What is GPS ?

The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.

The GPS project was launched by the U.S. Department of Defense in 1973 for use by the United States military and became fully operational in 1995. It was allowed for civilian use in the 1980s

Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System (OCX).

Announcements from Vice President Al Gore and the White House in 1998 initiated these changes

In 2000, the U.S. Congress authorized the modernization effort, GPS III. During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability"; this was discontinued in May 2000 by a law signed by President Bill Clinton.

The GPS system is provided by the United States government, which can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War, or degrade the service at any time

As a result, several countries have developed or are in the process of setting up other global or regional satellite navigation systems.

The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s.

GLONASS can be added to GPS devices, making more satellites available and enabling positions to be fixed more quickly and accurately, to within two meters (6.6 ft).

China's BeiDou Navigation Satellite System is due to achieve global reach in 2020

There are also the European Union Galileo positioning system, and India's NAVIC

Japan's Quasi-Zenith Satellite System (QZSS) is a GPS satellite-based augmentation system to enhance GPS's accuracy.

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

The latest stage of accuracy enhancement uses the L5 band and is now fully deployed. GPS receivers released in 2018 that use the L5 band can have much higher accuracy, pinpointing to within 30 centimetres or 11.8 inches.

The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems,[12] integrating ideas from several predecessors, including classified engineering design studies from the 1960s

The U.S. Department of Defense developed the system, which originally used 24 satellites

It was initially developed for use by the United States military and became fully operational in 1995

Civilian use was allowed from the 1980s. Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of The Aerospace Corporation, and Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it

The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s.

Friedwardt Winterberg proposed a test of general relativity – detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites

Special and general relativity predict that the clocks on the GPS satellites would be seen by the Earth's observers to run 38 microseconds faster per day than the clocks on the Earth.

There are a total of 31 satellites, including 2 reserve satellites, out of which at least 24 GPS satellites orbiting the earth. This means that each GPS satellite orbits the earth in every 12 hours.

The GPS calculated positions would quickly drift into error, accumulating to 10 kilometers per day (6 mi/d). This was corrected for in the design of GPS.

The basic idea of Geo Positioning System or GPS was based on LORAN and Decca Navigation System, which were used during World War II to help in the navigation of ships and planes at a long range.

Winterberg, Friedwardt (1956). “Relativistische Zeitdilatation eines künstlichen Satelliten (Relativistic time dilation of an artificial satellite)”