GPS, Global Positioing System, is managed and developed by the United States and it was originally intended for military applications. For a long time, the extensive use of technology and the mass market for GPS have made it increasingly important to guarantee also civilian applications.

U.S. satellite system

The development of GPS started already in the 1970s. The system is owned by the government of the United States and it is managed by the armed forces (US Air Force).

The normal satellite constellation of GPS means that at least 24 satellites are in operation, which was achieved in 1993. For a long time there have been 30–31 active satellites, evenly distributed over six separate orbital planes about 20,180 kilometres above the Earth's surface. The satellite orbits intersect the Earth's equatorial plane at an angle of 55 degrees (the inclination).

Satellite signals and modernisation

All GPS satellites transmit radio signals on two different frequencies, L1 and L2, both located in the L-band within the ultra-high frequency spectrum (around 1–2 GHz). New generations of satellites have brought modernisations such as improved lifespan, power supply and signal quality, as well as the transmission of new signals.

A signal called L2C allows for an improved access of the L2 signal and is available from all satellites launched since 2005. All satellites since 2010 have a brand new signal called L5 and all satellites since 2018 also have a modern access of L1 via the L1C signal.

The signals are modulated with satellite-specific codes, which allows a GPS receiver to quickly identify the satellite through Code-division multiple access (CDMA).

The GPS satellites are controlled and adjusted through approximately 17 ground-based control stations. The control stations track the satellites and can correct their orbits if necessary. The control stations also control which information the GPS satellites transmit in the so-called satellite message. The most important information in this message is the ephemerides of the satellites, which are given in the geodetic reference frame WGS 84. WGS 84 is since many years connected to ITRF. The realisation of WGS 84 that came in 2024, and is called WGS 84 (G2296), is connected to ITRF2020.

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