To fly above FL 180, aircraft will need to meet FAR Part 91 operating rules and carry a transponder with Mode S or altitude reporting, so air traffic control can maintain separation using radar skin tracks and transponder squawks. That is likely to limit access to medium-to-large aircraft. “Below 50 lb. they are unlikely to carry the equipment. Below 10 lb. they are unlikely to carry the equipment or reach the altitude,” Dann says.
Although the 2012 FAA reauthorization and defense appropriations bills have put pressure on the FAA to get activity underway by requiring integration of UAS into national airspace by September 2015, Dann says “it is impossible to do full file-and-fly by 2015 in all classes. But they could open airspace above FL 180 for properly equipped aircraft if they moved out now.”
How aircraft get to and from altitude through other classes of airspace is still an issue, but the ability to fly above FL 180 “will allow us to ferry aircraft and conduct many of the commercial ventures being considered, such as forest-fire monitoring, first responders, land-use planning—maybe even traffic surveillance if we are allowed to operate in congested airspace,” he says.
Under the proposed plan, access would work down to lower altitudes and eventually to the most-congested Class B airspace around major airports. This is likely to require UAS to carry a sense-and-avoid (SAA) system, or detect-and-avoid as it is becoming known. SAA is the combination of two functions: self-separation involving gentle maneuvers minutes from collision to stay well clear of other aircraft; and collision avoidance involving hard maneuvers seconds from impact.
Sense-and-avoid is no simple problem. Development of the algorithm for the traffic-alert and collision avoidance system (TCAS) cost $900 million and took several years, and it could be the same for SAA, says Dann. TCAS was government-developed, so industry could avoid liability for collisions. “The same has to happen with SAA,” he says, arguing algorithm and system development, testing and rulemaking involve “a decade's worth of work that could cost over $1 billion,” although the U.S. Air Force could get there earlier.
“Detect-and-avoid is not around the corner. Ten years is a fair assessment for a civil-certified algorithm that works autonomously,” says Andy Lacher, UAS integration lead at Mitre. He also agrees that liability for the logic should rest with the government. “I would suggest it is the right way to go, but I don't know who the decision-maker would be.”
Mitre is working with NASA Langley, University of North Dakota (UND) and Draper Laboratory to test a cooperative automatic sense-and-avoid system using automatic dependent surveillance-broadcast (ADS-B) as the sensor. The test involves flights of a NASA Cirrus SR22 surrogate UAS to test SAA algorithms developed by Draper, Mitre and UND.
“We are focused on ADS-B as the sensor because it is very accurate, with a high update rate. If the algorithm will not work with the best, it will not work with a less accurate sensor, or lower update rate,” Lacher says. “And we're focused on an automatic algorithm. An unmanned aircraft, with loss of link, needs a capability to detect and avoid without the pilot.”
Flights last September involved simple encounters. Tests last month involved more complex and dynamic encounters, one aircraft turning or climbing into conflict with another. A final series of flights this month will look at using primary radar tracks-broadcast via the traffic information system (TIS-B). “The FAA will never accept ADS-B only. But coupled with primary TIS-B it may help in a layered approach,” Lacher says.