GPS

Global Positioning System can give a very precise position of the vehicle based on a constellation of satellites orbiting the Earth. Inexpensive systems can give position within a few meters. With higher quality systems and corrections to the satellite signals, position can be determined to about 0.1 m (4 in).

INS

Inertial Navigation Systems have been used for years for vehicle navigation before the advent of GPS. INS uses inertial cues from sensors like accelerometers, speedometers and gyros to determine the vehicle position. With a good initial position (available from GPS), a well designed INS can keep very accurate position for a short time with loss of GPS coverage due to terrain or other environmental factors.