This paper is intended as an historical document describing the entry and success of the Decca Radar Company into the production of Doppler-based navigation units. It is based on an original personal paper written in about 1970 by Trevor Gray, then Head of Doppler Department, Decca Radar, updated and edited with additions by Walter Blanchard (past President of the RIN) assisted by Trevor Gray in May 2007. The paper first covers the basic theory and principles of Doppler radar systems, continues through the setting up of the Decca Doppler Department in the mid-1950s and the production of the first transistor-based Doppler system. Success led to the development of both civil and military Doppler systems culminating in the Decca Doppler 70 Series which was fitted to some 30 types of aircraft and helicopters, worldwide.