Ground Directed Bombing (GDB) is a type of attack mission performed by ground attack aircraft. Aircraft are remotely guided from the ground with the assistance of a height and direction-finding radar. In mature versions of the system, bomb release is controlled from the ground for precision. This enabled delivery of unguided bombs in poor weather and night, where pilots were unable to see the ground and identify targets. Until the introduction of GPS-guided bombs, this remained the primary method of bomb delivery in poor weather conditions. Over 75% of all bombs delivered in Vietnam, in particular all Operation Linebacker sorties, were performed with GDB.
The U.S. Marine Corps was the second to introduce this ability with the AN/MPQ-14, which first saw combat in Korea and proved the worth of such a system. It would be followed by the more reliable AN/TPQ-10 that saw widespread use in Iraq. The Air Force would introduce the AN/MSQ-77 in 1966 in Vietnam, a new role for the 1st Combat Evaluation Group. The Marine system was primarily used in the close air support role, with a CEP of 50 yards. The Air Force system, however, was primarily used for harassment and interdiction, and used only in emergencies for CAS - it claimed a 250-meter CEP.
Both systems were derivatives of radar systems designed for other uses. The Marine version was originally designed as a training aid to mimic the remote guidance of a V-1 buzz bomb. The Air Force system came out of the AN/MSQ-35 system used to score bombing runs.
Though rendered obsolescent with the introduction of the JDAM family of GPS-guided bombs, the system remains in use as a training aid for simulating bomb release.