The hub of the Air Force system was the Tactical Air Control Center (TACC) of the Fifth Air Force-EUSAK Joint Operations Center (JOC), known by the code name MELLOW. An aviator coming on duty called up MELLOW and received his instructions from JOC. FACs were assigned to U.S. Army and British units down to corps, division, and regimental levels, and to ROK corps and divisions. Further assignment to smaller front line units was possible but entailed a good deal of time and advance planning. And even the most urgent requests had to be channeled through division and regimental levels to JOC for approval. If a Marine FAC wasn’t able to control an air strike visually because of terrain conditions, he called for a “tactical air coordinator, airborne” (TACA) to locate the target from the air and direct planes to the attack. The Fifth Air Force also used special airborne coordinators. Known as “Mosquitoes,” they flew low-winged, two-seater North American training planes, designated T-6s by the Air Force and SNJs by the Navy. This plan was capable under favorable circumstances of providing the Fifth Air Force-EUSAK tactical air control system with a mobile and flexible means of directing air power at the front. Its chief weakness, according to Marine doctrine, lay in the separation of air power from ground force control. The Air Force claimed the advantage of projecting tactical air power deep into enemy territory; but as the Marines saw it, this was deep or interdictory support, and not to be compared to genuine close air support. _X Corps Conference at Kyongju_ The command and staff of the 1st Marine Division could only speculate during this interim period as to what the near future might hold for them. Rumors had been circulated, during the first week at Masan, that the Division would be employed as rearguard to cover an Eighth Army withdrawal from Korea, with Pusan serving as the port of debarkation. And while plans cannot be made on a basis of rumor, General Smith and Colonel Bowser went so far as to discuss the possibility seriously. At last, on 24 December, a more definite prospect loomed when the EUSAK staff requested the Division to furnish logistical data for a move by rail and truck to Wonju, some 130 miles north of Masan. It was not known whether an actual move was contemplated or the intention was merely to have available a plan for future use if the occasion warranted. General Smith sent the data but added a strong recommendation to the effect that any commitment of the Division be postponed until it was re-equipped and strengthened by replacements.[21] [21] 1stMarDiv _HD_ Dec 50, 12–13; Smith, _Notes_, 1258–1259.