Investigating Collision Avoidance for Small UAS using Cooperative Surveillance and ACAS X

Abstract

Small Unmanned Aircraft Systems (sUAS) have proliferated over the last decade. While these platforms offer many benefits to society, they pose a dangerous mid-air collision hazard. In order to safely integrate into airspace shared by other users, sUAS must be able to avoid collisions with manned aircraft.

To better understand sUAS flight behavior and inform Collision Avoidance (CA) systems for sUAS, over 600 active UAS platforms were reviewed. The mean climb rate capability was 720 feet per minute (fpm) for all reviewed sUAS, which suggests that CA systems currently used by manned aircraft (which require 2,500 fpm climb capability) would be inappropriate for implementation on sUAS. Novel CA systems are therefore required.

Next, to assess the feasibility of CA system equipage on sUAS, the Size, Weight, Power, and Cost (SWaP-C) of equipment necessary for CA systems were studied. It was determined that a complete CA system utilizing cooperative surveillance could weigh less than 70 grams and require less than 2 W of average input power. Because cooperative surveillance broadcasts from sUAS could overload the spectrum currently used to share aviation information, signal transmissions were simulated for a population of sUAS broadcasting alongside current users. While transmitting sUAS would quickly degrade performance on the busy 1090 MHz channel, the 978 MHz channel could potentially support about 1 transmitting sUAS per square kilometer if sUAS broadcast ADS-B signals at only 80 mW.

Finally, close encounters between sUAS and manned aircraft were simulated in the Mode C Veil environment to evaluate threat resolution options used by different CA systems. Manned aircraft using existing CA systems to avoid sUAS would achieve extremely high levels of safety (risk ratios below 0.05) but would experience high rates of alerts. Furthermore, sUAS are so small that manned aircraft without CA systems would be unlikely to visually see and avoid them. Novel CA systems were modeled on sUAS and were able to avoid manned aircraft with currently-accepted levels of safety (risk ratios below 0.18) even with limited or no vertical maneuvering by using horizontal escape maneuvers (i.e. turns). Alerting rates for horizontal maneuvers were high but may be acceptable for use on sUAS. The new sUAS CA systems cooperated well with existing systems for manned aircraft and resulted in extremely low collision risk (risk ratios below 0.02) in encounters where manned aircraft and sUAS both took action to avoid collisions.

Results therefore indicate that sUAS could utilize existing cooperative surveillance systems and prototype CA policies to mitigate close encounters with manned aircraft in Mode C Veils at safety levels that are currently accepted among manned aircraft.