Global Positioning System From Wikipedia, the free encyclopedia Jump to: to: navigation navigation,, search search "GPS" redirects here. For other uses, see see GPS (disambiguation). (disambiguation). Geodesy
Fig. Earth Earth
Geodetic system system
system Geog. coord. system
represent. Hor. pos. represent.
Lat. Long. Lat. / Long.
Map proj. proj.
Ref. ellipsoid ellipsoid
Satellite geodesy geodesy
system Spatial ref. system
80 GRS 80
geodesy History of geodesy NAVD29 NAVD29
Artist's conception of GPS Block II-F satellite in Earth orbit.
Civilian GPS receiver (" ( "GPS navigation device" device") in a marine application.
system in a taxicab. Automotive navigation system
GPS receivers GPS receivers are now integrated in many many mobile phones. phones.
U.S. Air Force chec klist during Global Positioning System Force Senior Airman runs through a checklist satellite operations. The Global Positioning System (GPS) is a space-based space-based satellite navigation navigation system that provides location and time information in all weather conditions, anywhere an ywhere on or near the Earth where there is an unobstructed line of sight to four or o r more GPS satellites. The system provides critical capabilities to military, civil and commercial users around the world. It is maintained by the United States government and is freely accessible to anyone with a a GPS receiver . The GPS project was developed in 1973 to overcome the limitations of previous navigation  systems,, integrating ideas from several predecessors, including a number of classified systems engineering design studies from the 1960s. GPS was created and realized by the the U.S. Department of Defense Defense (DoD) and was originally run with 24 satellites. It became fully operational in 1994. 1994. Roger L. Easton Easton is generally credited as its inventor.
Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next  Generation Operational Control System (OCX). (OCX). Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the modernization effort, GPS III. In addition to GPS, other systems are in use or developed under development. The Russian Global ) was contemporaneously with GPS, but Navigation Satellite System (GLONASS) GLONASS suffered from incomplete coverage of the globe until the mid-2000s. There are also the planned European Union Union Galileo positioning system, system, Chinese Chinese Compass navigation system system,, and and Indian Regional Navigational Satellite System. System.
1 History History 1.1 Predecessors o Predecessors 1.2 Development Development o 1.3 Timeline and modernization o modernization
Awards Awards 2 Basic1.4 concept of GPS GPS Structure 3 Structure 3.1 Space segment o segment 3.2 Control segment o segment segment 3.3 User segment o Applications 4 Applications Civilian o 4.1 Civilian use 4.1.1 Restrictions on civilian use Military 4.2 Military o 5 Communication Communication 5.1 Message format o format 5.2 Satellite frequencies frequencies o
o decoding 5.3 Demodulation and decoding equations 6 Navigation equations 6.1 Bancroft's method method o 6.2 Trilateration o Trilateration 6.3 Multidimensional Newton-Raphson calculations o calculations 6.4 Additional methods for more than four satellites o satellites 7 Error sources and analysis analysis surveying 8 Accuracy enhancement and surveying 8.1 Augmentation o Augmentation 8.2 Precise monitoring monitoring o 8.3 Timekeeping o 8.3.1 Leap seconds seconds 8.3.2 Accuracy Accuracy Format 8.3.3 Format o (surveying) 8.4 Carrier phase tracking (surveying)
9 Regulatory spectrum issues concerning GPS receivers receivers 10 Other systems systems 11 See also also Notes 12 Notes References 13 References 14 Further reading reading links 15 External links
History The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the the Decca Navigator , developed in the early 1940s and used during during World War II II.. LORAN
In 1956, the German-American physicist physicist Friedwardt Winterberg proposed proposed a test of general relativity (for time slowing in a strong relativity strong gravitational gravitational field) using accurate accurate atomic clocks clocks placed placed in orbit inside artificial satellites. (To achieve accuracy requirements, GPS uses principles of  general relativity to correct the satellites' atomic clocks. clocks. ) Additional inspiration for GPS came satellite, Sputnik , in 1957. Two American when the the Soviet Union Union launched the first man-made satellite, physicists, William Guier and George Weiffenbach, Weiffenbach, at Johns Hopkins's Hopkins's Applied Physics  Laboratory (APL), decided to monitor Sputnik's radio transmissions. Laboratory transmissions. Within hours they realized that, because of the the Doppler effect, effect, they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their UNIVAC UNIVAC to do the heavy calculations required. The next spring, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem — pinpointing the user's location given that of the satellite. (The Navy was developing the submarine-launched submarine-launched Polaris Polaris missile, which required  system.. them to know the submarine's location.) This led them and APL to develop the the Transit Transit system  ] [8 DARPA in 1972) also played a role in Transit Transit.. In 1959, ARPA (renamed (renamed DARPA
Official logo Emblem of for the 50th the NAVSTAR Space Wing Wing GPS The first satellite navigation system, system, Transit, Transit, used by the the United States Navy, Navy, was first successfully tested in 1960. It used a constellation of o f five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the the Timation Timation satellite that proved the ability to place accurate clocks in space, a technology technolog y required by GPS. In the 1970s, the ground-based ground-based Omega Navigation System, System, based on phase comparison of signal
became the first worldwide radio navigation system. transmission from pairs of stations, stations, became Limitations of these systems drove the need for a more universal navigation solution with greater accuracy. While there were wide needs for accurate navigation n avigation in military and civilian sectors, almost none of those was seen as justification for the billions of dollars it would cost in research, development, deployment, and operation constellation navigation satellites. thethat Cold War arms race, race, the nuclear threat to for the aexistence of theofUnited States was the During one need did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded. It is also the reason for the ultra secrecy at that time. The The nuclear triad triad consisted of missiles (SLBMs) along with with United the United States Navy's Navy's submarine-launched ballistic missiles States Air Force Force (USAF) strategic bombers and and intercontinental ballistic missiles missiles (ICBMs). Considered vital to the nuclear-deterrence posture, accurate determination det ermination of the SLBM launch position was a a force multiplier . Precise navigation would enable United States States submarines submarines to get an accurate fix of their  positions before they launched their SLBMs. SLBMs. The USAF, with two thirds of the nuclear triad, also had requirements for a more accurate and reliable navigation system. The Navy and Air Force were developing their own technologies technolo gies in parallel to solve what was essentially the same problem. To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (such as Russian Russian SS-24 SS-24 and and SS-25) SS-25) and so the need to fix the launch position had similarity to the SLBM situation. In 1960, the Air Force proposed prop osed a radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D 3-D LORAN. LORAN. A follow-on study, Project 57, was worked in 1963 and it was "in this study that the GPS concept was born". That same year, the concept was pursued as Project 621B, which had "many of the attributes that you now see in  GPS"" and promised increased accuracy for Air Force bombers as well as ICBMs. Updates GPS from the Navy Transit system were too slow for the high speeds sp eeds of Air Force operation. The Navy Research Laboratory continued advancements with their Timation (Time Navigation) satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clock  into orbit. orbit. Another important predecessor to GPS came from a different branch of the United States military. In 1964, the the United States Army Army orbited its first Sequential Collation of Range (SECOR ) satellite used for geodetic surveying. The SECOR ssystem ystem included three ground-based transmitters from known locations that would send signals to the satellite transponder in orbit. o rbit. A fourth ground-based station, at an undetermined position, could c ould then use those signals to fix its  location precisely. The last SECOR satellite was launched in 1969 1 969.. Decades later, during the early years of GPS, civilian surveying became one of the first fields to make use of the new technology, because surveyors could reap benefits of signals from the less-than-complete GPS constellation years before it was declared operational. GPS can be thought of as an evolution of the SECOR system where the ground-based transmitters have been migrated into orbit.
With these parallel developments in the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies techn ologies from 621B, Transit, Timation, and SECOR in a multi-service program. During Labor Day weekend in 1973, a meeting of about 12 military officers at the Pentagon discussed the creation of a Defense a Defense Navigation Satellite System (DNSS). (DNSS). It was at this meeting that "the real synthesis that became GPS was created." Later that year, DNSS program was named named Navstar Navstar . With the individual satellites being associated with thethe name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify  the constellation of Navstar satellites, Navstar-GPS satellites, Navstar-GPS , which was later shortened simply to GPS. GPS . Boeing 747 747 carrying 269 people, was shot down in 1983 After Korean Air Lines Flight 007, 007 , a Boeing  after straying into the USSR's USSR's prohibited prohibited airspace, airspace, in the vicinity of Sakhalin Sakhalin and and Moneron Islands,, President Islands President Ronald Reagan Reagan issued a directive making GPS freely available for civilian use,  once it was sufficiently developed, as a common good. good. The first satellite was launched in 1989, and the 24th satellite was launched in 1994. 1994. Roger L. Easton Easton is widely credited as the primary inventor of GPS. Initially, the highest quality signal was reserved for military use, and the signal sig nal available for Availability)). This changed with President President Bill civilian use was intentionally degraded (Selective Availability Clinton ordering Selective Availability to be turned off at midnight May 1, 2000, improving the Clinton precision of civilian GPS from 100 meters (330 ft) to 20 meters (66 ft). ft). The executive order signed in 1996 to turn off Selective Availability in 2000 was proposed by the U.S. Secretary of Defense, William Perry, Defense, Perry, because of the widespread growth of differential GPS GPS services to improve civilian accuracy and eliminate the U.S. military advantage. Moreover, the U.S. military was actively developing technologies to deny den y GPS service to potential adversaries on a regional  basis.. basis Over the last decade, the U.S. has implemented several improvements to the GPS service, including new signals for civil use and increased accuracy and integrity for all users, all while maintaining compatibility with existing GPS equipment. GPS modernization has now become an ongoing initiative to upgrade the Global Positioning System with new capabilities to meet growing military, civil, and commercial needs. The program is being implemented through a series of satellite acquisitions, acquisitions, including GPS Block III and the Next Generation Operational Control System S ystem (OCX). The U.S. Government continues to improve the GPS space and ground segments to increase performance and accuracy. GPS is owned and operated by the United States Government as a national resource. Department of Defense (DoD) is the steward of GPS. Interagency GPS. Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from 1996 to 2004. After that the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems. The executive committee is chaired jointly by b y the deputy secretaries of defense and Itshomeland membership includes officials from theComponents departmentsofof the state,transportation. commerce, and security, the equivalent-level joint chiefs of staff, and NASA.
executive office of the president participate as observers to the executive committee, and the FCC chairman participates as a liaison. The DoD is required by law to "maintain a Standard Positioning Service (as defined in the federal radio navigation plan and the standard positioning service signal specification) that will be available on a continuous, worldwide basis," and "develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses."
Timeline and moderniz modernization ation Main article: article: List of GPS satellite launches launches Summary of satellites satellites Satellite launches Block
Launch Currently in orbit Period Suc- Fail- In prep- Planand healthy cess ure aration ned
1978 – 1985 1985 10
1989 – 1990 1990 9
1997 19 IIA 1990 – 1997
2004 12 IIR 1997 – 2004
2009 8 IIR-M 2005 – 2009
From 2010 3
IIIA From 2014 0
(Last update: October 8, 2012) PRN 01 from Block IIR-M is unhealthy PRN 25 from Block IIA is unhealthy PRN 32 from Block IIA is unhealthy PRN 27 from Block IIA is unhealthy  For
a more complete list, see see list of GPS satellite launches launches
In 1972, the USAF Central Inertial Guidance Test Facilit Facility y (Holloman AFB), conducted developmental flight tests of two prototype GPS receivers over White Sands Missile [citation needed ] Range,, using ground-based pseudo-satellites. Range In 1978, the first experimental Block-I GPS satellite was launched. In 1983, after Soviet Soviet interceptor aircraft aircraft shot down the civilian airliner KAL 007 007 that airspace because because of navigational errors, killing all 269 people on strayed into into prohibited prohibited airspace
 Reagan that GPSbeen would be made published available for board, President Ronald Reagan although it had previously [in civilianU.S. usesPresident once it was completed completed, , announced Navigation magazine] that the CA code (Coarse Acquisition code) would be available to civilian users. By 1985, ten more experimental Block-I satellites had been launched to validate the concept. Command & Control of these satellites had moved from Onizuka AFS, CA and turned over to the 2nd Satellite Sa tellite Control Squadron (2SCS) located at Falcon Air Force  Station in Colorado Springs, Colorado. Colorado. On February 14, 1989, the first modern Block-II satellite was launched.  used.. The Gulf War from 1990 to 1991, was the first conflict where GPS was widely used The In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the the 50th Space Wing. Wing. By December 1993, GPS achieved initial operational capability (IOC), indicating a full
constellation (24 satellites) was available and providing the Standard Positioning Service  (SPS).. (SPS) Full Operational Capability (FOC) was declared by by Air Force Space Command Command (AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning  Service (PPS). (PPS). In 1996, recognizing the importance of GPS to civilian users as well as military users,  dual-use U.S. President President Bill Clinton Clinton issued a policy directive declaring GPS to be aa dual-use system and establishing an an Interagency GPS Executive Board Board to manage it as a national asset. In 1998, United States Vice President President Al Gore Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and a nd reliability, particularly with respect to aviation safety and in 2000 the the United States Congress Congress authorized the effort, referring to it as as GPS III . On May 2, 2000 "Selective Availability" was discontinued as a result of the 19 1996 96 executive order, allowing users to receive a non-degraded non -degraded signal globally. In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europe's planned planned Galileo system.. system In 2004, United States President President George W. Bush Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based  Positioning, Navigation, and Timing. Timing. November 2004, 2004, Qualcomm Qualcomm announced successful tests of assisted GPS GPS for mobile  phones.. phones In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance. On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the new Architecture Evolution Plan. Plan .
Office issued a report On May 19, 2009, the United States States Government Accountability Office  warning that some GPS satellites could fail as soon as 2010 20 10.. On May 21, 2009, the the Air Force Space Command Command allayed fears of GPS failure saying  "There's only a small risk we will not continue to exceed our performance standard." On January 11, 2010, an update of ground control systems caused a software incompatibility with 8000 to 10000 military receivers manufactured by a division of
 Trimble Navigation Limited Sunnyvale, Calif . the contract to develop the GPS On February 25, 2010, 2010 , theof U.S. Air Force awarded Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.  A GPS satellite was launched on May 28, 2010. 2010. The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on December  10, 1990. 1990. The GPS satellite, GPS IIF-2, was launched on July 16, 2011 at 06:41 GMT from from Space  Launch Complex 37B 37B at the the Cape Canaveral Air Force Station Station.. The GPS satellite, GPS IIF-3, was launched on October 4, 2012 at 12:10 GMT from  Space Launch Complex 37B 37B at the the Cape Canaveral Air Force Station. Station.
Awards On February 10, 1993, the the National National Aeronautic Association Association selected the GPS Team as winners Trophy,, the nation's most prestigious aviation award. This team of the 1992 1992 Robert J. Collier Trophy combines researchers from the the Naval Naval Research Laboratory Laboratory,, the USAF, the the Aerospace Corporation,, Rockwell International Corporation Corporation Corporation,, and and IBM IBM Federal Systems Company. The citation honors them "for the most significant development for safe and efficient navigation and radio navigation 50 years ago." surveillance of air and spacecraft since the introduction of radio Two GPS developers received the the National National Academy of Engineering Engineering Charles Stark Draper Prize Prize for 2003:
Ivan Getting, Corporation and an engineer at the Getting, emeritus president of The Aerospace Corporation Technology,, established the basis for GPS, improving on the Massachusetts Institute of Technology World War II II land-based radio system called called LORAN LORAN ( Long-range Long-range R Radio adio A Aid id to N avigation). avigation). Parkinson,, professor of aeronautics aeronautics and and astronautics astronautics at at Stanford University University,, Bradford Parkinson conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinson served twenty-one years in the Air Force, from 1957 to 1978, and retired with the rank of colonel. the National National Medal of Technology Technology on February GPS developer Roger L. Easton Easton received the  13, 2006. 2006.
In 1998, GPS technology was inducted into the the Space Foundation Foundation Space Technology Hall of  Fame.. Fame Francis Kane (Col. USAF, ret.) was inducted into the U.S. AirMarch Force2, Space Kane PioneersX. Hall of Fame at Lackland A.F.B., San Antonio, Texas, 2010and for Missile his role in
space technology development and the engineering design concept of GPS conducted as part of Project 621B. On October 4, 2011, the the International Astronautical Federation Federation (IAF) awarded the Global Positioning System (GPS) its 60th Anniversary Award, nominated by IAF member, the American Institute for Aeronautics and Astronautics (AIAA). The IAF Honors and Awards Committee recognized the uniqueness for of the program and the in building international collaboration theGPS benefit of humanity. humanit y. exemplary role it has played
Basic concept of GPS A GPS receiver calculates its position by precisely timing the signals sent by GPS GPS satellites satellites high above the Earth. Each satellite continually transmits messages that include
the time the message was transmitted satellite position at time of message transmission
The receiver uses the messages it receives to determine d etermine the transit time of each message and computes the distance to each satellite using the speed of light. Each of these distances and satellites' locations define a sphere. The receiver is on the surface of each of o f these spheres when the distances and the satellites' locations are correct. These distances and satellites' locations are used to compute the location of the receiver using the the navigation equations equations.. This location is then and longitude longitude;; elevation information displayed, perhaps with a moving map display or latitude latitude and may be included. Many GPS units show derived information such as direction and speed, calculated from position changes. In typical GPS operation, four or more satellites must be visible to obtain an accurate result. Four [a] sphere surfaces typically do not intersect. intersect. Because of this, it can be said with confidence that when the navigation equations are solved to find an intersection, this solution gives the p position osition of the receiver along with the difference between the time kept by the receiver's on-board clock and the true time-of-day, thereby eliminating the need for a very large, expensive, and power hungry clock. The very accurately computed time is used only for display or not at all in many GPS applications, which use only the location. A number of applications for GPS do make use of this cheap and highly accurate timing. These include include time transfer , traffic signal timing, and stations . synchronization of cell phone base stations. Although four satellites are required for normal operation, fewer apply appl y in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional reckoning, inertial navigation, navigation, clues or assumptions such as reusing the last known kno wn altitude, altitude, dead reckoning, or including information from the vehicle computer, to give a (possibly degraded) position when  ] [43 fewer than four satellites are visible. visible.