Job No. 1: Detect and avoid
By Debra Werner|May 2017
Read the rest of our coverage from May’s special report on drones.
If engineers succeed in winning FAA approval for sensors to prevent unmanned aerial systems from crashing into buildings and other aircraft, they will be clearing the way for UAS to perform jobs ranging from pipeline monitoring to package delivery. For now, UAS developers are wrestling with the challenge of making onboard detect-and-avoid technology small and lightweight enough to fit in the fastest growing segment of the market — UAS under 55 pounds.
I discussed detect-and-avoid technology and its implications for aircraft design with Jay Gundlach, a pioneer in UAS design, and Michael Guterres, a leader in efforts to integrate drones in national airspace. Gundlach established his own firm, FlightHouse Engineering, in 2016 to help commercial and government teams create unmanned aircraft of all sizes to perform specific missions. Guterres leads an FAA initiative to help BNSF Railway find ways to use drones to safely inspect tracks far beyond the view of their operators. Guterres also leads a multi-institution research partnership focused on integration of small drones in urban areas.
Q: What is challenging about developing detect-and-avoid systems for large and small unmanned aircraft?
Michael Guterres: Most of the larger unmanned aircraft that fly at higher altitudes are optionally piloted modified manned aircraft, military derivatives or military systems altogether. Those tend to have a lot of capacity in terms of internal volume, power and the ability to carry systems and equipment. Also, they fly in airspace that requires some type of equipment onboard. Smaller UAS fly at lower altitudes and have a different set of challenges: the necessity to avoid other aircraft, but also to negotiate ground obstacles such as buildings, cranes, trees and even people. Those small, light unmanned aircraft do not have the ability to carry or power a lot of equipment.
Q: What is happening in the large UAS category?
Guterres: There has been a pretty significant standards development effort. The RTCA [Radio Technical Commission for Aeronautics] brought together industry and government to develop a set of requirements and performance thresholds for larger aircraft, like Predator and Global Hawk, to transition to Class A airspace [above 18,000 feet]. Those standards, when published, will make it easier for operators using some of these larger systems to transition through those first 18,000 feet to Class A airspace. That transition phase from ground to 18,000 feet is a little bit riskier [than flying above 18,000 feet] because there are a lot of folks flying in that airspace that are not necessarily equipped. The UAS has a responsibility that normally would be handled in a manned aircraft by the pilot and co-pilot. When the aircraft are in Class A, they work with air traffic control. Some of the larger unmanned aircraft are able to carry TCAS [Traffic Alert and Collision Avoidance System], a system used by manned aircraft, and also onboard radars.
Q: Is it more challenging to provide detect-and-avoid capabilities for smaller unmanned aircraft?
Guterres: In some ways, yes. There are no defined performance requirements, no published standards for what a detect-and-avoid system should do around buildings, people, UAS, and to avoid manned aircraft. On the small side, I think you’ll have a combination of onboard systems with ground systems as well. For example, small UAS flying over infrastructure, pipelines or a rural railroad, where you have very little air traffic activity and low population density, may be able to use one type of detect-and-avoid solution. For UAS flying in a different type of environment, urban or suburban, close to people and buildings, the threshold for technology performance will be elevated. Then you may need different types of detect-and-avoid systems, onboard, ground or a combination of both. The technology solution may also depend on the operational concept and the risk level.
Q: In terms of onboard systems, what options are there for small UAS?
Guterres: There are quite a few that have been experimented with, from onboard radar to visual systems, using cameras and interpreting the imagery to identify objects and other aircraft, to acoustic sensors. These sensors are looking for aircraft that are not engaging with you. On the cooperative side, you have electronic communications: ADS-B [automatic dependent surveillance-broadcast] or Mode S transponders. If you have multiple aircraft using transceivers to communicate with each other, then they can communicate speed, heading and altitude electronically. This makes it a little bit easier and safer, but there is no mandate for all aircraft at low altitude to have those. So, you have to contend with other aircraft that are cooperating and those who are not.
Q: How do aircraft designers integrate detect-and-avoid systems in unmanned aircraft?
Jay Gundlach: There are a few different design considerations. The primary considerations are size, weight and power. Especially for small unmanned aircraft that weigh less than 55 pounds, which are covered by the current [FAA] Part 107 regulations, you really don’t have that much capacity. So, for example, a lot of unmanned aircraft tend to have payload capacities of about 5 to 20 percent of the takeoff gross weight. For a 50-pound [23 kilograms] unmanned aircraft, that’s only 2.5 to 10 pounds of payload. For a 10-pound UAV, that’s half a pound to 2 pounds of payload. So a 2-pound detect-and-avoid system has a tremendous impact on those vehicles.
Other considerations are the required field of regard, all the azimuth and elevation angles that the sensor can view. Then, there’s the field of view, where the sensor can see at any given instant. For a small unmanned aircraft operating at less than 400 feet, most of the air traffic would be above it. It would not need to look below for collision risk. For low altitude flight, it might need a bump on top of the aircraft that might look like a satellite communications antenna dish that you might see on a Predator or Global Hawk. But if the unmanned aircraft is flying at higher altitudes, it might need to look below itself as well. If it is a slow-moving unmanned aircraft, things may come at it from any orientation, including from behind. A fast-moving unmanned aircraft probably wants to look more in front. All these considerations dictate what the sensors need to see.
Another consideration is what kind of sensor the unmanned aircraft is operating. Is it RF-based or an optical sensor? If the sensor has line-of-sight obstructions from the wings, tails or fuselage, that can block where it can look. For a radar-based system, it might have some nonintuitive, non-line-of-sight interactions from the aircraft, especially if it is operating at low frequencies. There are a lot of competing requirements.
Some sensors may also be transponders, like ADS-B. The designer may have to consider frequency computability with other essential functions, such as the command-and-control link and the payload downlink. To a large extent, installing a detect-and-avoid system is not that much different than trying to install other payload types.
Q: Will every unmanned aircraft model need its own detect-and-avoid solution?
Gundlach: The detect-and-avoid system integration may be airframe-specific. For example, camera-based systems will need to provide a sufficient number of sensors with the correct positioning and orientation on the airframe to provide the necessary field of regard. The most convenient locations may be blocked by elements of the airframe, which may necessitate redesign. In contrast, an aircraft that is designed to accommodate a detect-and-avoid system upfront may avoid these difficulties.
Q: Is it better to think about detect-and-avoid capabilities as you design the aircraft?
Gundlach: Correct. Otherwise you can picture some of these sensors located remotely to the primary aircraft structure in order to get the required field of regard. Or having to locate multiple sensors to avoid obstructions, for example.
Q: As you develop and test detect-and-avoid technologies, are you focusing on size, weight and power?
Guterres: Size, weight and power are critical. Typically, you start with an existing UAS. The aircraft is a piece. You don’t often have the flexibility to modify the aircraft to accommodate some piece of equipment. So, system weight and power requirements are critical. That often becomes a funnel for picking options. Then you integrate it, figuring out how it works and make the best of it. However, if there is a significant enough impetus, there may be cases where the aircraft is modified to accommodate a certain system onboard.
Small unmanned aircraft systems bring a lot of advantages: low cost, easy transportability, and flexibility as far as supply chain. It has not been our observation that people are moving to bigger systems to accommodate onboard technology. There is an expectation that the onboard technology has to get smaller, lighter and use less power.
If you start with an existing aircraft, you are pretty limited in what you can do. If you can modify the aircraft, you are a little bit more able to accommodate things. Still, you quickly bump up into a load factor. If you add 10 pounds to the payload, you are most likely going to add quite a few more pounds to the aircraft itself. Staying with small aircraft is a very important thing, in general, to the operators we come across.
Gundlach: Larger detect-and-avoid systems can also make a difference in overall risk. If we were to add detect-and-avoid systems to an unmanned aircraft, then to achieve a similar level of performance in terms of payload capacity and endurance, we would need a larger aircraft. Now that new aircraft is less likely to collide with other air traffic. However, the consequences if there was an impact would be greater because it’s a heavier aircraft. Also, the risk to people on the ground may go up as well.
Generally, there is a trend toward unmanned aircraft going down in size, but depending on what the detect-and-avoid technology is, this could be a counterpressure that pushes toward larger and heavier aircraft.
Guterres: It’s a very interesting double-edged sword. An airplane can be a little heavier and much, much safer because of the technology you put on it. There is a bit of a fine balance.
It has not been our observation that people are moving to bigger systems to accommodate onboard technology. There is an expectation that the onboard technology has to get smaller, lighter and use less power.
Michael Guterres, Mitre Corp.
The FAA published rules in June 2016, known as Part 107, for drones weighing less than 55 pounds. They permit such aircraft to perform commercial jobs during the daytime at a maximum altitude of 400 feet and close enough for the operator to see it. The operator must pass an aeronautical test and receive an FAA remote pilot certificate. Firms can apply for FAA permission to waive the restrictions.
