A U.S. Army signals intelligence aircraft, the RC-12 GUARDRAIL (RC-12K/N/P/Q) is unusual among manned ISR aircraft in that it has no onboard capability to analyze the data it collects. The aircraft fly in units of three to give multiple bearings on a target, but transmit all data, in near-real-time, to a ground station.
It has been assigned to intelligence units at Corps level. Current versions include several systems:
- Improved GUARDRAIL V (IGR V)
- Communication High Accuracy Airborne Location System (CHAALS)
- Advanced QUICKLOOK (AQL) into the same SIGINT platform
These systems are collectors for the Precision SIGINT Targeting System (PSTS). PSTS is a joint-service and defense-agency effort that uses both tactical and national systems to provide a near real-time, precision-targeting, sensor-to-shooter capability. 
- "Guardrail Common Sensor", Globalsecurity.org