Global Positioning System

GPS determines a receiver's position by measuring the time it takes for signals from at least four satellites to arrive, using trilateration. Each satellite broadcasts precise timing signals based on onboard atomic clocks, and the receiver calculates its distance to each satellite. The system's space segment consists of a constellation of about 31 operational satellites in medium Earth orbit at an altitude of approximately 20,200 km, arranged in six orbital planes to ensure global coverage. The control segment, operated by the Space Force's 2nd Space Operations Squadron, includes a master control station at Schriever Space Force Base, Colorado, and a network of monitoring and ground antennas worldwide. The user segment encompasses all civilian and military receivers, from smartphones to aircraft navigation systems.

GPS originated from the U.S. Navy's Transit system and the Air Force's 621B project, with the first experimental satellite, Navstar 1, launched in 1978. The system was declared fully operational in 1995, following the deployment of 24 satellites. Initially, civilian signals were intentionally degraded under Selective Availability (SA), which was discontinued in 2000 by presidential order, improving civilian accuracy from about 100 meters to 20 meters. The system has undergone modernization with new signals (L2C, L5) and satellites (GPS III) that enhance accuracy and resilience. The U.S. government maintains GPS as a global public good, providing it free of charge, while also ensuring military superiority through encrypted M-code signals.

GPS has become indispensable across civilian and military domains. In transportation, it enables turn-by-turn navigation, fleet management, and autonomous vehicle testing. In agriculture, precision farming uses GPS for soil sampling and variable-rate irrigation. Emergency services rely on GPS for 911 location and disaster response. The system also underpins global financial networks, as it provides precise time synchronization for stock exchanges and cellular networks. The economic value of GPS is estimated at over $1.4 trillion since its inception, according to a 2019 study by the National Institute of Standards and Technology. However, GPS signals are weak and susceptible to jamming and spoofing, prompting the development of complementary systems like Europe's Galileo and Russia's GLONASS.

Beyond navigation, GPS has surprising niche uses. It is used to study plate tectonics by measuring millimeter-scale ground movements, and to track wildlife migration patterns. The system also aids in atmospheric science by measuring radio occultation, which provides temperature and humidity profiles. A lesser-known fact is that GPS satellites carry nuclear detonation detectors as part of the U.S. Nuclear Detonation Detection System. The first GPS satellite was launched in 1978, but the system was not opened to civilian use until the 1980s. The U.S. government has committed to maintaining GPS for at least 30 more years, with the next-generation GPS IIIF satellites expected to include laser retroreflectors for enhanced ranging. Additionally, GPS time is based on UTC but does not account for leap seconds, which can cause discrepancies over time.

Trilateration

A method of determining position by measuring distances to at least three known points.

Selective Availability

A deliberate degradation of civilian GPS signals that was discontinued in 2000.

GNSS

Global Navigation Satellite Systems, the collective term for satellite navigation systems like GPS, GLONASS, and Galileo.

M-code

A military GPS signal designed for enhanced security and resistance to jamming.