This mammoth air umbrella, called Operation NIAGARA, lasted from 22 January until 31 March and was truly an Allied effort. At one time or another, the Khe Sanh DASC utilized the assets of all services: 1st MAW, Seventh Air Force, Strategic Air Command, U. S. Navy Task Force 77, Vietnamese Air Force, and various U. S. Army aviation companies. The majority of the sorties, however, were flown by U. S. Marine, Navy, and Air Force crews. Their mission was to "destroy enemy forces in the SCOTLAND ... TAOR, interdict enemy supply lines and base areas, ... and provide maximum tactical ... air support to friendly forces." Generally, the type of strike fell into one of three categories: close air support, B-52 Arc Light strikes, or radar-controlled bombing.[49](138) Close air support missions were utilized against pinpoint targets in proximity of friendly troops. Along with radar-controlled bombing, this type of air strike was the most responsive to the needs of the ground commanders and the most accurate; the attack pilots, however, required reasonably good weather to be able to hit their targets. There were usually fighter/bombers overhead at Khe Sanh around the clock; if not, they could be quickly scrambled from hot pads or diverted from other missions. When the pilots arrived on station, they checked in with the DASC and were handed off to a Marine or Air Force Tactical Air Controller (Airborne) who personally directed the strike. There were seven TAC(A)s assigned to the 26th Marines; the Air Force personnel were members of the 20th Tactical Air Support Squadron and the Marines were from Headquarters and Maintenance Squadron 36 and Marine Observation Squadron 6. At least five of these pilots, flying O1-E Birddogs or UH-1E gunships, remained over the battlefield during the day and maintained direct communications with both the attack aircraft and the troops on the ground. In this manner, the TAC(A)s could rapidly employ the jets wherever they were needed the most and the close supervision reduced the chance of accidentally bombing friendly forces. During the day, the air around Khe Sanh was filled with the high-pitched shriek of jet engines: Marine, Navy, and Air Force F-4 Phantoms; Marine and Navy A-6 Intruders, A-4 Skyhawks, and F-8 Crusaders; Air Force F-105 Thunderchiefs and F-100 Super Sabers. In addition to the jets, the South Vietnamese prop-driven A-1 Skyraider, a rugged attack aircraft of Korean War vintage, was in evidence. At times, the sky overhead resembled a giant beehive. When a flight arrived on station, the Khe Sanh DASC normally directed it into a holding pattern until a TAC(A) or a Forward Air Controller on the ground was free to handle the strike. These patterns sometimes extended up to 35,000 feet with scores of planes gradually augering their way downward as each preceding flight unloaded its ordnance and scooted for home.