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Global Positioning System (GPS) is a satellite-based radio navigation system owned and operated by the United States Space Force, providing geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.

GPS does not require the user to transmit any data and operates independently of any telephonic or Internet reception, though these technologies can enhance the usefulness of the GPS positioning information.

Originally limited to use by the United States military, civilian use was allowed from the 1980s following an executive order from President Ronald Reagan after the Korean Air Lines Flight 007 disaster.

GPS positional accuracy was degraded by the United States government by a program called Selective Availability, which could selectively degrade or deny access to the system at any time, as happened to the Indian military in 1999 during the Kargil War.

When selective availability was lifted in 2000, GPS had about a five-meter (16 ft) accuracy.

GPS receivers that use the L5 band have much higher accuracy, pinpointing to within 30 centimeters (11.8 in).

Consumer devices, like smartphones, can be as accurate as to within 4.9 m (or better with assistive services like Wi-Fi positioning also enabled).

As of May 2021, 16 GPS satellites are broadcasting L5 signals, and the signals are considered pre-operational, scheduled to reach 24 satellites by approximately 2027.

There are other global or regional satellite navigation systems, including Russia's GLONASS, China's BeiDou Navigation Satellite System, the European Union Galileo navigation satellite system, and India's NavIC.

The U.S. Department of Defense is required by law to "maintain a Standard Positioning Service 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".

GPS is owned and operated by the United States government as a national resource.GPS: Timeline, Modernization, and Principles

Timeline and modernization:

• GPS Team won the prestigious 1992 Robert J. Collier Trophy for significant development in navigation and surveillance of air and spacecraft.
• Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003.
• Francis X. Kane was inducted into the U.S. Air Force Space and Missile Pioneers Hall of Fame in 2010.
• GPS technology was inducted into the Space Foundation Space Technology Hall of Fame in 1998.
• The International Astronautical Federation (IAF) awarded the GPS its 60th Anniversary Award in 2011.
• Gladys West was inducted into the Air Force Space and Missile Pioneers Hall of Fame in 2018.
• Four founding members of the GPS project were awarded the Queen Elizabeth Prize for Engineering in 2019.

Principles:

• GPS satellites carry atomic clocks that are synchronized with one another and with reference atomic clocks at ground control stations.
• Time delay between transmission of a signal and its reception by a ground station is proportional to the distance from the satellite to the ground station.
• GPS receiver calculates its own four-dimensional position in spacetime using data received from multiple GPS satellites.
• GPS receiver measures the time of arrival (TOA) of four satellite signals and computes its three-dimensional position and clock deviation from the four time of flight (TOF) values.
• The receiver's Earth-centered solution location is converted to latitude, longitude, and height relative to an ellipsoidal Earth model.
• The user location is not at the intersection of three spheres but at the point where three hyperboloids intersect.
• GPS units use measurements of the Doppler shift of the signals received to compute velocity accurately.
• GPS is used for non-navigation applications such as time transfer, traffic signal timing, and synchronization of cell phone base stations.

Structure:

• GPS consists of three major segments: the space segment, control segment, and user segment.

• The space segment is composed of 24 to 32 satellites in medium Earth orbit, arranged in six orbital planes with four satellites each.

• The control segment is composed of a master control station, an alternate master control station, and a number of dedicated and shared ground antennas and monitor stations.

• The user segment is composed of U.S. and allied military users of the secure GPS Precise Positioning Service and civil, commercial, and scientific users of the Standard Positioning Service.Overview of GPS Technology

• GPS receivers can receive differential corrections using the RTCM SC-104 format, but the accuracy of this signal is limited.

• Many GPS receivers can relay position data using the NMEA 0183 protocol or other proprietary protocols.

• GPS has civilian applications, including accurate timekeeping for everyday activities such as banking and mobile phone operations, as well as scientific uses, tracking, and surveillance.

• The U.S. government controls the export of some civilian receivers, including those capable of functioning above 60,000 ft or designed or modified for use with unmanned missiles and aircraft, which are classified as munitions.

• Military GPS applications include navigation, tracking, and surveillance.

• GPS signals are vulnerable to jamming for military purposes, and this threat continues to grow.

• GPS time is not corrected to match the rotation of the Earth, so it does not contain new leap seconds or other corrections that are periodically added to UTC.

• GPS time is theoretically accurate to about 14 nanoseconds, but most receivers are only accurate to about 100 nanoseconds.

• The GPS implements two major corrections to its time signals for relativistic effects: one for relative velocity of satellite and receiver, and one for the difference in gravitational potential between satellite and receiver.

• The GPS date is expressed as a week number and a seconds-into-week number, and the week number becomes zero again every 1,024 weeks.

• The navigational signals transmitted by GPS satellites encode a variety of information, including satellite positions, the state of the internal clocks, and the health of the network.

• All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal), and the system uses two distinct CDMA encoding types: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P(Y)) code, which is encrypted for military use.GPS Navigation Equations and Geometrical Interpretations

• GPS receivers use messages from satellites to determine satellite positions and time sent.

• The receiver uses at least four satellites to determine its position and clock bias.

• The geometric distance or range between the receiver and satellite i is designated as di.

• Pseudoranges are biased versions of the true range and are calculated as (ti-si)c.

• The GPS equations can be solved by algebraic or numerical methods.

• The amount of error in the results varies with the received satellites' locations in the sky.

• Receivers calculate a running estimate of the error in the calculated position.

• The receiver location is expressed in a specific coordinate system, such as latitude and longitude.

• The GPS equations can be solved using geometrical interpretations.

• The measured ranges represent the radii of spheres, each centered on one of the transmitting satellites.

• The receiver position can be interpreted as the center of an inscribed sphere.

• The clock in the receiver is usually not of the same quality as the ones in the satellites.