A low-power c.w. Doppler navigation aid is described which has been designed for use in maritime reconnaissance aircraft. Lowvoltage klystrons combined with high-gain fixed aerials provide adequate system gain for operating over all surfaces except the calmest water. Display is in the form of drum counters showing distance flown along heading and distance drifted at right angles to heading, port or starboard. Accuracy of measuring these distances is within ± 1 % of the total distance flown.(1) INTRODUCTION The difficulties which arise in designing a c.w. Doppler navigation system have been mentioned in a companion paper, 1 and a successful design using a pulse-modulated transmission and a receiver with self-coherent detection has been described by Beck and Thorne. 2 However, this latter design requires a complex modulation system and its receiver is comparatively insensitive. This technique has probably been pushed to the limit, and to improve the performance further and to simplify the design of Doppler navigation equipment the development of a c.w. system shows promise.In the equipment described the required performance has been achieved by using low-power low-voltage klystrons in conjunction with high-gain fixed-beam aerials and a sensitive receiver. The rearward-looking pencil beams are produced by aerials which are pendulously mounted in the pitch plane of the aircraft. Independent receivers detect the back-scattered energy and produce audio-frequency Doppler signals which are fed into two identical electronic tracking units. Low-frequency pulse trains from the tracking units are counted by magnetic stepping motors in an indicator unit which shows on counters the distance travelled along the heading and the distance drifted to port or starboard of the heading.The use of low-power klystrons enables a comparatively simple power supply to be used, and the fixed-beam aerials are simple to manufacture and maintain in service.(2) DESIGN REQUIREMENTS The equipment has been designed to meet the requirements of maritime reconnaissance aircraft, which fly for long periods over water at comparatively low altitudes and low speed. The accuracy of navigation obtained with the help of Doppler navigation equipment is less over water than over land because of the corrections which have to be applied for errors due to beam distortion and wave motion. 1 Furthermore, the uncertainty of the heading, as indicated by the magnetic compass, introduces navigational errors of the order of 1-2%. There is therefore little advantage in designing this particular Doppler navigation equipment with a greater accuracy than within about 0 • 5 %.Mr. a egg is at the Weapons Research Establishment, Australia (formerly at the Royal Ra dar Establishment.)Mr. Crompton is at the Weapons Research Establishment, Australia.
