AN/FPS-50 radars were the detection, as opposed to tracking, components of the Ballistic Missile Early Warning System (BMEWS). The operational units are located at Thule, Greenland, Flyingdales, United Kingdom and Clear, Alaska.
Initial versions, which became operational, in used fixed, 165 feet tall and 400 feet wide., but mechanically scanned antennas in the UHF frequency band, in the 400 MHz range. They have been replaced by solid-state phased array radars. Each system viewed 120 degrees of the horizon.
Shortly after its installation, the Thule radar reported a Soviet missile attack on the United States. When the overall BMEWS system continued to report more missiles than the Soviets possibly could have, and no points of impact were predicted, the warning was cancelled. Subsequent analysis found that the beam was hitting the Moon and scattering back at the receiver. While the radar was modified to prevent false alarms from the Moon, the principles were applied to lunar surface mapping.