system was handed over from the Neptunes to Air Force F-4 Phantom II fighter-bombers that had been specially equipped for the missions. U.S. strike aircraft were directed to predicted target areas by a variety of means. The first was for the ISC to relay target information to an airborne battlefield command and control center (ABCCC), which then routed bombers to a forward air control aircraft (FAC). The FAC then led the strike to the target. During inclement weather or complete darkness, aircraft could still attack the trail by utilizing either MSQ-77 Combat Skyspot (a radar-directed system) or LORAN (a radio-directed navigational system). As the program (and PAVN air defences) evolved, so did the relay aircraft. The EC-121Rs and their crews proved too vulnerable and were partly replaced in 1969 and 1970 by QU-22Bs (modified Beech A-36 Bonanzas) which were to be remotely piloted, and which had undergone primary mission equipment and PME flight tests at Eglin Air Force Base, Florida, in 1968. The aircraft suffered from mechanical difficulties, however, and were never flown during an operational mission without a pilot. They were replaced by C-130B models in December 1971. Operations in Laos On 25 November 1967, Muscle Shoals began to undergo field testing and evaluation in southeastern Laos. This process was stopped by the battle of Khe Sanh, when an estimated three PAVN divisions approached, and then surrounded the Marine outpost in western Quang Tri Province adjacent to Laos. The U.S. commander in Vietnam, General William C. Westmoreland, ordered Task Force Alpha to support the aerial effort to defend the base (Operation Niagara). On 22 January, the first of 316 sensors were implanted around and near Khe Sanh in 44 strings by Navy squadron VO-67. Although the Marine Direct Air Support Center was at first reluctant to trust the sensors (which in fact replaced ground patrolling), they were soon convinced of their utility. The Marines credited 40 percent of intelligence available to the Khe Sanh fire support coordination center to the sensors. General Westmoreland was ecstatic about Task Force Alpha's contribution to the victory at Khe Sanh. He had been one of the few high-ranking military officers that had supported the barrier concept since its planning stages. With the conclusion of aerial operations around Khe Sanh and the close out of the Marine base, the focus of Igloo White (as the operation was redesignated in June) turned once again to