The Type 901 radar was initially intended as the anti-aircraft Long RangeSystem 1 radar, but while it was being developed it was selected as the fire control radar for the Seaslug missile system. Although the anti-aircraft requirement was dropped in 1949, development continued for Seaslug.
Details
The single unit produced three co-axial beams; a high powered target tracking beam, a lower powered guidance beam for the missile and a wider gathering beam. The gathering antenna was finally sited on the right-hand side of the unit housing the other antennae; the gathering unit used a paraboloid reflector as a wide beam was required; the guidance and tracking units used a 9ft diameter dialectric lens. The Type 901 radar's tracking antenna could be set to one of 8 frequency bands within the range 9.1GHz to 9.8GHz, the guidance and gathering antennae shared a separate frequency, around 0.1GHz lower Initially the radar consisted of two offset paraboloid reflectors mounted side-by-side, on a modified wartime pom-pom mounting. The left-hand unit provided the main beam while the right-hand provided the gathering beam. This matured into the 'dustbin' type radar containing the tracking and guidance antennae with the gathering antenna attached to the right-hand side. Target acquisition was manual, with one crewman searching in elevation and azimuth, and on finding the target a second crewman found the range; at this stage the system was locked onto the target and would continue to track it. This process took 20 - 30 seconds The target-tracking and guidance beams produced were incredibly fine, being measured at 0.8° wide at 6dB down, this enabled the tracking radar to discriminate between targets as close as 20 minutes of arc or a range separation of 75 yards. Once properly gathered the missile typically remained within 35 feet of the centre of the beam for the duration of the engagement. The achieved power "was no mean feat, but undoubtedly the greatest advance made in this radar was the ultimate achievement of a tracking accuracy giving an rms aiming error of only 20 arc-seconds -the equivalent of 2·5 yards at a range of 25,000 yards. And this from a 4-axis-stabilised aerial with transmitter and receiver weighing some 12 tons in a ship that rolls, pitches and heaves." There were 3 flight modes:
LOSBR, in which the missile flew up a beam that tracked the target
CASWTD, with the missile climbing at a low angle and then diving onto a low-altitude target
MICAWBER, similar to CASWTD, but with a terminal low-level glide phase so that the Mark 2 variant could be used against ships. This mode suffered from problems associated with the surface of the water reflecting the guidance beam.
Type 901(M).
The upgraded radar associated with the Seaslug Mk2 system featured a Constant Wave tracking radar, and the discrimination was improved to 18 minutes of arc. It also featured a number of automatic scanning patterns for target acquisition
