A system of satellites that enables an accurate estimate to be made of the position of a receiving device on or near the earth’s surface. Twenty-four satellites are used in medium earth orbit at an altitude of approximately 20 000 km with their orbits inclined at 55.6° to the equator such that at least four are always visible from any point on earth. The satellites continuously transmit signals towards the earth: a receiving device monitors the difference in arrival time of the signals and thus calculates its position. The accuracy can be better than a metre horizontally, but is less in the vertical direction. See also Beidou; Galileo; GLONASS.