Seaslug Mk1 Fuzing.

A number of fuzing options were considered for Seaslug; both proximity based and what was called 'the ship borne fuze' ie command detonation. The latter made use of the type 901 radar's fine range discrimination to predict the point of coincidence (when the missile is at the same range as the target) and sent a command to detonate the warhead at that point.

The proximity fuze presented a greater problem. The first model, the GM1, was based on the VT fuze developed for anti-aircraft shells during the second world war and relied on the Doppler effect. As the missile approached the target the fuze measured the frequency of the signal reflected by the target. While approaching, that frequency will be greater than the transmitted frequency; when moving away it will be lower. It is only when there is no relative movement that the reflected frequency will be the same as the transmitted frequency, which is normally the point at which the missile is closest to the target and the warhead can be detonated. The disadvantages of this fuze were that it transmits in all directions and is easily jammed.

EMI Electronics Division (EMIED) refined this fuze and utilised directional antennae which gave the fuze a fixed 'look angle' rather than all-round coverage; this was the GM2. It was, however, still susceptible to jamming, and was prone to be triggered at low altitude (3-4000 feet) by the surface of the sea.

A passive infra-red fuze was also considered but it was realised that it could not be produced in time so it was put to one side; it was successfully developed for the Mk2 missile.

Two other radio fuses were considered, the Noise Range Gate and the Pulse Range Gate. The latter was successfully developed for Seaslug Mk1. It used a 6kw magnetron transmitting 0.15us pulses, after each pulse the receiver klystron tube pulsed for the same time; if an echo from the transmitted pulse was received during the klystron's pulse the warhead was detonated. The fuze was further tuned to have an effective range gate of 140 feet, and demonstrated a high resistance to ECM.

The PRG was nicknamed 'Pranger' (presumably from Pulse RANge GAte) by the EMIED staff and the MOD used it as the fuze's official name. I can remember seeing it listed in the Naval Armament stores listings as such; it was by beleived many of us (myself included) as being a direct-action impact fuze. Having read through the relevant files held by the National Archives I have confirmed it was a proximity fuze and that Seaslug Mk1 did not have an impact fuze at all.