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This paper, which was presented at an Ordinary Meeting of the Institute in London on 21 November 1979, with S. Ratcliffe in the Chair, traces the evolution of airborne doppler navigation systems over the past two decades and discusses the treatment of errors introduced when flying over water. Both authors are with Decca Radar Ltd.The object of this paper is to describe briefly the major developments which have taken place in doppler navigation systems during the past 15 years or so, to indicate the current state of the art and to examine in some detail the behaviour of doppler systems when flying over water. The year 1960 is taken as a starting point since by that time commercial doppler systems were established. A typical general purpose system of that generation (Fig. 1) consisted of the following units:1. An antenna, probably mechanically stabilized.2. A transmitter/receiver using either pulse or FMCW modulation.3. A tracking system.4. An analog computer.5. A display of ground-speed and drift-angle.6. A display of present position.

i. WEATHER RADARS. The presence of heavy rain and hailstones in cumulo-nimbus clouds is a natural phenomenon which is of vital importance to people who fly in aircraft. With a suitable radar fitted in the nose of an aircraft, this precipitation can be detected and the information presented to the pilot so that he can steer the aircraft to avoid the dangerous turbulent conditions associated with cumulo-nimbus clouds. This is, of course, the basis of weather radars which have been used for many years, and are now regarded as more or less essential for civil aircraft.In addition to detecting cumulo-nimbus clouds it is usual for these equipments to have a secondary role of ground mapping. The quality of the ground radar map presented on the plan position indicator (PPI) is determined by the aerial beamwidth. Installation space in the nose of an aircraft and frequency considerations prevent this being much less than 4°. As a result the resolution is not very good and its use as a navigation aid over land is very limited. However, it is quite effective in defining land/water boundaries and after a long sea crossing it enables an accurate fix to be made.A further role which is sometimes adopted is the measurement of drift angle. This is achieved by the addition of quite a small unit and over land drift angle can be measured to within i °. Over water, however, this simple scheme does not work.The basic design of the weather radar equipments now in use was laid down many years ago; since that time there has been a number of developments in the military airborne radar field which could have applications in civil aircraft. The purpose of this paper is to review the various functions it is now possible to perform with a radar fitted in the nose of an aircraft, allocate priorities and conclude what type should be developed to meet future civil requirements.2. POSSIBLE FUNCTIONS OF AN AIRCRAFT RADAR. The main basic feature of a radar is its ability to measure the range, in a given direction, of anything that reflects sufficient of the incident energy. For an aircraft radar the most common reflectors of energy are cumulo-nimbus clouds, terrain and other aircraft. Another basic feature is the ability to distinguish between various types of reflecting surfaces by the amplitude of the signals. This enables radar ground maps to be presented as an aid to navigation. From these two basic features it is possible to perform a number of different functions and these are listed in Table i. Also