South Vietnam. On 21 December 1961 SP4 James T. Davis of the 3rd RRU was operating a mobile PRD-1 receiver with an ARVN unit near Cầu Xáng when they were ambushed by VC and Davis was killed, becoming one of the first Americans killed in the Vietnam War. In early January 1962 the 3rd RRU's compound at Tan Son Nhut was renamed Davis Station. On 1 June 1966 the 3rd RRU was redesignated the 509th Radio Research Group. The 509th RR Group continued operations until 7 March 1973, when they were among the last US units to leave South Vietnam. 507th Tactical Control Group In late September 1961, the first permanent USAF unit, the 507th Tactical Control Group from Shaw Air Force Base deployed sixty-seven officers and airmen to Tan Son Nhut to install MPS-11 search and MPS-16 height-finding radars and began monitoring air traffic and training of RVNAF personnel to operate and service the equipment. Installation of the equipment commenced on 5 October 1961 and the unit would eventually grow to 314 assigned personnel. This organization formed the nucleus of South Vietnam's tactical air control system. Tactical Reconnaissance Mission On 18 October 1961, four RF-101C Voodoos and a photo processing unit from the 15th Tactical Reconnaissance Squadron of the 67th Tactical Reconnaissance Wing, based at Yokota AB Japan, arrived at Tan Son Nhut, with the reconnaissance craft flying photographic missions over South Vietnam and Laos from 20 October under Operation Pipe Stem. The RF-101s would depart in January 1962 leaving Detachment 1, 15th Tactical Reconnaissance Squadron to undertake photo-processing. In March 1962 a C-54 Skymaster outfitted for infrared reconnaissance arrived at the base and remained until February 1963, when it was replaced by a Brave Bull Boeing C-97 Stratofreighter. In December 1962 following the signing of the International Agreement on the Neutrality of Laos, which banned aerial reconnaissance over Laos, all four Able Marble RF-101Cs arrived at Tan Son Nhut from Don Muang Royal Thai Air Force Base. On 13 April 1963 the 13th Reconnaissance Technical Squadron was established at the base to provide photo interpretation and targeting information. Following the Gulf of Tonkin Incident on 4 August 1964, six additional RF-101Cs deployed to the base. The 67th TRW was soon followed by detachments of the 15th Tactical Reconnaissance Squadron of the 18th Tactical Fighter Wing, based at Kadena AB, Okinawa, which also flew RF-101 reconnaissance missions over Laos