The Doppler velocity and drift angle measuring device measures the parameters of aircraft vector of velocity: ground speed, i.e. speed relative to the earth's surface and drift angle - the angle between the direction of the longitudinal axis of aircraft and the actual direction of motion.
In Doppler measuring device the oblique irradiation from earth's surface is applied; it defines the parameters of the velocity vector for the frequency spectrum of the signal reflected by the ground. Due to Doppler frequency shift occurs between the emitted and reflected signals. To improve the accuracy Doppler measurement device emits not one, but three or four beams in different directions. Measurement error is typically less than 0.5% of the velocity and 0,2 ° drift angle. The frequency of the emitted signal is 13325 ± 75 MHz.
The system includes an antenna, receiver and computer that measures and calculates the frequency shift followed by ground speed and drift angle.
Since using GNSS make possible to measure ground speed, so Doppler measurement device does not use in avionics equipment of modern aircraft.