English: ARPA's HAPDAR (HArd Point Demonstration Array Radar) was a prototype of a high-performance radar system designed to generate tracking information on incoming ICBM warheads in seconds. The radar used a large number of active and passive antennas to form a sharp 1.5 degree beam that could be rapidly scanned to track multiple objects. HAPDAR was built onto the roof of the former Zeus Acquisition Radar receiver building, part of the earlier Nike Zeus program that had been cancelled in January 1963. Here one of the focussing elements is being installed by workers, behind the element in each hole is a phase shifter that allows the beam to be scanned.