GPS THE GLOBAL POSITIONING SYSTEM AND ITS APPLICATIONS
•GPS Wing (JPO)
The Global Positioning System (GPS) is a U.S. space-based radionavigation system that provides reliable positioning, navigation, and timing services to civilian users on a continuous worldwide basis -- freely available to all. For anyone with a GPS receiver, the system will provide location and time. GPS provides accurate location and time information for an unlimited number of people in all weather, day and night, anywhere in the world.
The GPS is made up of three parts: satellites orbiting the Earth; control and monitoring stations on Earth; and the GPS receivers owned by users. GPS satellites broadcast signals from space that are picked up and identified by GPS receivers. Each GPS receiver then provides three-dimensional location (latitude, longitude, and altitude) plus the time.
Individuals may purchase GPS handsets that are readily available through commercial retailers. Equipped with these GPS receivers, users can accurately locate where they are and easily navigate to where they want to go, whether walking, driving, flying, or boating. GPS has become a mainstay of transportation systems worldwide, providing navigation for aviation, ground, and maritime operations. Disaster relief and emergency services depend upon GPS for location and timing capabilities in their life-saving missions. Everyday activities such as banking, mobile phone operations, and even the control of power grids, are facilitated by the accurate timing provided by GPS. Farmers, surveyors, geologists and countless others perform their work more efficiently, safely, economically, and accurately using the free and open GPS signals.
For additional information about GPS, please explore the rest of this website, as well as the external sites referenced on this page.
The Global Positioning System
“The United States Government shall... Provide on a continuous, worldwide basis civil space-based positioning, navigation, and timing services free of direct user fees for civil, commercial, and scientific uses and for homeland security through the Global Positioning System and its augmentations, and provide open, free access to information necessary to develop and build equipment to use these services.”
President George W. Bush, U.S. National Space-Based Positioning, Navigation, and Timing Policy, December 2004
What is GPS?
The Global Positioning System (GPS) is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services. This system consists of three segments: the space segment, the control segment, and the user segment. The U.S. Air Force develops, maintains, and operates the space and control segments.
•The space segment consists of a nominal constellation of 24 operating satellites that transmit one-way signals that give the current GPS satellite position and time.
•The control segment consists of worldwide monitor and control stations that maintain the satellites in their proper orbits through occasional command maneuvers, and adjust the satellite clocks. It tracks the GPS satellites, uploads updated navigational data, and maintains health and status of the satellite constellation.
•The user segment consists of the GPS receiver equipment, which receives the signals from the GPS satellites and uses the transmitted information to calculate the user’s three-dimensional position and time.
GPS satellites provide service to civilian and military users. The civilian service is freely available to all users on a continuous, worldwide basis. The military service is available to U.S. and allied armed forces as well as approved Government agencies.
A variety of GPS augmentation systems and techniques are available to enhance system performance to meet specific user requirements. These improve signal availability, accuracy, and integrity, allowing even better performance than is possible using the basic GPS civilian service.
The outstanding performance of GPS over many years has earned the confidence of millions of civil users worldwide. It has proven its dependability in the past and promises to be of benefit to users, throughout the world, far into the future.
The Future of GPS
Stand-Alone GPS Notional Horizontal Performance with New Signals
The United States is committed to an extensive modernization program, including the implementation of a second and a third civil signal on GPS satellites. The second civil signal will improve the accuracy of the civilian service and supports some safety-of-life applications. The third signal will further enhance civilian capability and is primarily designed for safety-of-life applications, such as aviation.
The figure on the right depicts the improvement of the quality of service with the additional civilian signals.
Positioning, Navigation, and Timing Policy
U.S. law and policy on GPS emphasize continuity of service, open access to civil signals, and technological leadership. In 1996, the United States issued a national policy statement on the management and use of space-based positioning, navigation and timing services, which include GPS and augmentations. It underscored the dual use (civilian-military) nature of GPS and established a joint civil-military national management structure to oversee its operation.
U.S. policy was expanded in 2004 in response to changing international conditions and the incredible growth in the types and complexities of GPS applications. This policy reaffirms the United States commitment to provide reliable civil space-based positioning, navigation, and timing services through GPS on a continuous, worldwide basis -- freely available to all. The policy also calls for improving the performance of GPS and cooperating with other nations.
“One can imagine a 21st century world covered by an augmented GPS and laced with mobile digital communications in which aircraft and other vehicles travel through ‘virtual tunnels,’ imaginary tracks through space which are continuously optimized for weather, traffic, and other conditions. Robotic vehicles perform all sorts of construction, transportation, mining, and earth-moving functions working day and night with no need to rest.”
Bradford W. Parkinson, Stanford University, California, USA;
James J. Spilker Jr., Stanford Telecom, California, USA
To meet the specific user requirements for positioning, navigation, and timing (PNT), a number of augmentations to the Global Positioning System (GPS) are available. An augmentation is any system that aids GPS by providing accuracy, integrity, reliability, availability, or any other improvement to positioning, navigation, and timing that is not inherently part of GPS itself. Such augmentations include, but are not limited to:
•Nationwide Differential GPS System (NDGPS): The NDGPS is a ground-based augmentation system operated and maintained by the Federal Railroad Administration, U.S. Coast Guard, and Federal Highway Administration, that provides increased accuracy and integrity of the GPS to users on land and water. Modernization efforts include the High Accuracy NDGPS (HA-NDGPS) system, currently under development, to enhance the performance and provide 10 to 15 centimeter accuracy throughout the coverage area. NDGPS is built to international standards, and over 50 countries around the world have implemented similar systems.
•Wide Area Augmentation System (WAAS): The WAAS, a satellite-based augmentation system operated by the U.S. Federal Aviation Administration (FAA), provides aircraft navigation for all phases of flight. Today, these capabilities are broadly used in other applications because their GPS-like signals can be processed by simple receivers without additional equipment. Using International Civil Aviation Organization (ICAO) standards, the FAA continues to work with other States to provide seamless services to all users in any region. Other ICAO standard space-based augmentation systems include: Europe's European Geostationary Navigation Overlay System (EGNOS), India's GPS and Geo-Augmented Navigation System (GAGAN), and Japan's Multifunction Transport Satellite (MTSAT) Satellite Augmentation System (MSAS). All of these international implementations are based on GPS. The FAA will improve the WAAS to take advantage of the future GPS safety-of-life signal and provide better performance and promote global adoption of these new capabilities.
•Continuously Operating Reference Station (CORS): The U.S. CORS network, which is managed by the National Oceanic & Atmospheric Administration, archives and distributes GPS data for precision positioning and atmospheric modeling applications mainly through post-processing. CORS is being modernized to support real-time users.
•Global Differential GPS (GDGPS): GDGPS is a high accuracy GPS augmentation system, developed by the Jet Propulsion Laboratory (JPL) to support the real-time positioning, timing, and orbit determination requirements of the U.S. National Aeronautics and Space Administration (NASA) science missions. Future NASA plans include using the Tracking and Data Relay Satellite System (TDRSS) to disseminate via satellite a real-time differential correction message. This system is referred to as the TDRSS Augmentation Service Satellites (TASS).
•International GNSS Service (IGS): IGS is a network of over 350 GPS monitoring stations from 200 contributing organizations in 80 countries. Its mission is to provide the highest quality data and products as the standard for Global Navigation Satellite Systems (GNSS) in support of Earth science research, multidisciplinary applications, and education, as well as to facilitate other applications benefiting society. Approximately 100 IGS stations transmit their tracking data within one hour of collection.
There are other augmentation systems available worldwide, both government and commercial. These systems use differential, static, or real-time techniques.
U.S. Policy on International Cooperation
The U.S. Space-Based Positioning, Navigation, and Timing Policy underscores the importance that all global navigation satellite systems and their augmentations be compatible with GPS.
The agreement in 2004 between the United States and the European Union (E.U.) on GPS and Galileo recognized the benefits of interoperable systems. The parties agreed to pursue a common, open, civil signal on both Galileo and future GPS satellites, in addition to ongoing cooperation on the GPS-based EGNOS augmentation system.
The United States has a long cooperative relationship with Japan on GPS. In addition to the Multifunction Transport Satellite (MTSAT) Satellite Augmentation System (MSAS), the parties are working towards developing a GPS-compatible regional satellite "mini-" constellation known as the Quasi Zenith Satellite System (QZSS).
The United States is also consulting closely with India on its development of its GAGAN space-based augmentation system, and with the Russian Federation on compatibility and interoperability between GPS and Russia's satellite navigation system, GLONASS.
The U.S. Department of Defense also cooperates with numerous countries to ensure that GPS provides military space-based PNT service and interoperable user equipment to its coalition partners around the world.
Space-based PNT services must serve global users with transparent interfaces and standards. The U.S. policy is to provide space-based PNT services on a continuous worldwide basis, freely available to all for civil, commercial, and scientific uses, and provide open, free access, to information necessary to develop and build equipment to use these services.
http://www.ig.utexas.edu/research/overv ... SID=def1b9
The U.S. Global Positioning System (GPS) satellites provide data that are used in a broad spectrum of Earth science disciplines, including geodesy and geodynamics. Currently, there are 24 satellites, each travelling in a 12-hour, circular orbit 20,200 kilometers above Earth. The satellites are positioned so that at least six are nearly always observable from any point on Earth. The GPS satellites transmit ranging codes on two-radio frequency carriers at L-band frequencies, which can be detected by ground-based GPS receivers. Special ground-based stations perform satellite monitoring that permits the locations of GPS receivers to be determined with a high degree of accuracy.
Researchers at UTIG study the motions of tectonic plates, displacements associated with earthquakes, sea-level fluctuations, and Earth orientations by monitoring very precisely the change in the receiver locations over time. With the technology currently available, an accuracy of a few millimeters can be achieved.
UTIG has a number of projects using GPS all over the world, from the Southwest Pacific GPS Project to the Scotia Arc Project (SCARP) to the WAGN project. We have a number of GPS-based projects in the Caribbean:
•Collaborative Research: A GPS Study of Caribbean Plate Kinematics and Distributed Deformation Between the Caribbean and North American Plates
•Paleo-seismic Investigation of the North American-Caribbean Strike-Slip Plate Boundary, Dominican Republic
•GPS Investigation of the North America-Caribbean Plate Boundary (1994)
Publications on Caribbean wide GPS network including results from northeastern Caribbean CANAPE (Caribbean-North America Plate Experiment) network:
Focus on the GPS data from sites within the stable interior of the Caribbean plate:
DeMets, C., Jansma, P., Mattioli, G., Dixon, T., Farina, F., Bilham, R., Calais, E., and Mann, P., 2000, GPS geodetic constraints on Caribbean-North American plate motion, Geophysical Research Letters, v. 27, p. 437-440.
Focus on GPS data from the entire NSF-funded CANAPE network:
Dixon, T., Farina, F., DeMets, C., Jansma, P., Mann, P., and Calais, E., 1998, Relative motion between the Caribbean and North American plates and related boundary zone deformation based on a decade of GPS observations, Journal of Geophysical Research, v. 103, p. 15,157-15,182.
Focus on subset of CANAPE GPS data from the Puerto Rico-Virgin Islands area:
Jansma, P., Lopez, A., Mattioli, G., DeMets, C., Dixon, T., Mann, P., and Calais, E., 2000, Microplate tectonics in the northeastern Caribbean as constrained by Global Positioning (GPS) geodesy, Tectonics, v. 19, p. 1021-1037.
Focus on the subset of CANAPE GPS data from the Hispaniola-Bahama platform oblique collision area:
Calais, E., Mazabraund, Y, Mercier de Lepinay, B., Mann, P., Mattioli, G., and Jansma, P., 2002, Strain partitioning and fault slip rates in the northeastern Caribbean from GPS measurements: Geophysicsal Research Letters, v. 29, no. 18, 1856, doi:10:1029/2002GL015397, 2002.
Pollitz, F., and Dixon, T., 1998, GPS measurements across the northern Caribbean plate boundary zone; Impact of posteismic relaxation following historic earthquakes, Geophysical Research Letters, v. 25, p. 2233-2236.
Mann, P., Prentice, C., Burr, G., Pena, L., and Taylor, F. W., 1998, Tectonic geomorphology and paleoseismology of the Septentrional fault zone, Dominican Republic, in J. F. Dolan and P. Mann, editors, Active Strike-slip and Collisional Tectonics of the Northern Caribbean Plate Boundary Zone, Geological Society of America Special Paper 324, p. 63-123.
Mann, P., Calais, E.,Ruegg, J-C., DeMets, C., Jansma, P., and Mattioli, G., 2002, Oblique collision in the northeastern Caribbean from GPS measurements and geological observations: Tectonics, v. 21, no. 6, 1057, doi:10.1029?2001TC001304, 2002.
Focus on the subset of CANAPE GPS data from the Caribbean-Northern South America area:
Weber, J.C., Dixon, T.H., DeMets, C., Ambeh, W.B., Jansma, P., Mattioli, G., Saleh, J., Sella, G., Bilham, R., and Perez, O., 2001, GPS estimate of relative motion between the Caribbean and South American plates, and geologic implications for Trinidad and Venezuela, Geology, 29(1): 75-78.
Focus on northern South America using results from the GPS network operated by Perez, Bilham, et al.:
Perez, O.J., Bilham, R., Bendick, R., Velandia, J.R., Hernandez, N., Moncayo, C., Hoyer, M., and Kozuch, M., 2001, Velocity field across the southern Caribbean plate boundary and estimates of Caribbean/South-American plate motion using GPS geodesy 1994-2000; Geophysical Research Letters, v. 28, p. 2987-2990.
Publications - General
Calmant, S., Pelletier, B., Pillet, R., Régnier, M., Lebellegard, P., Maillard, D., Taylor, F.W., Bevis, M., and Recy, J., 1997, Aseismic and co-seismic motions in GPS series related to the Ms 7.3 July 13, 1994, Malekula earthquake, central New Hebrides subduction zone, Geophys. Res. Letts. 24, 3077-3080.
Taylor, F.W. et al. 1995, Geodetic measurements of convergence at the New Hebrides island arc indicate arc fragmentation due to an impinging aseismic ridge: Geology, 23, 1011-1014. Calmant, S., Lebellegard, P., Taylor, F., Bevis, M., Maillard, D., Recy, J., and Bonneau, J., 1995, Geodetic measurements of convergence across the New Hebrides subduction zone; Geophys. Res. Letts., 22, 2573-2576.
Bevis, M., Taylor, F.W., Schutz, B.E., Recy, J., Isacks, B.L., Helu, S., Singh, R., Kendrick, E., Stowell, J., Taylor, B., and Calmant, S., 1995, Geodetic observations of convergence and back-arc spreading at the Tonga island arc, Nature, 374, 249-251.
Taylor, F. W., Quinn, T. M., Gallup, C. G., and Edwards, R. L., 1994, Quaternary plate convergence rates at the New Hebrides arc from the chronostratigraphy of Bougainville Guyot (Site 831), Proc. ODP, Sci. Results, 134, College Station, TX (Ocean Drilling Program) 47-57.
Schutz, B. E., Bevis, M. G., Taylor, F. W., Kuang, D., Watkins, M., Recy, J., Perin, B., and Peyroux, O., 1993, The Southwest Pacific GPS Project: Geodetic results from burst 1 of the 1990 field campaign, Bull. Geodesique, 67, 224-240.
Taylor, F. W. , Recy, J., Larue, B.M., Bevis, M., and Schutz, B., 1991, A geodetically positioned shallow platform for tectonics and oceanographic research, in Kumar, M., and Maul, G. A. (eds.), Marine Positioning into the 1990's, Proc. International Symp. on Marine Positioning, October 15-19, 1990, (Rosenstiel School of Marine and Atmospheric Science) Miami, Florida, PIP Printing, Rockville, Maryland, 278-287.
Institute for Geophysics; J.J. Pickle Research Campus, Bldg. 196; 10100 Burnet Road (R2200); Austin TX 78758-4445
Phone: (512) 471-6156; FAX: (512) 471-8844
Last modified: 08 Aug 2006 13:22
Global Positioning System
From Wikipedia, the free encyclopedia
The Global Positioning System (GPS), is currently the only fully functional Global Navigation Satellite System (GNSS). More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the receiver's location, speed and direction.
Since the first experimental satellite was launched in 1978, GPS has become an indispensable aid to navigation around the world, and an important tool for map-making and land surveying. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.
Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year, including the replacement of aging satellites, and research and development. Despite this fact, GPS is free for civilian use as a public good.
Simplified method of operation
A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS radio signal gives the distance to each satellite, since the signal travels at a known speed. The signals also carry information about the satellites' location. By determining the position of, and distance to, at least three satellites, the receiver can compute its position using trilateration. Receivers typically do not have perfectly accurate clocks and therefore track one or more additional satellites to correct the receiver's clock error.
The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).
The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design calls for 24 SVs to be distributed equally among six circular orbital planes. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).
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.
As of February 2007, there are 30 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.
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). 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 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.
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 WAAS receivers.
Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000 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, USB or Bluetooth.
GPS broadcast signal
GPS satellites broadcast three different types of data in the primary navigation signal. The first is the almanac which sends coarse time information along with status information about the satellites. The second is the ephemeris, which contains orbital information that allows the receiver to calculate the position of the satellite. This data is included in the 37,500 bit Navigation Message, which takes 12.5 minutes to send at 50 bps.
The satellites also broadcast two forms of clock information, the Coarse / Acquisition code, or C/A which is freely available to the public, and the restricted Precise code, or P-code, usually reserved for military applications. The C/A code is a 1,023 bit long pseudo-random code broadcast at 1.023 MHz, repeating every millisecond. Each satellite sends a distinct C/A code, which allows it to be uniquely identified. The P-code is a similar code broadcast at 10.23 MHz, but it repeats only once a week. In normal operation, the so-called "anti-spoofing mode", the P code is first encrypted into the Y-code, or P(Y), which can only be decrypted by units with a valid decryption key. Frequencies used by GPS include:
•L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code.
•L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
•L3 (1381.05 MHz): Used by the Defense Support Program to signal detection of missile launches, nuclear detonations, and other high-energy infrared events.
•L4 (1379.913 MHz): Being studied for additional ionospheric correction.
•L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2008.
The coordinates are calculated according to the World Geodetic System WGS84 coordinate system. To calculate its position, a receiver needs to know the precise time. The satellites are equipped with extremely accurate atomic clocks, and the receiver uses an internal crystal oscillator-based clock that is continually updated using the signals from the satellites.
The receiver identifies each satellite's signal by its distinct C/A code pattern, then measures the time delay for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number as the satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange.
Overlapping pseudoranges, represented as curves, are modified to yield the probable position
The orbital position data from the Navigation Message is then used to calculate the satellite's precise position. Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. When four satellites are measured simultaneously, the intersection of the four imaginary spheres reveals the location of the receiver. Earth-based users can substitute the sphere of the planet for one satellite by using their altitude. Often, these spheres will overlap slightly instead of meeting at one point, so the receiver will yield a mathematically most-probable position (and often indicate the uncertainty).
Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism; if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite. In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators. RAIM features will not help, since RAIM only checks the signals from a navigational perspective.
Accuracy and error sources
The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.
To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about 1% of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only the GPS C/A signal.
Position accuracy can be improved by using the higher-speed P(Y) signal. Assuming the same 1% accuracy, the faster P(Y) signal results in an accuracy of about 30 centimeters.
Electronics errors are one of several accuracy-degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy.
Surveying and mapping
•Surveying — Survey-Grade GPS receivers can be used to position survey markers, 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.
•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 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.
•Archeology — As archaeologists excavate a site, they generally make a three-dimensional map of the site, detailing where each artifact is found.
•Precise time reference — Many systems that must be accurately synchronized use GPS as a source of accurate time. GPS can be used as a reference clock for time code generators or NTP clocks. Sensors (for seismology or other monitoring application), can use GPS as a precise time source, so events may be timed accurately. TDMA communications networks often rely on this precise timing to synchronize RF generating equipment, network equipment, and multiplexers.
•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.
•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 games — 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 waypoints.
•Aircraft passengers — Most airlines 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.
•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. 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.
•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.
•Photograph annotation — Combining GPS position data with photographs taken with a (typically digital) camera, allows one to lookup the locations where the photographs were taken in a gazeteer, and automatically annotate the photographs with the name of the location they depict. The GPS device can be integrated into the camera, or the timestamp of a picture's metadata can be combined with a GPS track log.
•Skydiving — Most commercial drop zones use a GPS to aid the pilot to "spot" the plane to the correct position relative to the dropzone that will allow all skydivers on the load to be able to fly their canopies back to the landing area. The "spot" takes into account the number of groups exiting the plane and the upper winds. In areas where skydiving through cloud is permitted the GPS can be the sole visual indicator when spotting in overcast conditions, this is referred to as a "GPS Spot".
The design of GPS is based partly on the similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS system came when the Soviet Union launched the first Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.
The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology the GPS system relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first world-wide radio navigation system.
The first experimental Block-I GPS satellite was launched in February 1978. The GPS satellites were initially manufactured by Rockwell International and are now manufactured by Lockheed Martin.
GPS THE GLOBAL POSITIONING SYSTEM A REVIEW
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