Theater Requirement. At the time of the Vietnam War, the United States did not have precision navigation capabilities such as the Global Positioning Satellites (GPS). Aircrews, especially B-52 bomber crews, were not able to "see" many of the ground targets that they were attacking, and navigation systems were not precise enough to conduct the types of missions directed by the Johnson administration. In addition, during the hours of darkness or in bad weather the performance of the FAC aircraft and more so the strike aircraft were limited. The loss of the Special Forces Camps at A Shau in March 1966 emphasized the need for a weapons system which could support ground forces at all times - day or night and in any kind of weather. Even during good weather and in daylight the availability or use of a FAC, albeit on rare occasions, was sometimes impractical.
Radar Bomb Scoring. The Strategic Air Command, to evaluate the proficiency of their aircrews, had been using a ground-based radar/computer unit designated MSQ-35. This system, called Radar Bomb Scoring, could predict the exact point of impact of a simulated bomb drop.
Radar Bomb Directing Central. Using this radar system, a test was conducted in 1965 at Matagorda Proving Range, Texas, using F-100 aircraft to deliver live ordnance. Factors such as altitude, wind speed, aircraft speed, temperature, and ordnance characteristics were introduced into the computer. The plotting board / computer operators could provide the pilot with heading, altitude, and airspeed instructions as the bomb-run progressed and was able to tell the aircrew to make minor corrections in their flight path. As the aircraft neared the target, a countdown was initiated and bomb release was accomplished on the controller's "3, 2, 1, hack." This type of mission was christened "Radar Bomb Directing Central" and the equipment was designated AN-MSQ-77. Some time later the technique became known as "Ground Directed Bombing (GDB)"
Range of the AN-MSQ-77. This radar depends on line of site and therefore was limited by earth curvature and other terrain obstructions. However, the radar portion of the AN-MSQ-77 was capable of tracking aircraft anywhere within 200 miles (320 km) of the radar site. (As a point of interest, the TSQ-96 using skin tracking the maximum range was 200,000 yards roughly 114 miles. In the beacon track mode, a beacon transponder on the aircraft was utilized and this doubled this doubled the maximum range to 400,000 yd. or 228 miles or 364 km for the TSQ-96.)
Technology. The unit was designed with a vacuum tube type computer (1950's technology) and a "Plotting Board", which could draw a precision map of where a tracked aircraft was flying. These maps could precisely determine where an aircraft was in relation to a chosen target. The computer continuously calculated the altitude, airspeed, wind drift correction, and ground elevation changes, using the ballistics of the bombs that were being carried by the aircraft. The USAF MSQ-77 ground-controlled radar system and the USMC equivalent, the TPQ-10 were not intended to replace visual bombing but rather as a complement to it.
Deployment of AN-MSQ-77 to Vietnam. The United States Air Force deployed sufficient of the MSQ-77 Bomb Directing Central Radar units to Southeast Asia to provide overlapping coverage of all the RVN. By Sep 66 the following sites had been activated:
Tasking and Operation. Preplanned radar-directed strikes were fragged by the TACC along with other missions. After takeoff, the strike aircraft were passed from the appropriate CRC/CRP to an MSQ-77 'Skyspot' or TPQ-10 radar station that directed the strike. The station provided heading, altitude, and airspeed directions to the pilot and, using bomb release tables and wind data, guided the strike aircraft by radar plot to a computed bomb release point.
Re-direction of Pre-planned Missions. Although mainly used for pre-planned night strikes in a harassment and interdiction role, aircraft were sometimes diverted to immediate strikes via radar direction after coordination between the DASC and the radar station. The DASC provided the target coordinates to eight digits (10-meter square), delivery heading and allowable deviation, position of the friendly ground commander, and TOT requirements to the controlling radar station. On several occasions, ordnance was delivered as close as 250 meters to friendly forces when they were in difficulty and visual strikes could not be made.