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Global Positioning System

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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 cm (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 positioning, RTK).23

GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic clocks 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.

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 repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137.24

GPS modernization

Having reached the program's requirements for Full Operational Capability (FOC) on July 17, 1995,25 the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to "modernize" the GPS system. Announcements from the Vice Presidential and the White House in 1998 heralded the beginning of these changes, and in 2000 the U.S. Congress reaffirmed the effort; referred to it as GPS III.

The project aims to improve the accuracy and availability for all users and involves new ground stations, new satellites, and four additional navigation signals. New civilian signals are called L2C, L5, and L1C; the new military code is called M-Code. Initial Operational Capability (IOC) of the L2C code is expected in 2008.26

Applications

Military

GPS allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions. 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).27

The GPS satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System.28

Navigation

  • Automobiles can be equipped with GPS receivers at the factory or as after-market equipment. Units often display moving maps and information about location, speed, direction, and nearby streets and landmarks.
  • 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 cockpits are appearing in general aviation aircraft of all sizes, using technologies such as WAAS or Local Area Augmentation System (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 waypoints such as alternate airports or mountain passes, to aid en route decision making for cross-country soaring.
  • Boats and ships 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 Chartplotters using the NMEA 0183 interface. GPS can also improve the security of shipping traffic by enabling 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.
  • 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.
  • Hikers, 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
A modern SiRF Star III chip based 20-channel GPS receiver with WAAS/EGNOS support.
  • Spacecraft are now beginning to use GPS as a navigational tool. The addition of a GPS receiver to a spacecraft allows precise orbit determination without ground tracking. This, in turn, enables autonomous spacecraft navigation, formation flying, and autonomous rendezvous. The use of GPS in MEO, GEO, HEO, and highly elliptical orbits is feasible only if the receiver can acquire and track the much weaker (15 - 20 dB) GPS side-lobe signals. This design constraint, and the radiation environment found in space, prevents the use of COTS receivers.

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 cm 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.

Other uses

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 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.29
  • 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.30 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.3132
  • 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."
  • Marketing-Some market research companies have combined GIS systems and survey based research to help companies to decide where to open new branches, and to target their advertising according to the usage patterns of roads and the socio-demographic attributes of residential zones.

History

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 1978.33 The GPS satellites were initially manufactured by Rockwell International and are now manufactured by Lockheed Martin.

Timeline

  • In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board, U.S. President Ronald Reagan announced that the GPS system would be made available for civilian uses once it was completed.
  • By 1985, ten more experimental Block-I satellites had been launched to validate the concept.
  • On February 14, 1989, the first modern Block-II satellite was launched.
  • In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.
  • By December 1993 the GPS system achieved initial operational capability34
  • 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 directive35 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.
  • On May 2, 2000 "Selective Availability" was discontinued, allowing users outside the US military to receive a full quality signal.
  • In 2004, U.S. President George W. Bush updated the national policy, replacing the executive board with the National Space-Based Positioning, Navigation, and Timing Executive Committee.
  • The most recent launch was on November 17, 2006. The oldest GPS satellite still in operation was launched in August 1991.

Awards

Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:

  • Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).
  • Bradford Parkinson, 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.36

On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious aviation award in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."

Other systems

  • GLONASS (GLObal NAvigation Satellite System) 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 percent of time. There are plans to restore GLONASS to full operation by 2008 with assistance from India.
  • 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 may be developed independently by China.37

Notes

  1. ↑ HowStuffWorks, How GPS Receivers Work. Retrieved May 14, 2006.
  2. ↑ Global Security, GPS. Retrieved June 20, 2007.
  3. ↑ Peter H. Dana, GPS Orbital Planes. Retrieved February 18, 2009.
  4. ↑ Metaresearch, What the Global Positioning System Tells Us about Relativity. Retrieved January 2, 2007.
  5. ↑ USCG Navcen, GPS Frequently Asked Questions. Retrieved January 3, 2007.
  6. ↑ Paul Massatt and Wayne Brady, Optimizing performance through constellation management, Crosslink. Retrieved June 20, 2007.
  7. ↑ US Coast Guard, General GPS News 9-9-05. Retrieved June 20, 2007.
  8. ↑ Britannica Concise Encyclopedia, Ephemeris. Retrieved June 20, 2007.
  9. ↑ USNO, NAVSTAR Global Positioning System. Retrieved May 14, 2006.
  10. ↑ Global Positioning System, Ephemeris and Clock Errors. Retrieved June 13, 2007.
  11. 11.0 11.1 Office of Science and Technology Policy, Presidential statement to stop degrading GPS. Retrieved May 1, 2000.
  12. ↑ GPS, Selective Availability. Retrieved June 13, 2007.
  13. ↑ Chris Rizos, GPS Satellite Signals, Univ. of New South Wales.
  14. ↑ ATI Courses, The Global Positioning System by Robert A. Nelson Via Satellite. Retrieved February 18, 2009.
  15. ↑ Neil Ashby, Relativity and GPS, Physics Today. Retrieved February 18, 2009.
  16. ↑ Space Environment Center, SEC Navigation Systems GPS Page. Retrieved February 18, 2009.
  17. ↑ GPS World, The hunt for an unintentional GPS jammer. Retrieved January 1, 2003.
  18. ↑ Phrack, Low Cost and Portable GPS Jammer. Retrieved June 20, 2007.
  19. ↑ American Forces Press Service, CENTCOM charts progress. Retrieved March 25, 2003.
  20. ↑ John Ruley, GPS jamming. Retrieved February 12, 2003.
  21. ↑ GPS Receivers, Facts for the Warfighter. Retrieved April 10, 2007.
  22. ↑ GPS, GPS Interference and Jamming. Retrieved June 13, 2007.
  23. ↑ GSP, Precise Monitoring. Retrieved June 13, 2007.
  24. ↑ GPS, GPS Time and Date. Retrieved June 13, 2007…
  25. ↑ U.S. Coast Guard, Global Positioning System Fully Operational. Retrieved June 20, 2007.
  26. ↑ GPS, GPS Modernization. Retrieved June 13, 2007.
  27. ↑ Arms Control Association, Missile Technology Control Regime. Retrieved May 17, 2006.
  28. ↑ Sandia National Laboratory, Nonproliferation programs and arms control technology. Retrieved June 20, 2007.
  29. ↑ Joe Mehaffey, Is it Safe to use a handheld GPS Receiver on a Commercial Aircraft? Retrieved February 18, 2009.
  30. ↑ JRC America, JLR-10 GPS Compass. Retrieved Jan. 6, 2007.
  31. ↑ Diomidis Spinellis, Position-annotated photographs: A geotemporal web. Retrieved February 18, 2009.
  32. ↑ K. Iwasaki, K. Yamazawa, and N. Yokoya, An indexing system for photos based on shooting position and orientation with geographic database. Retrieved June 20, 2007.
  33. ↑ Hydrographic Society Journal, Developments in Global Navigation Satellite Systems. Retrieved April 5, 2007.
  34. ↑ United States Department of Defense, Announcement of Initial Operational Capability. Retrieved February 18, 2009.
  35. ↑ National Archives and Records Administration, U.S. GLOBAL POSITIONING SYSTEM POLICY. Retrieved February 18, 2009.
  36. ↑ United States Naval Research Laboratory, National Medal of Technology for GPS. Retrieved February 18, 2009.
  37. ↑ The Inquirer, Chinese threaten to dump Galileo GPS. Retrieved June 20, 2007.

References

  • Brain, Marshall, and Tom Harris. How GPS Receivers Work. HowStuffWorks.com. Retrieved May 14, 2006.
  • Dana, Peter H. "Global Positioning System Overview." The Geographer's Craft Project. Department of Geography, The University of Colorado at Boulder, 1999. Retrieved May 28, 2007.
  • Nelson, Robert A. The Global Positioning System. Applied Technology Institute, 1999. Retrieved May 28, 2007.

External links

All links retrieved June 23, 2017.

  • GPS.gov-General public education website created by the U.S. Government.
  • U.S. Naval Observatory's GPS constellation status.
  • HowStuffWorks' Simplified explanation of GPS.
  • Trimble's Online GPS Tutorial.
  • Dana, Peter H. "Global Positioning System Overview".
  • Satellite Navigation: GPS & Galileo (PDF)-16-page paper about the history and working of GPS, touching on the upcoming Galileo.
  • Navstar, including information about each satellite's configuration and launch.
  • GPS Weapon Guidance Techniques.
  • GPS Anti-Jam Protection Techniques.

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