Special Electronic Mission Aircraft
The Guardrail airborne platform consists of a special electronic mission (SEMA) aircraft and its payload. The SEMA aircraft used in the Guardrail system were initially based on the U-21 Ute aircraft, until transitioning to the C-12 Huron aircraft as part of the Improved Guardrail V program. The airframe includes navigation, avionics and survivability equipment.
These aircraft were in a long line of SEMA that had made use of the U-21 Ute aircraft, beginning with the the Left Jab JU-21A aircraft deployed to Vietnam assigned to Army Security Agency Group, Vietnam conducting "Radio Research," a cover story for intelligence gathering operations. Thus began a long line of U-21 based SEMA, including Cefirm Leader RU-21A/B/C, the Laffing Eagle RU-21D, and the Left Foot RU-21E.
The Guardrail I, II, and IIA systems all utilized the specially modified RU-21G airframe, which was based on the Left Foot system's RU-21E airframe. Guardrail IV utilized a RU-21E airframe with fewer modifications.
In 1976, the Guardrail V system began development in order to develop and provide a supportable Corps level system to the Army, until the planned Cefly Lancer development was completed. The Guardrail V aircraft were converted from earlier Guardrail I/IIA aircraft, Laffing Eagle RU-21Ds, and unmodified U-21G airframes. These new aircraft were designated as RU-21H, the big improvement was the addition of wingtip pods to replace many of the blade antennas of the previous Guardrail aircraft.
The RU-21H aircraft, derived from the RU-21G aircraft or other older SEMA was upgraded with high flotation landing gear for a 10,200 gross maximum weight limit. The aircraft was certified with wing extensions/pods and the other mission airframe modifications. Two 200 amp generators and extra 400 hertz inverters supported the mission electronics and avionics. Twenty-one aircraft were converted to the RU-21H configuration. The mission enunciator panel located on the aircraft instrument panel has the Mission Active switch and indicator lights to warn the pilot of mission faults such as over temperature, loss of link synchronization and processor lock-up. The ARF Status panel located on the mission racks behind the co-pilot shows more detailed mission parameters and has the ARF (1, 2 or 3) designator control. The crews responsibility is only to fly the aircraft and to monitor the health of the mission equipment.
The US Army procured the first three King Air 200s, designated the RU-21J, in 1971 for use with the Cefly Lancer system. A number of U-21Js for non-SEMA missions were also acquired. Further variants of the King Air 200 series were designated as members of the C-12 series. When the RU-21J aircraft were retired and repurposed for non-SEMA missions, they were redesignated as C-12L.
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