The paper describes an experimental analogue computer used with a civil marine radar set for determining the closest point of approach of selected ship's echoes and the time to closest approach. The equipment was installed on shore and measurements made of shipping passing to and from Portsmouth and Southampton harbours. The projected track of a ship echo was displayed on a 5-in. diameter cathode ray tube and the closest point of approach read as the distance of this track from the centre of the tube. The results show a high probability that under these conditions errors in C.P.A. will not exceed ±0·5 n.m. for targets between 10 and 6 n.m. range, and ±0·3 n.m. for targets nearer than 6 n.m.