Most of the time, however, the low overcast precluded the use of either extraction system and the preponderance of supplies was delivered by paradrops. This technique called for close air/ground coordination and the C-130 pilots relied on the Marine Air Traffic Control Unit (MATCU) at Khe Sanh to guide them in to the drop zones. The Marine ground controller lined the aircraft up on the long axis of the runway for a normal instrument approach and when the Hercules passed a certain point over the eastern threshold of the field, the controller called "Ready, Ready, Mark." At "Mark," the pilot pushed a stop watch, activated his Doppler navigational system, turned to a predetermined heading and maintained an altitude of between 500 and 600 feet. The Doppler device indicated any deviation from the desired track to the drop zone, which was west of Red Sector, and the release point was calculated by using the stop watch--20 to 26 seconds from "Mark," depending on the winds. At the computed release point, the pilot pulled the C-130 into an 8-degree nose-up attitude and 16 parachute bundles, containing 15 tons of supplies, slid from the rear of the aircraft and floated through the overcast into the 300-meter-square drop zone. Under Visual Flight Rules (VFR), the average computed error for the drops was only 95 meters. Even when these missions were executed completely under Instrument Flight Rules (IFR), the average distance that the bundles landed from the intended impact point was 133 meters--well inside the drop zone. On a few occasions, however, the parachute bundles missed the zone and drifted far enough away from the base to preclude a safe recovery. In these rare instances, friendly artillery and air strikes were brought to bear on the wayward containers to keep them from falling into the hands of the enemy. During the siege, Air Force C-130 crews conducted a total of 496 paradrops at Khe Sanh.[38](122)