ABSTRACT
In the Doppler system of aircraft navigation two beams of radio waves are aimed at the ground from transmitters under the fuselage, the forward beam striking the ground slightly ahead of the aircraft and the back beam slightly astern; both alternate twice each second from port to starboard. The forward beam signals reflected back to the aircraft are increased in frequency in proportion to its speed over the ground, the back beam signals decrease. The difference in frequency is measured automatically to give the navigator an accurate indication of the speed of the aircraft and of the drift angle so that the course can be corrected accordingly. This in turn has led to the development of automatic flying and landing devices. The first pilot-less aircraft successfully completed an Atlantic crossing as long ago as 1947.
