A Doppler Collision Course Indicator for Use at Sea

Abstract

The usefulness of radar plotting, as at present practised, may be considerably enhanced by the application of a doppler radar technique which gives instantaneous measurement of relative velocity components and quickens the whole plotting process. The range and bearing of the target are given by an ordinary radar PPI, from which the rate of change of bearing may also be determined. The doppler radar gives an immediate measurement of the radial component of relative velocity and the derivative of radial velocity: from these data the course and speed of the target may readily be determined, using an automatic computer if so desired. Possible errors in deducing the true aspect and speed are discussed and the mode of operation, and principal design parameters of suitable doppler radar are given.i. INTRODUCTION. Although the number of collisions at sea between vessels equipped with radar continues to be a matter of grave concern, there has been little real progress in recent years in the development of anti-collision techniques. Whilst it is certainly true that so-called 'radar assisted collisions' are due to misapplication of the information supplied by radar, rather than to any shortcoming in the actual radar performance, there is frequent lament that information concerning the aspect of a target is seldom readily apparent, and many mariners would place greater confidence in their radar sets if aspect information were always immediately available as part of the normal performance.Captain F. J. Wylie (this Journal, 10, ^6) has postulated three factors for the reduction of collision risk in clear weather:

The Use of Self-contained Range and Azimuth Measuring Apparatus to Detect Collision Courses

“…SOME readers of the January issue of the Journal may have noticed rather strongly discrepant statements on the subject of self-contained detection devices in the several papers on collision avoidance. For example, on page 43 Gaudillere 1 suggests either constant bearing or constant closing speed as an indicator of impending collision between aircraft; and on pages 82-87 Whitfield and Cade 2 suggest that ships may determine transverse velocity (and hence may identify a collision course) by using a differentiated doppler radar return. But on pages 25-26 the author of this note states 3 that the derivatives of range and azimuth required for this use are in fact too small for successful physical measurement.…”

“…Equation 1shows the difference between closing speed r and relative speed v. It reflects the fact that the former is constant and equal to the latter only in the collision case m =0 (or the physically meaningless case r = oo); but that otherwise i is variable and always less than v. The facts we are after lie in Equations (2) and (3).…”