Safety analysis of TCAS on Global Hawk using airspace encounter models

Abstract

The U.S. Air Force's RQ-4 Global Hawk unmanned aerial vehicle (UAV) is a high altitude, long endurance aircraft used for surveillance and reconnaissance. Because of the potential for close proximity to manned aircraft in civil airspace, collision avoidance is a major concern, and the Air Force is seeking to equip Global Hawk with the Traffic Alert and Collision Avoidance System (TCAS) to reduce the probability of mid-air collision. Currently, Global Hawk is equipped with a Mode S transponder and uses chase aircraft, ground observers and/or ground radar contact to comply with the collision avoidance requirement. In order to evaluate TCAS effectiveness, a fast-time simulation tool has been developed at MIT Lincoln Laboratory that computes the mean probability of a near mid-air collision for a large number of close encounters between two aircraft. Airspace encounter models enable sets of encounters to be simulated that are statistically representative of the aircraft encounters that actually occur in the airspace. The TCAS logic is implemented in the simulation tool and the aircraft responses during the encounters, with and without TCAS, are simulated in parallel.

(cont.) By observing measured vertical miss distance at the closest point of approach between the two aircraft, it is possible to quantify the reduction in collision risk provided by TCAS, termed the risk ratio. Global Hawk's flight characteristics differ from a conventional aircraft. Its mission profile through civil airspace includes slow, steep climbs and descents, and shallower turns than a conventional aircraft. Its vertical acceleration and climb rate limits can hinder its response to a TCAS resolution advisory (RA). Communication latency also may occur. For this thesis, encounter models were developed that reflect Global Hawk's flight characteristics. The new encounter models were then implemented in the simulation tool, and millions of encounters between Global Hawk and a conventional aircraft were simulated. These encounters were compared against encounters between two conventional aircraft to observe how Global Hawk's flight characteristics changed the effectiveness of TCAS. Assuming a standard pilot response to TCAS RAs, TCAS provided a significant safety improvement to Global Hawk over a Mode S transponder alone, yielding risk ratios in the range of 0.003 to 0.079.

(cont.) Global Hawk's flight characteristics generally caused a decrease in TCAS effectiveness from the original encounter models. Encounters were also simulated where Global Hawk's response to TCAS RAs was delayed by increasing amounts to simulate communication latency. A delay of approximately 15 seconds was tolerable before TCAS provided less safety than a Mode S transponder alone.