Global Positioning System (GPS)

The Global Positioning System (GPS), the U.S. component of the Global Navigation Satellite System (GNSS). The GPS satellites, at an altitude of 20000 km, transmit down to the Earth carrier signals at two L-band frequencies (1:227 and 1:575 GHz) which are modulated by a pseudo-random noise code. When four satellites are in view, the user has enough information to solve for the station position and the clock offset from GPS time.

Source: IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)