Plain English Meaning

A way for an aircraft to figure out how fast it's moving over the ground and which way it's drifting, by bouncing radar beams off the earth below and measuring how the signal's pitch shifts due to the aircraft's motion.

Full Definition

A navigation technique that uses the Doppler effect — the apparent change in frequency of a signal caused by relative motion between source and receiver — to measure an aircraft's groundspeed and drift angle. By transmitting radar beams toward the ground and measuring the frequency shift of the returning signals, a Doppler navigation system continuously calculates the aircraft's over the earth without needing external ground stations.