Global Positioning System
The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR), is an intermediate circular orbit (ICO) satellite navigation system used for determining one's precise location and providing a highly accurate time reference almost anywhere on Earth or in Earth orbit.
The GPS system was designed by and is controlled by the United States Department of Defense and can be used by anyone, free of charge. The GPS system is divided into three segments: space, control, and user. The space segment is comprised of the GPS satellite constellation. The control segment is comprised of ground stations around the world that are responsible for monitoring the flight paths of the GPS satellites, synchronizing the satellites' onboard atomic clocks, and uploading almanac data for transmission by the satellites. The user segment is comprised of GPS receivers used for both military and civilian applications. A GPS receiver (GPSR) decodes time signal transmissions from multiple satellites and calculates its position by trilateration.
The cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites. The first of 24 satellites that form the current GPS constellation (Block II) was placed into orbit on February 14, 1989. The 50th GPS satellite since the beginning in 1978 was launched March 21, 2004 aboard a Delta II rocket (see article in External links section, below).
The system consists of a "constellation" of at least 24 satellites in 6 orbital planes. The GPS satellites were manufactured by Rockwell; the first was launched in February, 1978 (Block I), and the final (24th), satellite was launched in 1994. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the orbital elements of their own motion, as determined by a set of ground-based observatories.
The receiver does not need a precise clock, but does need to have a clock with good short-term stability and received signals from four satellites in order to find its own latitude, longitude, elevation, and the precise time. The receiver computes the distance to each of the four satellites by the difference between local time and the time the satellite signals were sent (this distance is called a pseudorange ). It then decodes the satellites' locations from their radio signals and an internal database. The receiver should now be located at the intersection of four 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. The receiver does not have a very precise clock and thus cannot know the time delays. However, it can measure with high precision the differences between the times when the various messages were received. This yields 3 hyperboloids of revolution of two sheets, whose intersection point gives the precise location of the receiver. This is why at least four satellites are needed: fewer than 3 satellites yield 2 hyperboloids, whose intersection is a curve; it's impossible to know where the receiver is located along the curve without supplemental information, such as elevation. If elevation information is already known, only signals from three satellites are needed (the point is then defined as the intersection of two hyperboloids and an ellipsoid representing the Earth at this altitude).
When there are n > 4 satellites, the n-1 hyperboloids should, assuming a perfect model and measurements, intersect on a single point. In reality, the surfaces rarely intersect, because of various errors. The question of finding the point P can be reformulated into finding its three coordinates as well as n numbers ri such that for all i, PSi-ri is close to zero, and the various ri-rj are close to C.Δij where C is the speed of light and Δij are the time differences between signals i and j. For instance, a least squares method may be used to find an optimal solution. In practice, GPS calculations are more complex (repeat measurements etc...).
There are several causes: The initial local time is a guess due to the relatively unprecise clock of the receiver, the radio signals move more slowly as they pass through the ionosphere, and the receiver may be moving. To counteract these variables, the receiver then applies an offset to the local time (and therefore to the spheres' radii) so that the spheres finally do intersect in one point. Once the receiver is roughly localized, most receivers mathematically correct for the ionospheric delay, which is least when the satellite is directly overhead and becomes greater toward the horizon, as more of the ionosphere is traversed by the satellite signal. Since it is common for the receiver to be moving, some receivers attempt to fit the spheres to a directed line segment.
The receiver contains a mathematical model to account for these influences, and the satellites also broadcast some related information which helps the receiver in estimating the correct speed of propagation. High-end receiver/antenna systems make use of both L1 and L2 frequencies to aid in the determination of atmospheric delays. Because certain delay sources, such as the ionosphere, affect the speed of radio waves based on their frequencies, dual frequency receivers can actually measure the effects on the signals.
In order to measure the time delay between satellite and receiver, the satellite sends a repeating 1,023 bit long pseudo random sequence; the receiver knows the seed of the sequence, constructs an identical sequence and shifts it until the two sequences match.
Different satellites use different sequences, which lets them all broadcast on the same frequencies while still allowing receivers to distinguish between satellites. This is an application of Code Division Multiple Access, CDMA.
There are two frequencies in use: 1575.42 MHz (referred to as L1), and 1227.60 MHz (L2). The L1 signal carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted P(Y) code. The L2 signal usually carries only the P(Y) code. The 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. In spite of not having the P(Y) code encryption key, several high-end GPS receiver manufacturers have developed techniques for utilizing this signal to increase accuracy and remove error caused by the ionosphere.
A minor detail is that the atomic clocks on the satellites are set to "GPS time", which is the number of seconds since midnight, January 5, 1980. It is ahead of UTC because it doesn't follow leap seconds. Receivers thus apply a clock correction factor, (which is periodically transmitted along with the other data), and optionally adjust for a local time zone in order to display the correct time. The clocks on the satellites are also affected by both special, and general relativity, which causes them to run at a slightly slower 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 dramatic proof of the theory of relativity in a real-world system, as it is exactly that predicted by the theory, within the limits of accuracy of measurement.
Sources of GPS measurement errors
Ideally, GPS receivers would easily be able to convert the C/A and P(Y)-code measurements into accurate positions. However, a system with such complexity leaves many openings for errors to affect the measurements. The following are several causes of error in GPS measurements.
Both GPS satellites and receivers are prone to timing errors. Satellites often possess cesium atomic clocks. Ground stations throughout the world monitor the satellites to ensure that the atomic clocks are accurate. Receiver clock error is unknown and often depends on the oscillator provided within the unit. However, it can be calculated and then eliminated once the receiver is tracking at least four satellites.
The ionosphere is one of the leading causes of GPS error. The speed of light varies due to atmospheric conditions. As a result, errors greater than 10 meters may arise. To compensate for these errors, the second frequency band L2 was provided. By comparing the phase difference between the L1 and L2 signals, the error caused by the ionosphere can be calculated and eliminated.
The antenna receives not only direct GPS signals, but also multipath signals: reflections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc). For long multipath signals, the receiver itself can filter the signals out. For shorter 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.
In the past, the civilian signal was degraded, and a more accurate Precise Positioning Service was available only to the United States military, its allies and other, mostly government users. However, on May 1, 2000, then US President Bill Clinton announced that this "Selective Availability" would be turned off, and so now all users enjoy nearly the same level of access, allowing a precision of position determination of less than 20 meters.
Techniques to improve GPS accuracy
The accuracy of GPS can be improved in a number of ways:
- Using a network of fixed ground based reference stations. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. This method is called Differential GPS or DGPS. DGPS was especially useful when GPS was still degraded (via the "Selective Availability" described below), since DGPS could nevertheless provide 5-10 meter accuracy. The DGPS network has been mainly developed by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries.
- 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).
- 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.
- 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 sytem. In this approach, accurate determinination of range signal can be resolved to an accuracy of less than 10 centimeters. 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 ).
The primary military purpose is to allow improved command and control of forces through an enhanced ability to accurately specify target locations for cruise missiles or troops. The satellites also carry nuclear detonation detectors.
The system is used by countless civilians as well, who can use the GPS's Standard Positioning Service worldwide free of charge. Low cost GPS receivers (price $100 to $200) are widely available. The system is used as a navigation aid in aeroplanes, ships and cars. The system can be used by computer controlled harvesters, mine trucks and other vehicles. Hand held devices are used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turnpoints in competitions.
On May 1, 2000, US President Bill Clinton announced that this "Selective Availability" would be turned off. However, for military purposes, "Selective Deniability" may still be used to, in effect, jam civilian GPS units in a war zone or global alert while still allowing military units to have full functionality. European concern about this and commercial issues has resulted in the planned GALILEO positioning system. Russia already operates an independent system called GLONASS (global navigation system), although with only 12 active satellites as of 2004 the system is of limited usefulness.
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). Commercial GPS receivers are also required to have limits on the velocities and altitudes at which they will report fix coordinates; this is to prevent them from being used to create improvised cruise or ballistic missiles.
Many synchronization systems use GPS as a source of accurate time, hence one of the commonest 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.
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.
- 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 Corporations, 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."
For a list of other systems, see satellite navigation system.
- Air traffic control
- Allan variance
- Degree Confluence Project
- GMS localization
- GPX - GPS eXchange Format
- Automatic Dependent Surveillance-Broadcast (also called ADS-B)
- Peter H. Dana: Global Positioning System Overview
- USCG Navigation Center: Status of the GPS constellation, government policy, and links to other references
- The FAA has more information on GPS, WAAS, LAAS, and DGPS at http://gps.faa.gov/FAQ/index.htm
- History of GPS, including information about each satellite's configuration and launch: http://www.astronautix.com/project/navstar.htm
- U.S. Army Corps of Engineers manual: NAVSTAR HTML and PDF (328 pages)
- Greg Goebel's "Navigation Satellites and GPS"
- Gpsdrive - GNU Map-based navigation system. It displays your position on a zoomable map provided from a NMEA-capable GPS receiver.
- USAPhotoMaps collects and displays aerial photos and topo maps from data which it downloads from Microsoft's TerraServer Web site, for use in GPS mapping.
- CNN article on 50th GPS satellite launch
- GPS and Galileo: where are we headed? (PDF) – Keynote speech from the 2004 European Navigation Conference, by David Last, Univ. of Wales
- Topcon's GPS Tutorial
- Trimble's Online GPS Tutorial
- GPS-World GPS-World is conceived of as a free and central forum for those who travel with GPS and would like to exchange resulting data with others.
- GPS Visualizer - An online utility that creates SVG maps and profiles from GPS waypoints and tracks.