Unmanned aircraft systems (UAS) are rapidly passing their initial "cool stuff for the military" phase. In fact, interest in UAS is growing worldwide, and many new civil applications are envisioned, such as homeland security, border protection, and crime scene surveillance, as well as commercial operations. In agriculture they could be used to check blight in crops and measure moisture levels in soil. UAS could add a whole new wrinkle to airborne photography, taking pictures in spaces otherwise inaccessible to manned aircraft.

Some operations, such as military training and border protection, are already being conducted. However, today these applications can be carried out only in compartmentalized airspace. There are operational, policy, and technical issues that prevent routine integration of UAS into civil airspace, and these must be solved if the industry is to grow.

Since 2004, MITRE has been researching potential consequences of unmanned aircraft operations in the National Airspace System. As new technologies like UAS develop, we work on an ongoing basis with the Federal Aviation Administration (FAA) to safely integrate them into the nation's airspace. The biggest problem in allowing unmanned systems in manned airspace is the ability to avoid collision with airborne objects, especially manned aircraft.

"The fundamental question is whether UAS can perform a 'sense-and-avoid' function that meets or exceeds the currently accepted 'see-and-avoid' capability of the human pilot," says David Maroney, a MITRE lead systems engineer.

The electric-powered Rotomotion SR-20 was acquired with a pan/tilt video camera mounted and integrated into a single downlink data stream. It's capable of fully autonomous flight with a safety operator to perform takeoff and landing and to engage and disengage the autonomous flight control system.

Maroney is working with Robert Bolling, a lead simulation modeling engineer who works on ground robotics, and on MITRE-sponsored research to see what kind of sensors could be used on UAS that would allow them to fly in manned airspace. Maroney and Bolling are focusing on small UAS that fly at low altitudes in uncontrolled airspace.

"Our research is intentionally focused on small UAS missions for remote sensing with a payload limitation of ounces to pounds," says Bolling. "It's reasonable that a solution may be scalable to larger UAS, although different missions and conditions may affect the scalability. Rather than scaling up, most approaches today are aimed at large UAS with the hope of scaling them down."

A Three-Part Problem

It's a difficult problem because of the wide range of UAS sizes, speeds, and maneuverability. Different kinds of sensors are available, but they vary in their ability to measure distance or angle to an obstacle, as well as closing rate and time to collision.

The operating environment for these UAS is expected to be in uncontrolled civil airspace where pilots use visual flight rules to see and avoid other aircraft or obstacles. "UAS operations in this airspace could encounter a variety of airborne obstacles such as small manned aircraft without transponders," explains Bolling. "Therefore, this research examines sensor-based solutions that don't use transponders where both aircraft cooperate in avoiding each other."

The team breaks the problem down into three parts:

1. Sense—Sensors on the UAS continuously collect data about the airspace.
2. Detect—Onboard computer determines if the data indicate a collision in the near future.
3. Avoid—Onboard computer calculates an action to avoid the collision.
   

These actions are passed to the flight manager for the UAS, directing it to make an immediate maneuver.

"We're putting the onus on the UAS to sense where the target is and avoid it," says Maroney. "The sensors have to be appropriate for identifying another plane, small or big, and give the UAS enough time to avoid a collision. Current small sensors can't consistently 'see' far enough. They are reasonable for detecting fixed or slow moving obstacles, but they aren't sufficient for fast moving or approaching obstacles.

"No one sensor provides a UAS with all the information it needs to avoid a collision. Video data will give great angular information, both in altitude and in azimuth [number of degrees from straight ahead], so you can see that the airplane is there. But it doesn't give good distance information —how far away it is, or how quickly the UAS is closing on it. Other sensors, like laser range finders, give good distance information. But they don't give good angular information [for direction and height] unless you're using a fully scanned laser range finder. And those are too heavy for our application."

Sensor Technologies to the Rescue

In order to find the best solution, the MITRE researchers are examining a variety of possibilities. Some of the sensor technologies tested by the team include:

• Video/electro-optical solutions provide only bearing or direction information, but no range information. Also, the detection algorithm requires a huge processing load. In addition, these sensors have to identify an object and react, even at slow approach speeds. They work well only in clear weather; performance degrades in any condition other than light and clear.



• Laser range finders measure distance accurately up to 1500 feet. These devices have a narrow bearing window unless they're in a scan mode; however, they work well when you know where to point them. They provide accurate distance information over large distances and are mostly a clear weather solution.



• Ultra-wideband radar sensors are limited in their range. They have a wide sector of detections (30-40 degrees), but give no bearing. Their range is limited by federal regulations that govern power output.



• Doppler radar can detect changes in rate but not distance. Detection areas are relatively small: they're bigger than laser range finders, but smaller than ultra-widebands. A UAS' speed is directly measured with a detection algorithm, and its distance to collision is inferred. Darkness or bad weather isn't a problem for Doppler radar.

Depending on the sensor, the sensing area and range of the sensor varies relative to the aircraft.

Maroney and Bolling found that radar promises to be the best supplement for electro-optic solutions, and will continue to research this area. They are also doing independent tests with acoustic detectors because there is a small but intense industry interest in these devices.

"We discovered there's a trade-off between sensor capabilities (sense/detect) and UAS capabilities (avoid)," explains Maroney. "Both drive overall sense-and-avoid requirements. Now, we're focusing on short timeframe reactive paradigms, not long lead collision-avoidance planning."

The team will continue to develop sense-and-avoid requirements that dovetail with other MITRE research on collision avoidance. "We'll also factor sense-and-avoid requirements into FAA airspace integration studies," says Maroney. "We believe some of our research may also be useful for research planning for the Next Generation Air Transportation System."

—by David A. Van Cleave

Related Information

Articles and News

• UAT Puts UAVs on the Radar
• MITRE Develops Portable ADS-B Radio for Small Unmanned Aircraft
• Collision Avoidance for UAS

Technical Papers and Presentations

• Sense and Avoid for Small Unmanned Aircraft
• Lightweight, Low-Cost ADS-B System for UAS Applications
• Collision Avoidance for Unmanned Aircraft: Proving the Safety Case

Websites

• MITRE Research Program Overview
• MITRE Research: Sensors and the Environment
• MITRE's Center for Advanced Aviation System Development (CAASD) FFRDC

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