GPS Navigation Systems

The Global Positioning System provides worldwide coverage with four satellites in each of six planes for a total of 24 satellites. The orbits are nearly circular and have an inclination of 55°. The ascending nodes of the six planes are equally spaced around the Equator. An altitude of 20,200 km (12,500 mi) gives the orbits a subsynchronous period such that they repeat their ground track every second orbit. The satellites are three-axis-stabilized by a combination of reaction wheels and magnetic torque applied against the Earth's magnetic field. The satellite is rotated so that the Earth panel with the antennas always faces the Earth and the solar panels are at right angles to the Sun.