Something had to be done in order to maintain the relevance of the UK's nuclear deterrent. Although developments in the ABM field had been watched throughout the 1960s, exactly what to do about the problem apparently remained a fairly low priority. In 1967 the US offered a newer version of the Polaris, the A3T design, which featured a "hardened" missile airframe intended to better protect it against ABMs. As the UK had not yet received their missiles, they agreed to use the A3T version as it required relatively few other changes.[citation needed]
In 1970 serious efforts to explore the ABM problem started. By this point the US and USSR had agreed in the ABM Treaty to deploy up to 100 ABMs at only two sites. MIRVs had so seriously upset the balance between ABM and ICBM that both parties agreed to limit ABM deployment largely as a way of avoiding a massive buildup of new ICBMs. The only good news for the UK in this development was that it clearly defined the problem they were facing; their attacks had to be able to credibly defeat a 100-interceptor ABM defense around Moscow. Thus started project KH.793, a one-year project to identify potential solutions.
One option would be to build additional Polaris platforms and keep more of them at sea. Two Resolutions would throw 96 warheads and almost guarantee penetration, while three would make it a certainty. This would require a fleet of at least five submarines to keep two on station at all times, as well as larger crew complements, training and logistics support, and appeared to be the most expensive option.
Several "low cost" options were also explored. Among these were "Topsy", the A3T missile the UK had already agreed to use. Another option was Antelope, which used hardened warheads along with penaids, although it reduced the payload to two warheads in order to save weight for the penaids. They also explored a "superhardened" version known as Super Antelope, a further improvement on the warhead that also used a manoeuvrable warhead "bus" to deploy the penaids further apart in space.
The Royal Navy preferred to upgrade to the Poseidon missile, increasing the number of warheads from three per missile to between ten and thirteen of a newer and lighter design. In this case a single Resolution could launch up to 208 warheads, guaranteeing that some would get through. This option also had the advantage of maintaining commonality with the US Navy, as well as offering greater range, and thus increasing the safety of the launch submarines.
The US also favoured the Poseidon, although for more technical reasons. They felt that while the decoy approach was useful against near-term ABMs deployed by the USSR, that it was considerably less useful against "point defence" type interceptors. This is because the decoys are so much lighter than warheads; when they started to hit the upper atmosphere the decoys would slow more than the warheads and thus "declutter", allowing the warheads to be attacked. This would require a much faster-reacting system than a long-range interceptor, one that could safely wait until only a few moments before detonation, but this was by no means impossible and was part of the US's own ABM systems.
As the UK had decided to produce its own warheads and bus, this option would require considerable development effort in order to produce a new MIRV bus of their own. Although in theory the UK might be able to use the US-designed bus, these options were being explored in the midst of the ABM Treaty, and it was not clear whether or not the treaty might forbid this technology being transferred. Some related options were explored, including "Option M" which used a simple "de-MIRVed" bus, "Hybrid" (or "Stag") which put the newer Poseidon warheads on the existing Polaris A3T missiles, and "Mini Poseidon", a similar adaptation with a smaller six-Poseidon-warhead payload on the A3T.
In the end the higher levels of the British political system decided against the urgings of their own Chiefs of Staff and went with the penaid approach on the existing A3T missile. This decision was made official late in 1973 by the Edward Heath administration, who changed the name from Super Antelope to Chevaline. The name 'Chevaline' was the result of a telephone call to London Zoo from an official at the Ministry of Defence. Prompted by a request for a name-change from his boss, the Secretary of State for Defence, Lord Carrington, the official asked the zoo to 'imagine an animal like a large antelope' and inquired whether there was such a beast. The zoo told him there was a South African creature called a Chevaline, and the official 'thought that sounded rather good'.[2] (The creature to which the zoo was probably referring was the Roan Antelope, Hippotragus equinus which is known in French as 'Antilope chevaline'.)