A radio-navigation system used by aircraft. A radio beam transmitted from the aircraft and reflected back by various landscape features is received at the aircraft. Because the aircraft transmitter and receiver are in motion, the signal undergoes a frequency shift (the Doppler effect) which is measured and converted to aircraft velocity. A computer on the aircraft determines the plane’s position by integrating the velocity over time. Actually, two transmitter-receivers beam to the ground ahead of and behind the aircraft to determine the forward speed and two transmitters beam to the right and left of the aircraft to determine the crosstrack speed. The arrangement is similar to that used with Doppler sonar (Figure D-21b). The Doppler data combined with a compass heading give the direction of the aircraft. Velocity can be measured to about 0.5%, somewhat less over water because the surface of the water may itself be in motion.