The AN/MSQ-77 was a United States Air Force ground-based system for directing bombing of targets, also known as COMBAT SKYSPOT. It was developed from the AN/MSQ-35 radar used for verifying the accuracy of Strategic Air Command bombers in training. For combat operations, it had a 175-mile range, which had to be in line of sight to the target; this made high ground preferable for its installation.
AN/MSQ-77 systems were extensively used during the Vietnam War, most importantly to direct hombing from B-52 heavy bombers at high altitude. It was also used to direct tactical aircraft during all weather and during darkness, in situations including attacks on supply lines, close air support, parachute delivery of supplies and personnel, and combat search and rescue. It could back up Ground Controlled Approach landing systems for aircraft with damaged onboard electronics, and also mark areas to attack, relative to an airborne spotter that discovered a new target.
While more accurate attack direction systems replaced it in that role, it continued to be used for training until the AN/TPQ-43 system was introduced.