Airborne Common Sensor
The Airborne Common Sensor (ACS) was a planned U.S. Army program to replace the Airborne Reconnaissance Low (AR-L), RC-12 GUARDRAIL, and possibly other sensors. As opposed to the GUARDRAIL, it will carry four operators who will guide sensors including communications intelligence (COMINT), direction finding and full geolocation of electronic intelligence (ELINT) targets. It is unclear if the ACS might converge with the now-cancelled Navy EP-X program, intended to replace its existing EP-3 ARIES II land-based tactical SIGINT aircraft. The ACS, however, was cancelled in 2006. At least some of its functions will be replaced by the Airborne Signals Intelligence Payload on drones.
One of the COMINT sensors was to be an upgraded GUARDRAIL package, derived from systems on the U-2 Dragon Lady manned and MQ-4 GLOBAL HAWK unmanned reconnaissance aircraft. Unlike the RC-12, the airborne crew will be able to control the sensors, rather than relying on a ground station. It will go beyond the RC-12, which flies missions in groups of three, in that it will interoperate with heterogeneous groups of battlefield surveillance sensors and processors. ACS will be the manned collection component of the Army , which integrates Army and Air Force sensors.
Like the AR-L but not the RC-12, it will carry some type of imaging radar. The format of the output of the radar has been agreed; it will comply with NATO STANAG 4607 for moving target indicator data.  There are discussions about the applicability of the Raytheon Advanced Stand-Off Radar (ASTOR), which was developed for the Royal Air Force and recently passed acceptance tests.  ASTOR is undergoing testing for interoperability with U.S. Air Force systems.
It is assumed that the actual aircraft will be a commercial-off-the-shelf (COTS) small turboprop or jet business aircraft; the choice will be up to the overall system integrator that wins the project award.
- U.S. Army Revives the Airborne Common Sensor Program, August 2008
- Distributed Common Ground System (DCGS)
- Emerging Standard for MTI Distribution, 14 Sep 2005
- Pocock, Chris (July 15, 2008), ASTOR passes big test, formal acceptance next