FAA regulations require that pilots be able to "see and avoid" other aircraft, but UAVs don't have pilots on board. Thus, when a UAV flies through national air space, a "chase plane" must follow it, or a trained ground observer watches it, with strict requirements for how close the observer must be at all times. This also means that drones can't be flown at night.
The Army and the Air Force are developing "sense-and-avoid" systems designed to automate these maneuvering functions. The military branches have different mission requirements for UAVs, including size of the vehicles and potential payloads. Rather than pursuing their own solutions, however, they're working together on short- and long-term technologies.
The Army is taking the lead in developing a ground-based sense-and-avoid (GBSAA) system. Earlier this year, the Army's Unmanned Systems Airspace Integration program held a two-week demonstration at the Dugway Proving Ground in Utah to validate the system's design and functionality.
[ DARPA's competition to develop a more sophisticated spy drone ended with disappointing results. Read about it here: Drones Fail 'Perch And Stare' Contest. ]
Army drones are generally smaller, lighter, and fly at lower altitudes than those used by the Air Force, which presents a design challenge for adding radar, sensors, and power equipment. "For the most part, our operations are taking off from an airfield and operating in restricted airspace," said Viva Austin, product director for Army unmanned systems airspace integration concepts.
The Army's ground-based program is further along than the Air Force program, which is developing an airborne system, Austin said. The Air Force issued a "sources sought" announcement on July 3 for a "sensor fusion and avoidance maneuver product." The Air Force wants the technology to be "sensor- and platform-agnostic."
"Our larger aircraft have longer ranges," said Paul Schaeffer, the Air Force's air borne sense and avoid (ABSAA) program manager. For example, a Global Hawk UAV might take off from a base in California and cross the Pacific Ocean at an altitude of 60,000 feet.
The airborne sense-and-avoid system will incorporate two types of sensors, including cooperative sensors that work with the Traffic Collision Avoidance System used in civil aviation and with the Automatic Dependent Surveillance-Broadcast (ADS-B) system that is being developed as part of the FAA Next Gen air traffic system. The other sense type is an electrical optical system that looks for other aircraft and provides tracking information to the centralized computer on the UAV.
Ground and airborne systems have their strengths and weaknesses, Schaeffer said. The ground system is geographically constrained, because radar is only able to see a limited distance. On the other hand, an airborne system can go virtually anywhere, but lacks visibility as it's approaching the ground because the sensors are looking down at the clutter. "So the systems generally are designed to work collaboratively," he said.
Sensors that detect other aircraft already exist. The work underway focuses on the development of software that coordinates the information provided by the sensors, then plotting course and altitude to avoid collision.
"One of the things we found is that the math behind the algorithms always does the most efficient thing. It determines probabilities, looks ahead in time," and acts on the safest and most efficient course of action, Austin said. The drawback is that those mathematically derived actions aren't necessarily the same ones that would be made by human pilots.
"The system will only do what it's told to do," Schaeffer said. "A pilot can be more creative, but can also forget something. The system will never fall asleep, never look out the window. Some pilots will tell you it's advantageous to be able to make it up on the fly, but I would say it wouldn't be necessary if they followed the rules to begin with."
Austin and Schaeffer agreed that a ground-based system is a short- to intermediate-term solution, easier to implement because it doesn't have the same size, weight, and power restrictions. An airborne sense-and-avoid system that can work on all classes of UAV is a longer-term solution, as the technology developed for the Air Force gets reengineered to be smaller.
Other federal agencies interested in UAV sense-and-avoid systems include the U.S. Border Patrol, Commerce Department, and NASA, and there is commercial interest as well, according to Schaeffer and Austin.
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