The Type 901 radar project started during the second world war as part of the projected 'Long Range System 1' (LRS1) gunnery control system. LRS1 was never completed but the radar was utilised for the Seaslug missile project. Operating between 9.0GHz and 9.8GHz, it first tracked an aircraft as early as November 1946. At that stage the system looked very different to the tapered drum of the operational system; instead it consisted of two offset feed dishes on a wartime 2pdr pom-pom mounting. One dish was for the tracking beam and the other dish was for the gathering/guiding beams. By mid 1951 experimental missiles were riding a stationary beam, and by the end of that year they were riding a jitter-free moving beam. By the spring of 1952 missiles had been gathered at 'a relatively high elevation' and had successfully ridden a beam in the presence of jitter.

Later trials confirmed the tracking and guidance beams had a 0.8° semi cone angle (at 6db) with a 13° control cone although it was felt that a 5° cone would be better. Minimum elevation was 2° with a smooth sea surface and 1.5° with waves of 6” RMS or more, and sea reflections would be reduced with a 5° gathering deflection. This upward gathering deflection was, in fact, subsequently adopted; further improvements to the aerial system reduced the minimum elevations to 1.25° for a smooth sea and 0.75° for 6” waves.

In 1960-61 exercises were carried out to establish the tracking discrimination of the type 901 radar. These proved that the radar required a separation of 20 minutes of arc (0.33°) between two targets to be able to track either of them without jitter; this was half the predicted value, and was later reduced to 18 minutes of arc. The radar could similarly fully discriminate between targets with a range separation of only 75 yards.