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Global Positioning System: Difference between revisions
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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 | 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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==== 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 | 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 | 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). | ||
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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 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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*'''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. | *'''[[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. | ||
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*'''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 | 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. | ||
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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 | *[[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 | *[[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> | ||
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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 | * [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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