Polaris had not been designed to penetrate ABM defences, but the Royal Navy had to ensure that its small Polaris force operating alone, and often with only one submarine on patrol, could penetrate the ABM screen around Moscow. The Wilson government publicly ruled out the purchase of Poseidon missiles in June 1967, and without such a commitment, the Americans were unwilling to share information about warhead vulnerability. The result was Chevaline, an Improved Front End (IFE) that replaced one of the three warheads with multiple decoys and other defensive countermeasures. in what was known as a Penetration Aid Carrier (PAC). It was the most technically complex defence project ever undertaken in the United Kingdom. Chevaline's existence, along with its formerly secret codename, was revealed by the Secretary of State for Defence, Francis Pym, during a debate in the House of Commons on 24 January 1980. By this time the project had gone on for a decade. The final cost reached £1,025 million.