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The orbital planes are centered on the Earth, not rotating with respect to the distant stars.<ref>[http://metaresearch.org/cosmology/gps-relativity.asp What the Global Positioning System Tells Us about Relativity]. Accessed [[January 2]], [[2007]].</ref> The six planes have approximately 55° [[inclination]] (tilt relative to Earth's [[equator]]) and are separated by 60° [[right ascension]] of the [[orbital node|ascending node]] (angle along the equator from a reference point to the orbit's intersection).<ref name="GPS overview from JPO">[http://gps.losangeles.af.mil/jpo/gpsoverview.htm GPS Overview from the NAVSTAR Joint Program Office]. Accessed [[December 15]], [[2006]].</ref>
The orbital planes are centered on the Earth, not rotating with respect to the distant stars.<ref>[http://metaresearch.org/cosmology/gps-relativity.asp What the Global Positioning System Tells Us about Relativity]. Accessed [[January 2]], [[2007]].</ref> The six planes have approximately 55° [[inclination]] (tilt relative to Earth's [[equator]]) and are separated by 60° [[right ascension]] of the [[orbital node|ascending node]] (angle along the equator from a reference point to the orbit's intersection).<ref name="GPS overview from JPO">[http://gps.losangeles.af.mil/jpo/gpsoverview.htm GPS Overview from the NAVSTAR Joint Program Office]. Accessed [[December 15]], [[2006]].</ref>


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 anywhere on Earth.<ref>[http://www.navcen.uscg.gov/faq/gpsfaq.htm USCG Navcen: GPS Frequently Asked  Questions]. Accessed [[January 3]], [[2007]].</ref>
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 anywhere on Earth.<ref>[http://www.navcen.uscg.gov/faq/gpsfaq.htm USCG Navcen: GPS Frequently Asked  Questions]. Accessed [[January 3]], [[2007]].</ref>


[[As of February 2007]], there are 30 actively broadcasting satellites in the GPS [[satellite constellation|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.<ref>Massatt, Paul and Brady, Wayne. "[http://www.aero.org/publications/crosslink/summer2002/index.html Optimizing performance through constellation management]", ''Crosslink'', Summer 2002, pages 17-21.</ref>
[[As of February 2007]], there are 30 actively broadcasting satellites in the GPS [[satellite constellation|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.<ref>Massatt, Paul and Brady, Wayne. "[http://www.aero.org/publications/crosslink/summer2002/index.html Optimizing performance through constellation management]", ''Crosslink'', Summer 2002, pages 17-21.</ref>


==== Control segment ====
==== 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).<ref>US Coast Guard [http://www.navcen.uscg.gov/gps/gps_news_090905.htm General GPS News 9-9-05]</ref> The tracking information is sent to the [[Air Force Space Command]]'s master control station at [[Schriever Air Force Base]], [[Colorado Springs, Colorado]], 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 [[1 E-6 s|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 other various inputs.<ref>[[USNO]]. [http://tycho.usno.navy.mil/gpsinfo.html NAVSTAR Global Positioning System]. Accessed [[May 14]], [[2006]].</ref>
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).<ref>US Coast Guard [http://www.navcen.uscg.gov/gps/gps_news_090905.htm General GPS News 9-9-05]</ref> The tracking information is sent to the [[Air Force Space Command]]'s master control station at [[Schriever Air Force Base]], Colorado Springs, Colorado, 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 [[1 E-6 s|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 other various inputs.<ref>[[USNO]]. [http://tycho.usno.navy.mil/gpsinfo.html NAVSTAR Global Positioning System]. Accessed [[May 14]], [[2006]].</ref>


[[Image:GPS Receivers.jpg|thumb|left|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 (left to right).]]
[[Image:GPS Receivers.jpg|thumb|left|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 (left to right).]]
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==== User segment ====
==== 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.  
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.  


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 bps speed. Data is 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.
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 bps speed. Data is 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]]<ref>[[NMEA]] [http://www.nmea.org/pub/2000/index.html NMEA 2000]</ref> 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]] protocol.  Receivers can interface with other devices using methods including a serial connection, [[Universal Serial Bus|USB]] or [[Bluetooth]].
Many GPS receivers can relay position data to a PC or other device using the [[NMEA 0183]] protocol. [[NMEA 2000]]<ref>[[NMEA]] [http://www.nmea.org/pub/2000/index.html NMEA 2000]</ref> 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]] protocol.  Receivers can interface with other devices using methods including a serial connection, [[Universal Serial Bus|USB]] or [[Bluetooth]].


=== Navigation signals ===
=== Navigation signals ===
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The effects of the ionosphere are generally slow-moving, 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 System]]s 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.  
The effects of the ionosphere are generally slow-moving, 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 System]]s 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 much more localized, and changes more quickly than the ionospheric effects, making precise compensation for humidity more difficult. Altitude also causes a variable delay, as the signal passes through less atmosphere at higher elevations. Since the GPS receiver measures altitude directly, this is a much simpler correction to apply.
Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the [[troposphere]]. This effect is much more localized, and changes more quickly than the ionospheric effects, making precise compensation for humidity more difficult. Altitude also causes a variable delay, as the signal passes through less atmosphere at higher elevations. Since the GPS receiver measures altitude directly, this is a much simpler correction to apply.


==== Multipath effects ====
==== Multipath effects ====
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SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 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.  In order to improve the usefulness of GPS for civilian navigation, '''[[Differential GPS]]''' was used by many civilian GPS receivers to greatly improve accuracy.
SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 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.  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 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 0 meters (effectively the same thing) presented a clear benefit.
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 0 meters (effectively the same thing) presented a clear benefit.


In the 1990s, the [[Federal Aviation Administration|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, but SA was eventually "discontinued"; the amount of error added was "set to zero"<ref>[[Office of Science and Technology Policy]]. [http://www.ostp.gov/html/0053_2.html Presidential statement to stop degrading GPS]. [[May 1]], [[2000]].</ref> in 2000 following an announcement by U.S. President Bill Clinton, allowing users access to an undegraded 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]],<ref>[[FAA]], [http://gps.faa.gov/gpsbasics/SA-text.htm ''Selective Availability'']. Retrieved Jan. 6, 2007.</ref> have stated that it is not intended to be reintroduced.   
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, but SA was eventually "discontinued"; the amount of error added was "set to zero"<ref>[[Office of Science and Technology Policy]]. [http://www.ostp.gov/html/0053_2.html Presidential statement to stop degrading GPS]. [[May 1]], [[2000]].</ref> in 2000 following an announcement by U.S. President Bill Clinton, allowing users access to an undegraded 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]],<ref>[[FAA]], [http://gps.faa.gov/gpsbasics/SA-text.htm ''Selective Availability'']. Retrieved Jan. 6, 2007.</ref> 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.
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.
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[[Jamming]] of any radio navigation system, including satellite based navigation, is possible. The U.S. Air Force conducted GPS jamming exercises in 2003 and they also have GPS anti-spoofing capabilities. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in [[Phrack]] issue 60<ref>[[Phrack]]. [http://www.phrack.org/archives/60/p60-0x0d.txt Issue 0x3c (60), article 13]. [[December 28]], [[2002]].</ref> by an anonymous author. There has also been at least one well-documented case of unintentional jamming, tracing back to a malfunctioning TV antenna preamplifier.<ref>GPS World. [http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=43404&&pageID=1 The hunt for an unintentional GPS jammer]. [[January 1]], [[2003]].</ref> If stronger signals were generated intentionally, they could potentially interfere with aviation GPS receivers within line of sight. According to John Ruley, of AVweb, "IFR pilots should have a fallback plan in case of a GPS malfunction".<ref>Ruley, John. AVweb. [http://www.avweb.com/news/avionics/182754-1.html GPS jamming]. [[February 12]], [[2003]].</ref> [[RAIM|Receiver Autonomous Integrity Monitoring]] (RAIM), a feature of some aviation and marine receivers, is designed to provide a warning to the user if jamming or another problem is detected. GPS signals can also be interfered with by natural [[geomagnetic storm]]s, predominantly at high latitudes.<ref>[[Space Environment Center]]. [http://www.sec.noaa.gov/nav/gps.html SEC Navigation Systems GPS Page]. [[August 26]], [[1996]].</ref>
[[Jamming]] of any radio navigation system, including satellite based navigation, is possible. The U.S. Air Force conducted GPS jamming exercises in 2003 and they also have GPS anti-spoofing capabilities. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in [[Phrack]] issue 60<ref>[[Phrack]]. [http://www.phrack.org/archives/60/p60-0x0d.txt Issue 0x3c (60), article 13]. [[December 28]], [[2002]].</ref> by an anonymous author. There has also been at least one well-documented case of unintentional jamming, tracing back to a malfunctioning TV antenna preamplifier.<ref>GPS World. [http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=43404&&pageID=1 The hunt for an unintentional GPS jammer]. [[January 1]], [[2003]].</ref> If stronger signals were generated intentionally, they could potentially interfere with aviation GPS receivers within line of sight. According to John Ruley, of AVweb, "IFR pilots should have a fallback plan in case of a GPS malfunction".<ref>Ruley, John. AVweb. [http://www.avweb.com/news/avionics/182754-1.html GPS jamming]. [[February 12]], [[2003]].</ref> [[RAIM|Receiver Autonomous Integrity Monitoring]] (RAIM), a feature of some aviation and marine receivers, is designed to provide a warning to the user if jamming or another problem is detected. GPS signals can also be interfered with by natural [[geomagnetic storm]]s, predominantly at high latitudes.<ref>[[Space Environment Center]]. [http://www.sec.noaa.gov/nav/gps.html SEC Navigation Systems GPS Page]. [[August 26]], [[1996]].</ref>


The [[Federal government of the United States|U.S. government]] believes that such jammers were also used occasionally during the [[2001 war in Afghanistan]]. Some officials believe that jammers could be used to attract the precision-guided munitions towards [[non-combatant]] infrastructure; other officials believe that the jammers are completely ineffective.  In either case, the jammers may be attractive targets for [[anti-radiation missile]]s. During the [[Iraq War]], the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb.<ref>American Forces Press Service. [http://www.defenselink.mil/news/Mar2003/n03252003_200303254.html CENTCOM charts progress]. [[March 25]], [[2003]].</ref>
The [[Federal government of the United States|U.S. government]] believes that such jammers were also used occasionally during the [[2001 war in Afghanistan]]. Some officials believe that jammers could be used to attract the precision-guided munitions towards non-combatant infrastructure; other officials believe that the jammers are completely ineffective.  In either case, the jammers may be attractive targets for [[anti-radiation missile]]s. During the [[Iraq War]], the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb.<ref>American Forces Press Service. [http://www.defenselink.mil/news/Mar2003/n03252003_200303254.html CENTCOM charts progress]. [[March 25]], [[2003]].</ref>


==== Relativity ====
==== Relativity ====
According to the [[theory of relativity]], due to their constant movement and height relative to the Earth-centered inertial [[Special relativity#Reference frames.2C coordinates and The Lorentz transformation|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, by about 7,200 ns per day, than stationary ground clocks. When combined, the discrepancy is 38 [[microseconds]] per day; a difference of 4.465 parts in 10<sup>10</sup>.<ref>Rizos, Chris. [[University of New South Wales]]. [http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/312.htm GPS Satellite Signals]. 1999.</ref>. To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly more slowly than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.<ref>[http://www.aticourses.com/global_positioning_system.htm The Global Positioning System by Robert A. Nelson Via Satellite], November 1999</ref>  
According to the [[theory of relativity]], due to their constant movement and height relative to the Earth-centered inertial [[Special relativity#Reference frames.2C coordinates and The Lorentz transformation|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, by about 7,200 ns per day, than stationary ground clocks. When combined, the discrepancy is 38 microseconds per day; a difference of 4.465 parts in 10<sup>10</sup>.<ref>Rizos, Chris. [[University of New South Wales]]. [http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/312.htm GPS Satellite Signals]. 1999.</ref>. To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly more slowly than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.<ref>[http://www.aticourses.com/global_positioning_system.htm The Global Positioning System by Robert A. Nelson Via Satellite], November 1999</ref>  


Another relativistic effect to be compensated for in GPS observation processing is the [[Sagnac effect]]. The GPS time scale is defined in an [[inertial]] system but observations are processed in an [[ECEF|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.<ref>Ashby, Neil [http://www.ipgp.jussieu.fr/~tarantola/Files/Professional/GPS/Neil_Ashby_Relativity_GPS.pdf Relativity and GPS].  [[Physics Today]], May 2002.</ref>
Another relativistic effect to be compensated for in GPS observation processing is the [[Sagnac effect]]. The GPS time scale is defined in an [[inertial]] system but observations are processed in an [[ECEF|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.<ref>Ashby, Neil [http://www.ipgp.jussieu.fr/~tarantola/Files/Professional/GPS/Neil_Ashby_Relativity_GPS.pdf Relativity and GPS].  [[Physics Today]], May 2002.</ref>
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Receivers that have the correct decryption key can relatively easily decode the P(Y)-code transmitted on both L1 and L2 to measure the error. Receivers that do not possess the key can still use a process called ''codeless'' to compare the encrypted information on L1 and L2 to gain much of the same error information. However, this technique is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies. When these become operational, non-encrypted users will be able to make the same comparison and directly measure some errors.
Receivers that have the correct decryption key can relatively easily decode the P(Y)-code transmitted on both L1 and L2 to measure the error. Receivers that do not possess the key can still use a process called ''codeless'' to compare the encrypted information on L1 and L2 to gain much of the same error information. However, this technique is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies. When these become operational, non-encrypted users will be able to make the same comparison and directly measure some 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 [[Pseudorandom number generator|PRN]] is not instantaneous, and thus the [[cross-correlation|correlation]] (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a [[Periodicity|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 [[Differential GPS|DGPS]] normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.
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 [[cross-correlation|correlation]] (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a [[Periodicity|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 [[Differential GPS|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|real-time kinematic positioning]], RTK).
'''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|real-time kinematic positioning]], RTK).


=== GPS time and date ===
=== GPS time and date ===
While most clocks are synchronized to [[Coordinated Universal Time]] (UTC), the [[Atomic clock]]s 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 synchronized with the [[International Atomic Time]] (TAI).  
While most clocks are synchronized to [[Coordinated Universal Time]] (UTC), the [[Atomic clock]]s 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 synchronized with the [[International Atomic Time]] (TAI).  


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 'local' time.  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.
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 'local' time.  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]]). In order to determine the current [[Gregorian calendar|Gregorian]] date, a GPS receiver must be provided with the approximate date (to within 3,584 days) in order 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.
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]]). In order to determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) in order 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.


=== GPS modernization ===
=== GPS modernization ===
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GPS allows accurate targeting of various military weapons including [[cruise missile]]s and [[precision-guided munition]]s. To help prevent GPS guidance from being used in enemy or improvised weaponry, the US Government controls the export of civilian receivers.  A US-based manufacturer cannot generally export a receiver unless the receiver contains limits restricting it from functioning when it is simultaneously (1) at an altitude above 18 kilometers (60,000ft) and (2) traveling at over 515 m/s (1,000 knots).<ref>Arms Control Association. [http://www.armscontrol.org/documents/mtcr.asp Missile Technology Control Regime]. Accessed [[May 17]], [[2006]].</ref>
GPS allows accurate targeting of various military weapons including [[cruise missile]]s and [[precision-guided munition]]s. To help prevent GPS guidance from being used in enemy or improvised weaponry, the US Government controls the export of civilian receivers.  A US-based manufacturer cannot generally export a receiver unless the receiver contains limits restricting it from functioning when it is simultaneously (1) at an altitude above 18 kilometers (60,000ft) and (2) traveling at over 515 m/s (1,000 knots).<ref>Arms Control Association. [http://www.armscontrol.org/documents/mtcr.asp Missile Technology Control Regime]. Accessed [[May 17]], [[2006]].</ref>


The GPS satellites also carry nuclear detonation detectors, which form a major portion of the [[United States Nuclear Detonation Detection System]].<ref>Sandia National Laboratory's [http://www.sandia.gov/LabNews/LN03-07-03/LA2003/la03/arms_story.htm Nonproliferation programs and arms control technology].</ref>
The GPS satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System.<ref>Sandia National Laboratory's [http://www.sandia.gov/LabNews/LN03-07-03/LA2003/la03/arms_story.htm Nonproliferation programs and arms control technology].</ref>


=== Navigation ===
=== Navigation ===
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{{main|Automotive navigation system}}
{{main|Automotive navigation system}}


*'''[[Aircraft]]''' navigation systems usually display a "moving map" and are often connected to the [[autopilot]] for en-route navigation.  Cockpit-mounted GPS receivers and [[glass cockpit]]s are appearing in [[general aviation]] aircraft of all sizes, using technologies such as [[WAAS]] or [[LAAS]] to increase accuracy.  Many of these systems may be certified for [[instrument flight rules]] navigation, and some can also be used for final approach and landing operations.  [[Glider]] pilots use [[GNSS Flight Recorders]] to log GPS data verifying their arrival at turn points in [[gliding competitions]]. Flight computers installed in many gliders also use GPS to compute wind speed aloft, and glide paths to [[waypoint]]s such as alternate airports or mountain passes, to aid en route decision making for cross-country [[soaring]].
*'''Aircraft''' navigation systems usually display a "moving map" and are often connected to the [[autopilot]] for en-route navigation.  Cockpit-mounted GPS receivers and [[glass cockpit]]s are appearing in [[general aviation]] aircraft of all sizes, using technologies such as [[WAAS]] or [[LAAS]] to increase accuracy.  Many of these systems may be certified for [[instrument flight rules]] navigation, and some can also be used for final approach and landing operations.  [[Glider]] pilots use [[GNSS Flight Recorders]] to log GPS data verifying their arrival at turn points in [[gliding competitions]]. Flight computers installed in many gliders also use GPS to compute wind speed aloft, and glide paths to [[waypoint]]s such as alternate airports or mountain passes, to aid en route decision making for cross-country [[soaring]].


*'''[[Boat]]s and [[ship]]s''' can use GPS to navigate all of the world's lakes, seas and oceans.  Maritime GPS units include functions useful on water, such as “man overboard” (MOB) functions that allow instantly marking the location where a person has fallen overboard, which simplifies rescue efforts.  GPS may be connected to the ships [[self-steering gear]] and [[Chartplotter]]s using the [[NMEA|NMEA 0183]] interface. GPS can also improve the security of shipping traffic by enabling [[Automatic Identification System|AIS]].
*'''[[Boat]]s and [[ship]]s''' can use GPS to navigate all of the world's lakes, seas and oceans.  Maritime GPS units include functions useful on water, such as “man overboard” (MOB) functions that allow instantly marking the location where a person has fallen overboard, which simplifies rescue efforts.  GPS may be connected to the ships [[self-steering gear]] and [[Chartplotter]]s using the [[NMEA|NMEA 0183]] interface. GPS can also improve the security of shipping traffic by enabling [[Automatic Identification System|AIS]].


*'''Heavy Equipment''' can use GPS in construction, mining and [[precision agriculture]].  The blades and buckets of construction equipment are controlled automatically in GPS-based [[machine guidance]] systems.  [[Agricultural equipment]] may use GPS to steer automatically, or as a visual aid displayed on a screen for the driver.  This is very useful for controlled traffic and row crop operations and when spraying.  Harvesters with yield monitors can also use GPS to create a yield map of the paddock being harvested.   
*'''Heavy Equipment''' can use GPS in construction, mining and [[precision agriculture]].  The blades and buckets of construction equipment are controlled automatically in GPS-based [[machine guidance]] systems.  Agricultural equipment may use GPS to steer automatically, or as a visual aid displayed on a screen for the driver.  This is very useful for controlled traffic and row crop operations and when spraying.  Harvesters with yield monitors can also use GPS to create a yield map of the paddock being harvested.   


*'''[[Cycling|Bicycles]]''' often use GPS in racing and touring.  GPS navigation allows cyclists to plot their course in advance and follow this course, which may include quieter, narrower streets, without having to stop frequently to refer to separate maps.  Some GPS receivers are specifically adapted for cycling with special mounts and housings.
*'''[[Cycling|Bicycles]]''' often use GPS in racing and touring.  GPS navigation allows cyclists to plot their course in advance and follow this course, which may include quieter, narrower streets, without having to stop frequently to refer to separate maps.  Some GPS receivers are specifically adapted for cycling with special mounts and housings.
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*'''[[Hiking|Hikers]]''', [[mountain climbing|climbers]], and even ordinary pedestrians in urban or rural environments can use GPS to determine their position, with or without reference to separate maps.  In isolated areas, the ability of GPS to provide a precise position can greatly enhance the chances of rescue when climbers or hikers are disabled or lost (if they have a means of communication with rescue workers).
*'''[[Hiking|Hikers]]''', [[mountain climbing|climbers]], and even ordinary pedestrians in urban or rural environments can use GPS to determine their position, with or without reference to separate maps.  In isolated areas, the ability of GPS to provide a precise position can greatly enhance the chances of rescue when climbers or hikers are disabled or lost (if they have a means of communication with rescue workers).


*'''[[GPS for the visually impaired|GPS equipment for the visually impaired]]''' is available.  For more detailed information see the  article [[GPS for the visually impaired]]
*'''GPS equipment for the visually impaired''' is available.  For more detailed information see the  article GPS for the visually impaired


[[Image:GPS receiver (mouse).jpg|thumb|right|A modern SiRF Star III chip based 20-channel GPS receiver with WAAS/EGNOS support.]]
[[Image:GPS receiver (mouse).jpg|thumb|right|A modern SiRF Star III chip based 20-channel GPS receiver with WAAS/EGNOS support.]]
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*'''Surveying''' — Survey-Grade GPS receivers can be used to position [[survey marker]]s, buildings, and [[road construction]]. These units use the signal from both the L1 and L2 GPS frequencies.  Even though the L2 code data are [[encrypted]], the signal's [[carrier wave]] enables correction of some [[ionospheric]] errors.  These dual-frequency GPS receivers typically cost US$10,000 or more, but can have positioning errors on the order of one centimeter or less when used in carrier phase [[differential GPS]] mode.
*'''Surveying''' — Survey-Grade GPS receivers can be used to position [[survey marker]]s, buildings, and [[road construction]]. These units use the signal from both the L1 and L2 GPS frequencies.  Even though the L2 code data are [[encrypted]], the signal's [[carrier wave]] enables correction of some [[ionospheric]] errors.  These dual-frequency GPS receivers typically cost US$10,000 or more, but can have positioning errors on the order of one centimeter or less when used in carrier phase [[differential GPS]] mode.


*'''[[Cartography|Mapping]] and [[geographic information system]]s (GIS)''' — Most mapping grade GPS receivers use the carrier wave data from only the L1 frequency, but have a precise [[crystal oscillator]] which reduces errors related to receiver clock [[jitter]]. This allows positioning errors on the order of one meter or less in real-time, with a differential GPS signal received using a separate radio receiver. By storing the carrier phase measurements and differentially [[post-processing]] the data, positioning errors on the order of 10 centimeters are possible with these receivers.
*'''Mapping and geographic information systems (GIS)''' — Most mapping grade GPS receivers use the carrier wave data from only the L1 frequency, but have a precise [[crystal oscillator]] which reduces errors related to receiver clock [[jitter]]. This allows positioning errors on the order of one meter or less in real-time, with a differential GPS signal received using a separate radio receiver. By storing the carrier phase measurements and differentially [[post-processing]] the data, positioning errors on the order of 10 centimeters are possible with these receivers.


*'''Geophysics and geology''' — High precision measurements of [[crust (geology)|crustal]] strain can be made with differential GPS by finding the relative displacement between GPS sensors.  Multiple stations situated around an actively deforming area (such as a [[volcano]] or [[fault zone]]) can be used to find strain and ground movement. These measurements can then be used to interpret the cause of the deformation, such as a dike or sill beneath the surface of an active volcano.
*'''Geophysics and geology''' — High precision measurements of [[crust (geology)|crustal]] strain can be made with differential GPS by finding the relative displacement between GPS sensors.  Multiple stations situated around an actively deforming area (such as a [[volcano]] or [[fault zone]]) can be used to find strain and ground movement. These measurements can then be used to interpret the cause of the deformation, such as a dike or sill beneath the surface of an active volcano.
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=== Other uses ===
=== Other uses ===
[[Image:GPS roof antenna dsc06160.jpg|thumb|right|This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.]]
[[Image:GPS roof antenna dsc06160.jpg|thumb|right|This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.]]
*'''Precise time reference''' — Many systems that must be accurately [[synchronization|synchronized]] use GPS as a source of accurate time. GPS can be used as a [[Radio clock|reference clock]] for [[time code]] generators or [[Network Time Protocol|NTP]] clocks. [[Sensor]]s (for [[seismology]] or other monitoring application), can use GPS as a precise time source, so events may be timed accurately. [[Time division multiple access|TDMA]] communications networks often rely on this precise timing to synchronize RF generating equipment, network equipment, and [[multiplexer]]s.
*'''Precise time reference''' — Many systems that must be accurately [[synchronization|synchronized]] use GPS as a source of accurate time. GPS can be used as a [[Radio clock|reference clock]] for time code generators or [[Network Time Protocol|NTP]] clocks. [[Sensor]]s (for [[seismology]] or other monitoring application), can use GPS as a precise time source, so events may be timed accurately. [[Time division multiple access|TDMA]] communications networks often rely on this precise timing to synchronize RF generating equipment, network equipment, and [[multiplexer]]s.


*'''Mobile Satellite Communications''' — Satellite communications systems use a directional antenna (usually a "dish") pointed at a satellite.  The antenna on a moving ship or train, for example, must be pointed based on its current location. Modern antenna controllers usually incorporate a GPS receiver to provide this information.
*'''Mobile Satellite Communications''' — Satellite communications systems use a directional antenna (usually a "dish") pointed at a satellite.  The antenna on a moving ship or train, for example, must be pointed based on its current location. Modern antenna controllers usually incorporate a GPS receiver to provide this information.


*'''[[E911|Emergency]] and [[Location-based services]]''' — GPS functionality can be used by [[emergency services]] to locate cell phones.  The ability to locate a mobile phone is required in the United States by [[E911]] emergency services legislation. However, as of September 2006 such a system is not in place in all parts of the country.  GPS is less dependent on the telecommunications network [[topology]] than [[radiolocation]] for compatible phones.  [[Assisted GPS]] reduces the power requirements of the mobile phone and increases the accuracy of the location.  A phone's geographic location may also be used to provide location-based services including advertising, or other location-specific information.
*'''[[E911|Emergency]] and [[Location-based services]]''' — GPS functionality can be used by [[emergency services]] to locate cell phones.  The ability to locate a mobile phone is required in the United States by [[E911]] emergency services legislation. However, as of September 2006 such a system is not in place in all parts of the country.  GPS is less dependent on the telecommunications network topology than [[radiolocation]] for compatible phones.  Assisted GPS reduces the power requirements of the mobile phone and increases the accuracy of the location.  A phone's geographic location may also be used to provide location-based services including advertising, or other location-specific information.


*'''[[Location-based game]]s''' — The availability of hand-held GPS receivers has led to games such as [[Geocaching]], which involves using a hand-held GPS unit to travel to a specific [[longitude]] and [[latitude]] to search for objects hidden by other geocachers. This popular activity often includes walking or hiking to natural locations.  [[Geodashing]] is an outdoor sport using [[waypoint]]s.
*'''[[Location-based game]]s''' — The availability of hand-held GPS receivers has led to games such as [[Geocaching]], which involves using a hand-held GPS unit to travel to a specific [[longitude]] and latitude to search for objects hidden by other geocachers. This popular activity often includes walking or hiking to natural locations.  [[Geodashing]] is an outdoor sport using [[waypoint]]s.


*'''Aircraft passengers''' — Most [[airline]]s allow passenger use of GPS units on their flights, except during landing and take-off when other electronic devices are also restricted. Even though consumer GPS receivers have a minimal risk of interference, a few airlines disallow use of hand-held receivers during flight.  Other airlines integrate aircraft tracking into the seat-back television entertainment system, available to all passengers even during takeoff and landing.<ref>Joe Mehaffey. [http://gpsinformation.net/airgps/gpsrfi.htm Is it Safe to use a handheld GPS Receiver on a Commercial Aircraft?]. Accessed [[May 15]], [[2006]].</ref>
*'''Aircraft passengers''' — Most [[airline]]s allow passenger use of GPS units on their flights, except during landing and take-off when other electronic devices are also restricted. Even though consumer GPS receivers have a minimal risk of interference, a few airlines disallow use of hand-held receivers during flight.  Other airlines integrate aircraft tracking into the seat-back television entertainment system, available to all passengers even during takeoff and landing.<ref>Joe Mehaffey. [http://gpsinformation.net/airgps/gpsrfi.htm Is it Safe to use a handheld GPS Receiver on a Commercial Aircraft?]. Accessed [[May 15]], [[2006]].</ref>


*'''Heading information''' — The GPS system can be used to determine heading information, even though it was not designed for this purpose. A "GPS compass" uses a pair of antennas separated by about 50 cm to detect the phase difference in the carrier signal from a particular GPS satellite.<ref>[http://www.jrcamerica.com/product.asp?Product_Id=17778 ''JLR-10 GPS Compass'']. Accessed Jan. 6, 2007.</ref> Given the positions of the satellite, the position of the antenna, and the phase difference, the orientation of the two antennas can be computed.  More expensive GPS compass systems use three antennas in a triangle to get three separate readings with respect to each satellite.  A GPS compass is not subject to [[magnetic declination]] as a magnetic compass is, and doesn't need to be reset periodically like a [[gyrocompass]]. It is, however, subject to multipath effects.
*'''Heading information''' — The GPS system can be used to determine heading information, even though it was not designed for this purpose. A "GPS compass" uses a pair of antennas separated by about 50 cm to detect the phase difference in the carrier signal from a particular GPS satellite.<ref>[http://www.jrcamerica.com/product.asp?Product_Id=17778 ''JLR-10 GPS Compass'']. Accessed Jan. 6, 2007.</ref> Given the positions of the satellite, the position of the antenna, and the phase difference, the orientation of the two antennas can be computed.  More expensive GPS compass systems use three antennas in a triangle to get three separate readings with respect to each satellite.  A GPS compass is not subject to magnetic declination as a magnetic compass is, and doesn't need to be reset periodically like a gyrocompass. It is, however, subject to multipath effects.


*'''[[GPS tracking]]''' systems use GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a log of movements.  The data can be stored inside the unit, or sent to a remote computer by radio or cellular modem.  Some systems allow the location to be viewed in [[real-time]] on the Internet with a web-browser.
*'''[[GPS tracking]]''' systems use GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a log of movements.  The data can be stored inside the unit, or sent to a remote computer by radio or cellular modem.  Some systems allow the location to be viewed in [[real-time]] on the Internet with a web-browser. There are various applications for GPS tracking, namely in public safety and crime prevention.<ref>''[http://gpstrackit.com/gps-tracking-the-future/ GPS Tracking: The Future]''</ref>


*'''Weather Prediction Improvements''' — Measurement of atmospheric bending of GPS satellite signals by specialized GPS receivers in orbital satellites can be used to determine atmospheric conditions such as air density, temperature, moisture and electron density. Such information from a set of six micro-satellites, launched in April 2006, called the Constellation of Observing System for Meteorology, Ionosphere and Climate [[COSMIC]] has been proven to improve the accuracy of weather prediction models.
*'''Weather Prediction Improvements''' — Measurement of atmospheric bending of GPS satellite signals by specialized GPS receivers in orbital satellites can be used to determine atmospheric conditions such as air density, temperature, moisture and electron density. Such information from a set of six micro-satellites, launched in April 2006, called the Constellation of Observing System for Meteorology, Ionosphere and Climate [[COSMIC]] has been proven to improve the accuracy of weather prediction models.
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== History ==
== History ==
The design of GPS is based partly on the similar ground-based radio navigation systems, such as [[LORAN]] and the [[Decca Navigator System|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 program|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 converse is also true: if the satellite's position were known, they could identify their own position on Earth. (commented because I am not sure of this.  At most, they would know the rate at which the distance between themselves and the satellite was changing.  There would be at least two points (one each north and south of the equator) for which that would be true, and practically one would not get an exact position, especially with 1950s electronics, even if one knew the satellite's exact orbit, and the exact time -->
The design of GPS is based partly on the similar ground-based radio navigation systems, such as [[LORAN]] and the [[Decca Navigator System|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 program|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 converse is also true: if the satellite's position were known, they could identify their own position on Earth. (commented because I am not sure of this.  At most, they would know the rate at which the distance between themselves and the satellite was changing.  There would be at least two points (one each north and south of the equator) for which that would be true, and practically one would not get an exact position, especially with 1950s electronics, even if one knew the satellite's exact orbit, and the exact time -->


The first satellite navigation system, [[Transit (satellite)|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 satellite navigation system, [[Transit (satellite)|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.<ref>Hydrographic Journal. [http://www.hydrographicsociety.org/Articles/journal/2002/104-1.htm Developments in Global Navigation Satellite Systems]. April 2002. Accessed [[May 14]], [[2006]].</ref>  The GPS satellites were initially manufactured by [[ArvinMeritor|Rockwell International]] and are now manufactured by [[Lockheed Martin]].
The first experimental Block-I GPS satellite was launched in February 1978.<ref>Hydrographic Journal. [http://www.hydrographicsociety.org/Articles/journal/2002/104-1.htm Developments in Global Navigation Satellite Systems]. April 2002. Accessed [[May 14]], [[2006]].</ref>  The GPS satellites were initially manufactured by [[ArvinMeritor|Rockwell International]] and are now manufactured by [[Lockheed Martin]].
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*By December 1993 the GPS system achieved initial operational capability<ref>United States Department of Defense. [http://www.navcen.uscg.gov/ftp/gps/ARCHIVES/gpsdoc/IOCLTR.TXT Announcement of Initial Operational Capability]. [[December 8]], [[1993]].</ref>   
*By December 1993 the GPS system achieved initial operational capability<ref>United States Department of Defense. [http://www.navcen.uscg.gov/ftp/gps/ARCHIVES/gpsdoc/IOCLTR.TXT Announcement of Initial Operational Capability]. [[December 8]], [[1993]].</ref>   
*By [[January 17]], [[1994]] a complete constellation of 24 satellites was in orbit.
*By [[January 17]], [[1994]] a complete constellation of 24 satellites was in orbit.
*In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President [[Bill Clinton]] issued a policy directive<ref>[[National Archives and Records Administration]]. [http://clinton4.nara.gov/textonly/WH/EOP/OSTP/html/gps-factsheet.html U.S. GLOBAL POSITIONING SYSTEM POLICY]. [[March 29]], [[1996]].</ref> declaring GPS to be a dual-use system and establishing an [[Interagency GPS Executive Board]] to manage it as a national asset.
*In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive<ref>[[National Archives and Records Administration]]. [http://clinton4.nara.gov/textonly/WH/EOP/OSTP/html/gps-factsheet.html U.S. GLOBAL POSITIONING SYSTEM POLICY]. [[March 29]], [[1996]].</ref> 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.
*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, allowing users outside the US military to receive a full quality signal.
*On [[May 2]], [[2000]] "Selective Availability" was discontinued, allowing users outside the US military to receive a full quality signal.
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Two GPS developers have received the [[United States National Academy of Engineering|National Academy of Engineering]] [[Charles Stark Draper]] prize year 2003:
Two GPS developers have received the [[United States National Academy of Engineering|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]] ('''Lo'''ng-range '''R'''adio '''A'''id to '''N'''avigation).
*[[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]] ('''Lo'''ng-range '''R'''adio '''A'''id to '''N'''avigation).
*[[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.
*[[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]].<ref>[[United States Naval Research Laboratory]]. [http://www.eurekalert.org/pub_releases/2005-11/nrl-par112205.php National Medal of Technology for GPS]. [[November 21]], [[2005]]</ref>
One GPS developer, [[Roger L. Easton]], received the [[National Medal of Technology]] on [[February 13]] [[2006]] at the White House.<ref>[[United States Naval Research Laboratory]]. [http://www.eurekalert.org/pub_releases/2005-11/nrl-par112205.php National Medal of Technology for GPS]. [[November 21]], [[2005]]</ref>


On [[February 10]], [[1993]], the [[National Aeronautic Association]] selected the Global Positioning System Team as winners of the 1992 [[Collier Trophy|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|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."
On [[February 10]], [[1993]], the [[National Aeronautic Association]] selected the Global Positioning System Team as winners of the 1992 [[Collier Trophy|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|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."
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* '''[[GLONASS]]''' ('''GLO'''bal '''NA'''vigation '''S'''atellite '''S'''ystem) is operated by Russia, although with only twelve active satellites [[as of 2004]]. In Russia, Northern Europe and Canada, at least four GLONASS satellites are visible 45% of time.  There are plans to restore GLONASS to full operation by 2008 with assistance from India.
* '''[[GLONASS]]''' ('''GLO'''bal '''NA'''vigation '''S'''atellite '''S'''ystem) is operated by Russia, although with only twelve active satellites [[as of 2004]]. In Russia, Northern Europe and Canada, at least four GLONASS satellites are visible 45% of time.  There are plans to restore GLONASS to full operation by 2008 with assistance from India.


*'''[[Galileo positioning system|Galileo]]''' is being developed by the [[European Union]], joined by China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine planned to be operational by 2010.   
*'''[[Galileo positioning system|Galileo]]''' is being developed by the European Union, joined by China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine planned to be operational by 2010.   


*'''[[Beidou navigation system|Beidou]]''' may be developed independently by China.<ref>[http://www.theinquirer.net/default.aspx?article=35624 Chinese threaten to dump Galileo GPS]</ref>
*'''[[Beidou navigation system|Beidou]]''' may be developed independently by China.<ref>[http://www.theinquirer.net/default.aspx?article=35624 Chinese threaten to dump Galileo GPS]</ref>
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{{nautical portal}}
{{nautical portal}}
*[[Degree Confluence Project]] Use GPS to visit integral degrees of latitude and longitude.
*Degree Confluence Project Use GPS to visit integral degrees of latitude and longitude.
*[[Geo (microformat)]] — a markup system for WSG84 coordinates in (X)HTML
*[[Geo (microformat)]] — a markup system for WSG84 coordinates in (X)HTML
*[[GPS Drawing]] Digital mapping and drawing with GPS tracks.
*[[GPS Drawing]] Digital mapping and drawing with GPS tracks.
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* [http://www.rand.org/publications/MR/MR614/MR614.appb.pdf RAND history of the GPS system (PDF)]
* [http://www.rand.org/publications/MR/MR614/MR614.appb.pdf RAND history of the GPS system (PDF)]
* [http://www.defense-update.com/products/g/gps-aj.htm GPS Anti-Jam Protection Techniques]
* [http://www.defense-update.com/products/g/gps-aj.htm GPS Anti-Jam Protection Techniques]
* [http://www.aero.org/publications/crosslink/summer2002/index.html Crosslink] Summer 2002 issue by [[The Aerospace Corporation]] on satellite navigation.
* [http://www.aero.org/publications/crosslink/summer2002/index.html Crosslink] Summer 2002 issue by The Aerospace Corporation on satellite navigation.
* [http://www.ucar.edu/communications/staffnotes/0409/cosmic.html Improved weather predictions from COSMIC GPS satellite signal occultation data].
* [http://www.ucar.edu/communications/staffnotes/0409/cosmic.html Improved weather predictions from COSMIC GPS satellite signal occultation data].
* [http://users.erols.com/dlwilson/gps.htm David L. Wilson's GPS Accuracy Web Page] A thorough analysis of the accuracy of GPS.
* [http://users.erols.com/dlwilson/gps.htm David L. Wilson's GPS Accuracy Web Page] A thorough analysis of the accuracy of GPS.
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[[af:GPS]]
[[af:GPS]]
[[bs:GPS]]
bs:GPS
[[bg:Глобална система за позициониране]]
[[bg:Глобална система за позициониране]]
[[ca:GPS]]
[[ca:GPS]]
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et:GPS
[[el:Global Positioning System]]
[[el:Global Positioning System]]
[[fa:جی‌پی‌اس]]
[[fa:جی‌پی‌اس]]
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[[lb:Global Positioning System]]
[[lb:Global Positioning System]]
[[lt:GPS]]
[[lt:GPS]]
[[hu:GPS]]
[[ms:Sistem Kedudukan Sejagat]]
[[ms:Sistem Kedudukan Sejagat]]
[[no:Global Positioning System]]
[[no:Global Positioning System]]
[[nrm:Pliaich'chie globale à satellite]]
[[nrm:Pliaich'chie globale à satellite]]
[[pl:Global Positioning System]]
pl:Global Positioning System
[[ru:Спутниковая система навигации]]
ru:Спутниковая система навигации
[[sk:Global Positioning System]]
[[sk:Global Positioning System]]
[[sl:GPS]]
[[sl:GPS]]
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[[de:GPS]]
[[de:GPS]]
[[eo:Tutmonda loktrova sistemo]]
[[eo:Tutmonda loktrova sistemo]]
[[es:GPS]]
[[fi:GPS]]
[[fi:GPS]]
[[fr:GPS]]
[[fr:GPS]]
[[nl:Global Positioning System]]
[[nl:Global Positioning System]]
[[sv:GPS]]
[[sv:GPS]]
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