English: This image shows ARPA's HAPDAR radar under construction on 21 April 1965. The camera is looking to the south-west.

HAPDAR (Hard Point Demonstration Array Radar) was a prototype of a high-performance phased-array radar that was designed to quickly generate tracks on ICBM warheads within seconds of them appearing above the defended site. The warheads would then be attacked by a high-performance missile, either Sprint or a new design known as HiBEX. The idea was to wait until the last possible moment before firing, to be sure they only attacked warheads that would actually hit the Minuteman missile silos, ignoring those that would fall outside their lethal range.

To save construction costs, HAPDAR was built on the pre-existing but by this time disused Zeus Acquisition Radar (ZAR) receiver, which had been part of the earlier Nike Zeus ABM system. The actual HAPDAR radar would be installed on the now-empty face, pointing north towards the missile launchers.