidiaries is located in Taiwan. Including Japanese and European orders, Taiwan's GPS sales were around NT$171 billion. The total growth rate was 70%, which makes its growth very noticeable compared to the 2007 ICT industry growth rate of 4.6%, or 730 billion dollars.

Even though this year, European and U.S. consumer electronic product sales are still being effected by the current economy situation, IEK is still very pretty positive about the GPS market in Taiwan. The Taiwan GPS industry market will grow strongly once the market moves from Europe and the, U.S. to the newly developed countries.

GPS for Hiking

What's a suitable GPS for cycling? Color mapping, long life battery or replaceable batteries, topo, street/road, & bike trail mapping, text-to-speech (gives me the street name), prefer selectable car vs bicycle mode, a way to pick future routes, possibly software that would use steet & topo to avoid large hilly areas on long distance biking, large SD card, fairly waterproof, screen can be seen in sunlight. One of the Taiwan GPS receiver suppliers announced their product as an all-in-one GPS receiver.

Dedicated massive-correlator signal parameter search engine within the baseband enables rapid search of all the available satellites and acquisition of very weak signal. An advanced track engine allows weak signal tracking and positioning in harsh environments such as urban canyons and under deep foliage.

With very fast signal acquisition speed, it has very low average power consumption for locate on demand type of applications.

GPS for Golfing

Golf GPS systems are rapidly evolving from only player enhancement to complete course management systems. The primary goal of a golf GPS system is to provide players with distances to all kind of features on the course and especially to the green they are playing. Some courses are better fit to use the capabilities of GPS than others. In order to obtain a satisfactory accuracy the GPS receiver must have a non-obstructed view of the sky. This makes that courses with lots of trees are less suitable for golf GPS.



Golf GPS system should contain the following:

Provides you with GPS distances from "any" location on the hole to the 200, 150, 100 yard / meter markers, to the front, centre, and back of the Green.

Record the GPS distance, club used and landing position for every stroke you make.

Stroke length Statistics: tells you the average and longest distance you hit each club in your bag. You know exactly how far you drive the ball.

Club Selector! Suggest which club to use for your next stroke based on the historical stroke data already stored from previous scorecards!



Taiwan GPS suppliers, Mobile Action Technology Inc and Polar have released their new GSP products for sport.

GPS for Cycling

Whether you are a road biker or a mountain biker, there's a GPS that will work well for you, and it can mount right on your handlebars. All of them have high-sensitivity chipsets for superior satellite reception in urban canyons and under canopy. While they are excellent cyclometers, they aren't all that great as navigation devices. If you are primarily wanting to track your performance though, Taiwan suppliers who make sporting GPS receivers would be your excellent choice. They are suitable for whoever wants data acquisition and a dashboard lap timer.

To get a complete report on Taiwan GPS, please go to http://gps-receiver.ready-online.com/index.html



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Global Positioning System

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

[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"



Career Solutions and Recruitment Services

Map Capabilities

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



Career Solutions and Recruitment Services

Jeff Sanders

Garmin GPS Navigation Systems

http://www.GpsFrontier.com

04/05/09

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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