The AN/MSQ-77 radar system has a very distinguished history. 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.
The United States Air Force developed the AN-MSQ-77 radar system to guide the aircraft to the target. This type of mission was called Ground Directed Bombing (GDB). The radar portion of the AN-MSQ-77 was capable of tracking aircraft anywhere within 200 miles of the radar system, so one radar system could cover the whole of North Vietnam and South Vietnam. To reflect this GDB role, the AN/MSQ-77 was originally called the "Radar Bomb Directing Central". It 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 plotting board / computer operators would tell the aircrew to make minor corrections in their flight path, and then the exact momment when to drop their bombs, to ensure that the bombs would be on target.
Over 75% of all the bombs dropped in Vietnam, such as Operation Rolling Thunder, which lasted over three years, and the Linebacker missions were done with precision GDB.
Even after the Vietnam Conflict was over, the 77 was still used by the USAF for training purposes, and to conduct Radar Bomb Scoring (RBS) into the early 90's, when it was replaced by the AN/TPQ-43 "Seek Score" radar system that is currently in use.
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