Ground-based sense-and-avoid (GBSAA) could find a role in terminal airspace, where ATC coordinates traffic flow but does not provide positive separation. “Airborne sense-and-avoid has to be there, but GBSAA might have to be used at dense airports where SAA could get into difficulty seeing aircraft and things on the ground,” Dann says.
Fielding of an Army-developed GBSAA system will begin next year at an initial five U.S. sites where MQ-1C Gray Eagle units will be based. The system is needed “to get Gray Eagles up and away from Army airfields in national airspace and over into restricted airspace or military operating areas,” says Viva Kelley, product director for UAS airspace integration. Design of the GBSAA system was “locked down” after a demonstration at Dugway Proving Grounds, Utah, in June 2012. “We are now writing artifacts for software certification,” she says.
GBSAA uses three ground-based radars to track aircraft. Most sites require two SRCTech LSTAR three-dimensional radars in addition to the airfield's existing ATC radar. “We have done the site surveys and have radar locations for the first two,” Kelley says. System integration will begin about a year from now, with acceptance testing at Massachusetts Institute of Technology (MIT) Lincoln Laboratory and live testing at Dugway. Initial fielding will be Block 0 capability, in which a ground-based observer recommends maneuvers based on system-generated alerts.
A Block 1 upgrade is planned that will allow the observer to recommend maneuvers generated by the system itself. Block 1 introduces collision-avoidance functionality based on the maneuver algorithms under development for the FAA's planned ACAS-X replacement for the TCAS. “We are coordinating with the FAA . . . and our algorithms are developed from ACAS-X, which is being developed by MIT Lincoln Labs as well,” says Kelley.
The FAA is developing ACAS X as a family of collision-avoidance systems for commercial, general-aviation and unmanned aircraft flying in NextGen airspace. ACAS Xu is the variant intended for UAS, and requires a Mode S transponder, but has no interrogation capability, instead using received ADS-B messages for passive surveillance and coordination with other ACAS/TCAS-equipped aircraft.
Where TCAS works only with a transponder, ACAS Xu is designed to accept additional surveillance sources such as radar or electro-optical (EO) sensors to track non-cooperative traffic. “We are working with the FAA to integrate an early version of ACAS Xu on to the Predator B,” says Brandon Suarez, lead for sense-and-avoid activity at GA-ASI.
Flights are planned for this month or next at Edwards AFB, Calif., using a company aircraft and NASA's Ikhana (also a Predator B) on a self-separation test that will also provide risk mitigation for a flight-test of prototype ACAS Xu logic planned with the FAA for 2014, using ADS-B to track cooperative intruders and alternative surveillance sources for non-cooperative traffic.
GA-ASI is developing an active, electronically scanned array “due regard” air-to-air radar on internal funds. “We flew a prototype on a Predator B and later this year will wrap it up into the self-separation test,” he says. “All the sensors and hardware will fly on the Predator B so we can get early maturation on surveillance and maneuvers.”
GA-ASI's vision for self-separation is to “keep the pilot in the loop” and help them make better decisions. “Collision avoidance could be done automatically, but we would still give the pilot a chance to respond. In a situation like losing the data link, the aircraft would have to respond automatically,” Suarez says. Automatic versus pilot-in-the-loop is “still being worked on” for ACAS Xu.