The controlling agency at Khe Sanh for these strikes was Air Support Radar Team-Bravo (ASRT-B), Marine Air Support Squadron 3 which had moved from Chu Lai on 16 January. The ground controllers operated from a heavily reinforced van which housed their sensitive computer equipment and used the TPQ-10 radar to guide aircraft to their target; thus, the missions were called TPQs.[55] [(147)] The radar emitted a pencil-shaped beam which detected and locked on to the aircraft. Using target coordinates provided by the FSCC, the controller programmed the enemy position, ballistic characteristics of the bombs, current winds, and other pertinent data into a computer which was connected to the radar. The computer also received inputs from the radar and, in turn, provided corrections in airspeed, altitude, and heading which the operator passed on to the pilot. The controller closely monitored his set and, at a predetermined release point, called a "Mark" to the pilot who "pickled" his bombs.[56] In specially-equipped aircraft, such as the A-4 Skyhawk and the A-6 Intruder, the bombs could be released automatically from the ground. One ground controller could handle a single plane, a section (two planes), or a division (four planes) on the same pass as long as the pilots flew in a tight formation and the radar did not break lock. One of the controllers' favorite aircraft was the A-6 because it packed such a heavy wallop; a single Intruder usually carried 28 500-pounders. Any fighter/bomber, however, could be used as long as it carried low-drag ordnance and the pilot could make a smooth run.[(148)]
Even though most TPQs were conducted from around 14,000 feet, the accuracy of ASRT-B was phenomenal. When new personnel arrived at Khe Sanh, they were given several check drops on a nearby hill to test their proficiency before the newcomers were allowed to conduct strikes near friendly troops. The first drop was always within 40 meters of the target and, after they adjusted there was virtually no error. Calibration drops were also conducted twice weekly to ensure the accuracy of the equipment. One member of the FSCC stated that, if he were in a foxhole and under attack, he would have no qualms about calling an ASRT-B controlled TPQ within 35 meters of his position. The rule of thumb which the FSCC generally applied when determining a safe distance for normal operations, however, was one meter from the friendlies for every pound of conventional ordnance being delivered. Thus, for TPQs, a 250-pound bomb would not normally be dropped within 250 meters of Allied troops, a 500-pounder within 500 meters, and so on. This criteria was not established because the men on the ground lacked confidence in the system but because of the large fragmentation pattern produced by the bombs. Besides, anything inside the prescribed radius could be handled just as effectively by artillery, mortars, and direct fire weapons. In an emergency, the regimental commander would have undoubtedly lifted the restriction. Concerning the quality of support he received from ASRT-B, Colonel Lownds said, "Anything but the highest praise would not be enough."[(149)]
A-6 Intruder, under TPQ control, provides precision bombing around Khe Sanh despite poor weather. (USMC Photo A422000)
B-52 Stratofortresses flew strikes daily in support of the 26th Marines. (Photo courtesy USAF)
In addition to its accuracy, the TPQ system was extremely flexible. A strike could be programmed and executed within 10 or 12 minutes utilizing any available aircraft. Most of the missions were at night when it was inefficient and dangerous to conduct dive-bombing strikes. As a matter of routine, two Marine and three Air Force flights were scheduled every hour unless an emergency developed. On 18 February, ASRT-B set a new squadron record for a single 24-hour period by controlling aircraft which delivered 486 tons of ordnance on 105 separate targets. After that, the record was approached frequently but never broken. During the siege, ASRT-B controlled 4,989 TPQs in support of the 26th Marines.[(150)]
Beginning on 20 February, ASRT-B also assisted with supply drops whenever the Khe Sanh MATCU was inoperable. Normally, the controllers could have guided the transport pilots to an exact release point but, at Khe Sanh, the C-130s had to fly directly over the station and the TPQ-10 would break lock.[57] [(151)] Therefore, the ASRT personnel used the same technique as the MATCU controllers and called a "Mark" when the Hercules was over the eastern threshold and the pilots completed the runs with Doppler navigation and stop watches. The only problem was that, when the ASRT conducted supply drops, it was drawn away from the primary mission of handling TPQs.[(152)]
While air support was vital to the defense of the base, Colonel Lownds felt that his artillery played an equally important role. When the fighting first broke out, the colonel surmised that the side which managed to keep its artillery intact would win the battle. The Marine artillery emerged almost unscathed. Many incoming rounds landed within the battery positions, however, very few actually hit the gun pits and throughout the operation only three artillery pieces at the base were destroyed; one was a 155mm howitzer parked alongside the loading ramp awaiting airlift to Dong Ha.[58] Generally the pieces, were tucked away inside heavily sandbagged revetments and, while the crews were often showered with fragments, it would have taken a direct hit, squarely on top of the weapon, to knock out a howitzer. Fortunately for the Marine gunners, the North Vietnamese scored only one such a hit which led the regimental commander to the conclusion: "Either they were amazingly inaccurate or we were amazingly lucky."[(153)]