GPS satellite in orbit, image courtesy of NASA
- GPS redirects here. For other uses of the acronym GPS, see GPS (disambiguation).
The Global Positioning System, usually called GPS, is not the only fully-functional satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio to electronic GPS receivers which allow them to accurately determine their location (longitude, latitude, and altitude) in real time, day or night, in any weather, anywhere on Earth.
Since GPS was declared fully operational in 1993, it has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making, and land surveying. GPS also provides an extremely precise time reference, required for some scientific research, including the study of earthquakes.
The Wide-Area Augmentation System (WAAS), available since August 2000, increases the accuracy of GPS signals to within 2 meters (6 ft) [1] for compatible receivers. GPS accuracy can be improved further, to about 1 cm (half an inch) over short distances, using techniques such as Differential GPS (DGPS).
United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System). The satellite constellation is managed daily by the 50th Space Wing at Schriever Air Force Base. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good.
Magellan GPS receiver in a marine application.
Over fifty
GPS satellites such as this NAVSTAR have been launched since 1978.
Applications
GPS allows accurate targeting of cruise missiles and precision-guided munitions (or "smart bombs"), as well as improved command and control of forces through improved locational awareness. GPS increases the accuracy of submarine launched ballistic missiles, since knowing the exact launching position allows for more accurate targeting of the missile. The satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System. Commercial civilian GPS receivers are required to have limits on the velocities and altitudes at which they will report coordinates; this is to prevent them from being used to create improvised missiles.
This taxi in Kyoto, equipped with GPS navigation, is an example of how
GPS technology can be applied in routine activities.
- Main article: Automotive navigation system
GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. The system can also be used by computer controlled harvesters, mine trucks and other vehicles. Hand-held GPS receivers can be used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turnpoints in competitions. Low cost GPS receivers are often combined in a bundle with a PDA, car computer, or vehicle tracking system.
More costly and precise receivers are used by land surveyors to locate boundaries, structures, and survey markers, and for road construction.
- GPS for the visually impaired
For information about navigation systems for the visually impaired, including MoBIC, Drishti, Brunel Navigation System for the Blind, NOPPA, BrailleNote GPS, and Trekker, refer to the main article GPS for the visually impaired.
GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such those shown here from manufacturers Trimble, Garmin and Leica (respectively, left to right).
The availability of hand-held GPS receivers for a cost of about $90 and up (as of March 2005) has led to recreational applications including Geocaching. Geocaching involves using a hand-held GPS unit to travel to a specific longitude and latitude to search for objects deliberately hidden there by other Geocachers. Geocaching often includes walking or hiking to natural locations, and is very popular.
Most airlines allow private use of ordinary GPS units on their flights, except during landing and take-off, like all other electronic devices. Additionally, some airline companies disallow use of hand-held receivers for security reasons, such as unwillingness to let ordinary passengers track the flight route. On the other extreme, some airlines integrate GPS tracking of the aircraft into their aircraft's seat-back television entertainment systems, available even during takeoff and landing to all passengers.
Even fixed systems may use GPS, in order to get precise time. This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.
Many synchronization systems use GPS as a source of accurate time, hence one of the most common applications of this use is that of GPS as a reference clock for time code generators or NTP clocks. For instance, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with some precise time source, so that the time of events may be recorded accurately.
The atomic clocks on the satellites are set to "GPS time", which is the number of seconds since 00:00:00 UTC, January 6, 1980. Today, GPS time is 14 seconds ahead [2] of UTC, because it does not follow leap seconds. Receivers thus apply a clock-correction offset (which is periodically transmitted along with the other data) in order to display UTC correctly, and optionally adjust for a local time zone. New GPS units will initially show the incorrect time after achieving a GPS lock for the first time. However, this is usually corrected on the display within 15 minutes once the UTC offset message is received for the first time.
History
The inspiration for the GPS system came when the Soviets 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, due to 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. It was only a small leap of logic to realize that the converse was also true; if the satellite's position was known then they could identify their own position on Earth.
The first satellite navigation system, Transit, used by the US Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour.
The first experimental Block-I GPS satellite was launched in February 1978 [3]. The GPS satellites were initially manufactured by Rockwell and now manufactured by Lockheed Martin.
In 1983, after Soviet jet interceptors shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board, 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. The first modern Block-II satellite was launched on 14th February 1989, and a complete constellation of 24 satellites was in orbit by 17th January 1994.
[4]. The most recent launch was in September 2005. The oldest GPS satellite still in operation was launched in February 1989.
Technical description
Satellites
GPS satellite on test rack
The GPS system uses a satellite constellation of 24 satellites in intermediate circular orbits. The orbits are designed so at least four satellites are always within line of sight from almost any place on earth. [5] Each satellite circles the Earth twice each day at an altitude of 20,200 kilometres (12,600 miles). There are four satellites in each of six orbital planes. The constellation also includes three spare satellites in orbit. Each orbit is inclined 55 degrees from the equatorial plane, and the right ascension of the ascending nodes are separated by sixty degrees. [6]
The flight paths of the satellites are measured by five monitor stations around the world (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs). The master control station, at Schriever AFB, processes their combined observations and sends updates to the satellites through the stations at Ascension Island, Diego Garcia, Kwajalein. The updates synchronize the atomic clocks onboard each satellite to within one microsecond, and also adjust the ephemeris of the satellites' internal orbital model to match the observations of the satellites from the ground. [7]
Each satellite repeatedly re-broadcasts the exact time according to it's internal atomic clock along with a digital data packet. The data includes the orbital elements of the satellite's precise position, satellite status messages, and an almanac of the approximate position of every other active GPS satellite. The almanac lets GPS receivers use data from the strongest satellite signal to locate other satellites.
Receivers
GPS receivers calculate their current position (latitude, longitude, elevation), and the precise time, using the process of trilateration after measuring the distance to at least four satellites by comparing the satellites' coded time signal transmissions. The receiver calculates the orbit of each satellite based on information encoded in their radio signals, and measures the distance to each satellite, called a pseudorange, based on the time delay from when the satellite signals were sent until they were received.
In order to measure the delay, the satellite repeatedly sends a 1,023 bit long pseudo random sequence; the receiver calculates an identical sequence from a known seed number, and shifts it until the two sequences match. Each satellite uses a different sequence, which lets them share the same radio frequencies, using Code Division Multiple Access, while still allowing receivers to identify each satellite.
Once the location and distance of each satellite is known, the receiver should theoretically be located at the intersection of four imaginary spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. In practice, GPS calculations are more complex for several reasons. One complication is that GPS receivers do not have atomic clocks, so the precise time is not known when the signals arrive. Fortunately, even the relatively simple clock within the receiver provides an accurate comparison of the timing of the signals from the different satellites. The receiver is able to determine exactly when the signals were received by adjusting its internal clock (and therefore the spheres' radii) so that the spheres intersect near one point.
One of biggest problems for GPS accuracy is that changing atmospheric conditions change the speed of the GPS signals unpredictably as they pass through the ionosphere. The effect is minimized when the satellite is directly overhead and becomes greater toward the horizon, as the satellite signals must travel through the greater "thickness" of the ionosphere as the angle increases. Once the receiver's rough location is known, an internal mathematical model can be used to estimate and correct for the error.
Because ionospheric delay affects the speed of radio waves differently based on their frequencies, a second frequency band was added to help eliminate this type of error. Newer GPS receivers can compare the phase difference between the L1 and L2 frequencies to actually measure the atmospheric effects on the signals and apply precise corrections.[citation needed]
GPS signals can also be affected by multipath reflections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc). For long delay multipath signals, the receiver itself can filter the signals out. A variety of receiver techniques, most notably Narrow Correlator spacing, have been developed to mitigate multipath errors. For shorter delay multipath signals that result from reflections from the ground, special antenna features may be used such as a ground plane, or a choke ring antenna. Shorter multipath signals from ground reflections can often be very close to the direct signals, and can greatly reduce precision.
Frequencies
Several frequencies make up the GPS electromagnetic spectrum:
- L1 (1575.42 MHz):
Carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted precision P(Y) code.
- L2 (1227.60 MHz):
Usually carries only the P(Y) code. The encryption keys required to directly use the P(Y) code are tightly controlled by the U.S. government and are generally provided only for military use. The keys are changed on a daily basis. In spite of not having the P(Y) code encryption key, several high-end GPS receiver manufacturers have developed techniques for utilizing this signal (in a round-about manner) to increase accuracy and remove error caused by the ionosphere. Recognizing the civilian need for increased accuracy, "modernized" IIR-M (IIR-14 (M) and later) satellites carry a civilian signal interleaved with an improved military signal on both the L1 and L2 frequencies.
- L3 (1381.05 MHz):
Carries the signal for the GPS constellation's alternative role of detecting missile/rocket launches (supplementing Defense Support Program satellites), nuclear detonations, and other high-energy infrared events.
- L4 (1841.40 MHz):
Being studied for additional ionospheric correction.
- L5 (1176.45 MHz):
Proposed for use as a civilian safety-of-life (SoL) signal. 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 2007.
GPS and relativity
The clocks on the satellites are also affected by both special and general relativity, which causes them to run at a slightly faster rate than do clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day, which is corrected by electronics on each satellite. This offset is a practical demonstration of the theory of relativity in a real-world system; it is exactly that predicted by the theory, within the limits of accuracy of measurement.
Neil Ashby presented a good account of how these relativistic corrections are applied, why, and their orders of magnitude, in Physics Today (May 2002) [8]. Whether relativity must be considered as a mere correction to a Newtonian GPS theory, or, rather, as the necessary foundation of a cleaner (and more fundamental) GPS theory, is currently under debate. Bartolomé Coll has recently developed the basic notions necessary for a fully relativistic theory of Positioning Systems [9].
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, teacher of aeronautics and astronautics at Stanford University developed the system.
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."
Techniques to improve GPS accuracy
The accuracy of GPS can be improved in a number of ways:
- Differential GPS (DGPS) can improve the normal GPS accuracy of 4-20 meters to 1-3 meters.[10] DGPS uses a network of stationary GPS receivers to calculate the difference between their actual known position and the position as calculated by their received GPS signal. The "difference" is broadcast as a local FM signal, allowing many civilian GPS receivers to "fix" the signal for greatly improved accuracy.
- The Wide Area Augmentation System (WAAS). This uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers, including information on ionospheric delays, individual satellite clock drift, and suchlike. Although only a few WAAS satellites are currently available as of 2004, it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation (landing an airplane in conditions of little or no visibility). The current WAAS system only works for North America (where the reference stations are located), and due to the satellite location the system is only generally usable in the eastern and western coastal regions. However, variants of the WAAS system are being developed in Europe (EGNOS, the Euro Geostationary Navigation Overlay Service), and Japan (MSAS, the Multi-Functional Satellite Augmentation System), which are virtually identical to WAAS.
- A Local Area Augmentation System (LAAS). This is similar to WAAS, in that similar correction data are used. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. These correction data are typically useful for only about a thirty to fifty kilometer radius around the transmitter.
- Exploitation of DGPS for Guidance Enhancement (EDGE) is an effort to integrate DGPS into precision guided munitions such as the Joint Direct Attack Munition (JDAM).
- A Carrier-Phase Enhancement (CPGPS). This technique utilizes the 1.575 GHz L1 carrier wave to act as a sort of clock signal, resolving ambiguity caused by variations in the location of the pulse transition (logic 1-0 or 0-1) of the C/A PRN signal. The problem arises from the fact that the transition from 0-1 or 1-0 on the C/A signal is not instantaneous, it takes a non-zero amount of time, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. A successful correlation could be defined in a number of various places along the rising/falling edge of the pulse, which imparts timing errors. CPGPS solves this problem by using the L1 carrier, which has a period 1/1000 that of the C/A bit width, to define the transition point instead. The phase difference error in the normal GPS amounts to a 2-3 m ambiguity. CPGPS working to within 1% of perfect transition matching can achieve 3 mm ambiguity; in reality, CPGPS coupled with DGPS normally realizes 20-30 cm accuracy.
- Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers.
- Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, accurate determination of range signal can be resolved to an accuracy of less than 10 centimetres. 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 testspossibly with processing in real-time (real-time kinematic positioning, RTK).
- Many automobile GPS systems combine the GPS unit with a gyroscope and speedometer pickup, allowing the computer to maintain a continuous navigation solution by dead reckoning when buildings, terrain, or tunnels block the satellite signals. This is similar in principle to the combination of GPS and inertial navigation used in ships and aircraft, but less accurate and less expensive because it only fills in for short periods.
Selective availability
When it was first deployed, GPS included a feature called Selective Availability (or SA) that introduced intentional errors of up to a hundred meters into the publicly available navigation signals, making it difficult to use for 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 m horizontally and 30 m vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern US area might read 30 m off, but 30 m off everywhere and in the same direction. In order 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 disabling the Selective Availability. 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, less accurate, radio navigation systems. The military resisted for most of the 1990s, but SA was eventually turned off in 2000 following an announcement by then US President Bill Clinton, allowing all users to enjoy nearly the same level of access. [11]
The US military maintains the ability to use a more advanced version of Selective Availability, called "Selective Deniability", to reduce the accuracy of civilian GPS units in a specific area without affecting the rest of the world. The original SA system could only limit the accuracy of GPS signals world-wide, or not at all. Authorized military units are still able to decrypt the corrected signals using a tamper-resistant hardware module called an SAASM, Selective Availability / Anti-Spoofing Module.
Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere (see above).
GPS tracking
- Main article: GPS tracking
A GPS tracking system uses GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a track file or log of activities. The recorded data can be stored within the tracking unit, or it may be transmitted to a central location, or internet-connected computer, using a cellular modem, 2-way radio, or satellite. This allows the data to be reported in real-time, using either web browser based tools or customized software.
GPS jamming
A large part of modern munitions, the so-called "smart bombs" or precision-guided munitions, use GPS. GPS jammers are available, from Russia, and are about the size of a cigarette box. The U.S. government believes that such jammers were used occasionally during the U.S. invasion of Afghanistan. Some officials believe that jammers could be used to attract the precision-guided munitions towards noncombatant infrastructure, other officials believe that the jammers are completely ineffective. In either case, the jammers are attractive targets for anti-radiation missiles.
The U.S. Air Force conducted GPS jamming exercises in 2003. A detailed description of how to build a GPS jammer was posted on a hackers' site by an anonymous author. And there has been at least one well-documented case of unintentional jamming; if similar, but stronger, signals were generated on purpose, they could interfere with aviation GPS receivers at a range of 50 km. According to the reference below, "IFR pilots should have a fallback plan in case of a GPS malfunction".
There were also incidents of unintentional jamming, traced back to malfunctioning TV antenna preamplifiers.
Other systems
Russia operates an independent system called GLONASS (global navigation system), although with only twelve active satellites as of 2004, the system is of limited usefulness. There are plans to restore GLONASS to full operation by 2008. The European Union is developing Galileo as an alternative to GPS, planned to be in operation by 2010. China and France are also developing other satellite navigation systems.
| Satellite navigation systems |
 Transit | GPS |  GLONASS |  Galileo |  Beidou |
| Related topics: EGNOS | WAAS | LAAS |
See also
Wikimedia Commons has media related to:
- Geographic coordinate system
- EGNOS - European equivalent of WAAS
- Wikipedia Geographical coordinates project - adding geographic coordinates to WikiPedia articles
- Air traffic control
- Allan variance
- Automatic Dependent Surveillance-Broadcast (also called ADS-B)
- AGPS
- Degree confluence project Use GPS to visit integral degrees of latitude and longitude. Pictures and narrative for each one.
- Geodashing
- GSM localization
- GPX - GPS eXchange Format
- Location based media
- Mobile phone integration
- Trilateration - the mathematical technique used for GPS location
- Commercialization of space
- Open Geospatial Consortium (OpenGIS)
- RAIM
- Waypoint
- WikiGPS, Wikimedia proposed project.
- World Geodetic System - WGS 84 defines a global reference frame for GPS
Software
Makers of popular GPS hardware and vehicle navigation systems
GPS software for car navigation
Hardware
Usenet newsgroups
Other information
Wearable GPS solutions
Categories: Articles lacking sources | GPS | Camping equipment | Backpacking | Navigational equipment | Navigation
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