The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system, which is a type of Global Navigation Satellite System (GNSS), owned by the United States Space Force and operated by Mission Delta 31. The project was launched by the U.S. Department of Defense in 1973 to create a universal navigation solution with greater accuracy than previous systems. The full constellation of 24 satellites became operational in 1993. Civilian use was authorized by President Ronald Reagan in 1983 after the shoot-down of Korean Air Lines Flight 007, which had strayed off course due to navigational errors.
The system works using a constellation of at least 24 operational satellites, each carrying atomic clocks to provide extremely accurate time. A GPS receiver calculates its position using a technique called trilateration by measuring the time it takes to receive signals from multiple satellites. To determine a three-dimensional position (latitude, longitude, and altitude), the receiver must lock onto the signals of at least four satellites.
A key related concept for an Indian reader is India's regional satellite navigation system, NavIC (Navigation with Indian Constellation), formerly known as IRNSS, developed by the Indian Space Research Organisation (ISRO). NavIC provides coverage over India and up to 1500 km around it, and using both GPS and NavIC together significantly improves accuracy and signal reliability in the Indian subcontinent. A major change in the system's history was the termination of Selective Availability (SA), a deliberate error introduced for civilian users, by President Clinton on May 1, 2000, which dramatically increased civilian GPS accuracy. Modernization continues with the launch of the GPS III satellites, the first of which was launched in 2018.