Zeus. Adding to the problems, as the warhead weight dropped, existing missiles had leftover throw weight that could be used for various radar decoys, which Zeus proved unable to distinguish from the actual RV. The Army calculated that as many as twenty Zeus' would have to be fired to ensure a single incoming missile was destroyed. Nike-X Faced with these problems, both the Eisenhower and Kennedy administrations turned to the newly created ARPA to suggest solutions. ARPA noted that because the decoys were lighter than the actual warhead, they would slow down more rapidly as they reentered the lower atmosphere. They proposed a system using a short range missile that could wait until the warhead was below altitude, at which point the decoys would have been decluttered. Desiring to destroy the missile before it was below altitude, combined with the per second terminal speed of the RV meant there were only 2 to 3 seconds to develop a track and shoot the interceptor. This would demand extremely fast missiles, high-performance radars and advanced computers. In 1963, Robert McNamara cancelled the upcoming deployment of Zeus and announced that money would instead be provided for research into this new system, now known as Nike-X. Construction on the new phased array radar and its associated computer systems began at the MAR-I site in White Sands Missile Range. MAR could track hundreds of warheads and the interceptor missiles sent to attack them, meaning the Soviets would have to launch hundreds of missiles if they wanted to overwhelm it. And now that decoys were no longer an issue, the cost-exchange ratio fell back to reasonable levels. The Nike-X developed the same sort of trouble as the Zeus before it. In this case it was ironically due to the Soviet's own ABM system which was very similar to Zeus. To ensure they could defeat it, the US Air Force began equipping their own ICBMs with decoys, which would defeat the Soviet's Zeus-like system. However, concerned that the Soviets would upgrade their system to Nike-X like performance, they instead began replacing their warheads with lighter ones, and carrying three of them. This meant that every US missile would require three (or six accounting for redundancy) interceptors. When the same calculations were run for Nike-X, it was calculated that they would have to deploy 7,000 Sprint missiles, and the cost-exchange ratio was 20-to-1 in favor of the Soviets.