When there is relative movement between an observer and a wave source the observed frequency changes by an amount depending on the relative velocity. This is the doppler effect, and it has been realized for a number of years that it might be the means of measuring the speed of an aircraft over the ground, using radio waves transmitted obliquely and received again at the aircraft after being scattered at the surface of the Earth. By 1937 radio techniques had developed so far that letters patent were granted for systems which then appeared to be practical. A number of severe problems remained, however, and not until the close of the war did development programmes to resolve these problems make headway. Since that time equipments have been successfully produced, both here and in the United States of America, and sufficient operational experience has been gained for the value of the process as an aid to navigation to be assessed.