The 3rd air wing to operate EC-121s was 553rd Reconnaissance Wing, a Vietnam war organization activated in 1967 and based in Thailand until inactivation in 1970 (See BatCat below). In 1966 Lockheed modified 30 ex-USN Super Constellations (2 EC-121P/WV-3 and 28 EC-121K/WV-2) aircraft to EC-121Rs for the specialized recon mission flown by the 553rd. Aircraft were delivered during the course of 1967. 553rd RW flew over land and off the coast of Vietnam, over Laos and Cambodia, monitoring and retransmitting low-power signals. Usually they orbited the Ho Chi Minh Trail in 8-hour shifts. As it was expensive to operate and it exposed a large crew to enemy fire it was replaced in December 1970 by the much smaller (and eventually unmanned drone) QU-22 Pave Eagle sensor monitor.