The US's first anti-satellite system was Program 505, based at the Kwajalein Missile Range, about halfway between Hawaii and the Philippine islands. This system became operational in 1962, based on the Nike Zeus anti-ballistic missile that had been tested at that site. These missiles had been modified for greater range, but even with these modifications the system could only intercept satellites that flew very close to the base, with a maximum altitude around . A system with longer range was an obvious requirement.