Collision Avoidance for UAS

By Bob Bolling, Dave Maroney, and Andy Zeitlin

There are many obstacles to allowing routine operation of unmanned aircraft systems (UAS) in civil airspace. However, one of the greatest engineering challenges is a technology for collision avoidance that mimics an on-board pilot's ability to "see and avoid" other aircraft, while also being compatible with existing collision avoidance technology.

This is not only important for routine access to civil airspace by UAS but also for military unmanned aircraft in theaters of operations. Currently, manned aircraft are largely excluded from airspace where unmanned aircraft are operating. For manned and unmanned military aircraft to operate in the same airspace in combat, we will need highly-reliable collision-avoidance solutions.

MITRE is exploring a variety of collision-avoidance technologies, mechanisms to assess safety risks, and issues associated with integrating into existing collision avoidance systems such as the Traffic Alert and Collision Avoidance System (TCAS). We are also investigating ways to prevent collisions with non-transponding aircraft, including the innovative use of passive and active sensors.

Transponding Collision Avoidance

TCAS II (known internationally as Airborne Collision Avoidance System or ACAS) is an airborne system that uses range and altitude data to detect potential collision threats and to suggest evasive vertical maneuvers to the pilot. In an effort to reduce mid-air collisions, the International Civil Aviation Organization (ICAO) has recommended that all commercial aircraft world-wide should be equipped with TCAS II if they weigh more than 5,700 kg (approximately 12,500 lbs) or are authorized to carry more than 19 passengers. This is the current standard in Europe. In the United States, TCAS II transponders are required in all air carrier aircraft having more than 30 passenger seats or weighing more than 33,000 lbs.

The variety and distinctiveness of unmanned aircraft make installing TCAS II problematic. Some of today's UAS are not as maneuverable as piloted aircraft and cannot climb or descend at the rates assumed by TCAS II. Also, TCAS is designed as an alerting system for pilots. Given that an unmanned aircraft pilot is not located on board the aircraft, there is some added degree of latency associated with the pilot's decision, his flight control commands, and the aircraft's response.

MITRE is tapping its extensive experience with aircraft collision avoidance (dating back to the very beginnings of TCAS) to explore collision avoidance solutions for UAS. We are leveraging advanced simulation and visualization capabilities to refine UAS collision detection systems requirements. MITRE's models can simulate literally millions of potential encounters by modeling air-to-air surveillance, collision warning advice, data links to a remote operator, and vehicle response.

This analysis capability can be used to evaluate collision avoidance architecture alternatives such as autonomous response to advisories (i.e., direct on-board coupling of collision avoidance capability with flight controls) and the addition of surveillance data from sensors that can detect non-transponding aircraft.

Non-Transponding Collision Avoidance

In addition to avoiding collisions with aircraft that are transponding, there is a fundamental requirement for a pilot to see and avoid objects and other aircraft under nearly all circumstances. Aircraft can also communicate position information with Automatic Dependent Surveillance-Broadcast technology, which transmits an aircraft's position in three dimensions (e.g., GPS horizontal position and barometric altitude). Thus, unmanned aircraft would have to either actively or passively sense potential targets, detect whether a collision risk exists, and determine an appropriate maneuver to avoid a collision.

Given that small UAS will be operating in airspace with a significant number of non-transponding aircraft, an aircraft's size, weight, and power for sense and avoid solutions will be significant issues. Unmanned aircraft must behave in a predictable fashion when executing an avoidance maneuver if they are to be permitted to fly with manned aircraft.

If manned and unmanned aircraft are going to operate in the same airspace, there must be a capability to avoid collisions. For civil airspace access, collision avoidance solutions must be highly reliable, ensuring that they satisfy a specific level of safety that is roughly equivalent to manned operations.

Autonomous Collision Avoidance

To help unmanned aircraft meet the see and avoid requirement, MITRE is investigating solutions and evaluating technologies that would be appropriate for collision avoidance with non-transponding aircraft. The MITRE project involves integrating sensor technology and developing prototype algorithms, then testing with computer simulation, bench-testing in the lab, and flight testing in the field using a fleet of home-grown UAS platforms.

MITRE's study investigates various sensor technologies, including optical, thermal, sonar, radar, and laser. We will test selected sensors in the prototyping environment; study detection and avoidance geometries and methods, in part based on TCAS; and develop quick reactive algorithms.

Collision avoidance is emerging as a key enabler to UAS civil airspace access, as well as an important capability for the integration of manned and unmanned missions in military theaters of operation. Most experts believe that UAS collision avoidance capabilities must be interoperable and compatible with existing collision avoidance and separation assurance capabilities (including TCAS), and must meet the requirement for a pilot to see and avoid other aircraft.

MITRE is working with the aviation community and leveraging its expertise in collision avoidance technology to help develop the appropriate enabling solutions and provide the best technical advice to our sponsors.

For more information, please contact Bob Bolling, Dave Maroney or Andy Zeitlinusing the employee directory.

Solutions That Make a Difference.®
Copyright © 1997-2013, The MITRE Corporation. All rights reserved.
MITRE is a registered trademark of The MITRE Corporation.
Material on this site may be copied and distributed with permission only.

Privacy Policy | Contact Us