UAS Integration in the NAS: Detect, Sense and Avoid

See and avoid is a concept to abate aircraft collisions. Integration of air traffic, in different classes of airspace and operating under different rules, rely on it to provide a safe flight environment. It is preferred that Unmanned Aircraft Systems (UAS) have the same ability when it comes to see and avoid; however, it is supplemented with the phrases detect or sense, and avoid. Information that governs see and avoid (SAA) are found in the 14 Code of Federal Regulations (CFR) and numerous products produced by the FAA and organizations like: Radio Technical Conference of Aeronautics (RTCA). These standards are applied to UAS because they need to satisfy the same standards as manned aircraft for proper integration

Regulations

The 14 CFR, Federal Aviation Administration Regulation, Parts 91.111, 91.113 and 91.115 (water) represent the main guidance for Sense and Avoid (Electronic Code of Federal Regulations, 2017). Specifically, Part 91.113 states that “When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft. When a rule of this section gives another aircraft the right-of-way, the pilot shall give way to that aircraft and may not pass over, under, or ahead of it unless well clear" (Skybrary, 2016). Right of way rules are a set of standards or prescribed maneuvers that aid the pilot in executing the safest and most effective method to avoid a collision. They are defined according to certain categories of operation and are used to justify giving way to slower moving objects in the aerospace environment. These protocols are standard operating procedures for all pilots. The Radio Technical Conference of Aeronautics (RTCA) defined UAS see and avoid as: “The ability of a pilot to see traffic which may be a conflict, evaluate flight paths, determine traffic right-of-way, and maneuver to avoid the traffic” (FAA, 2009). Guidance for UAS operating in the NAS is given in FAA Order 7610.4K with the intention that UAS operations provide an equivalent level of safety to that intended by Title 14 CFR Part 91 requirements for manned aircraft SAA (FAA, 2009).

Layered Defense to Collision Avoidance

See and avoid is all but one of the methods used to de-conflict traffic from sparse to high-density air traffic environments, with others being procedural control, specific vectors or traffic advisories from a controlling agency’s radar depending on airspace class and position reports from the aircraft themselves (avoidance for non-cooperative traffic), and notifications from traffic avoidance systems that like users have from TAS, to TCAS and ADS-B (defined as cooperative traffic) (Bergqvist, 2017, NASA Access 5, 2008, Rosenkrang, 2008, & Skybrary, 2016).

Figure 1: UAS Safety Layers Under Study for Collision Avoidance. Rosenkrang, Wayne. 2008. Flight Tech: Detect, Sense and Avoid. Aviation Safety World Magazine. Retrieved from http://flightsafety.org/asw/july08/asw_july08_p34-39.pdf?dl=1

Currently, see and avoid is the last line of defense in a layered approach to prevent a collision. Sometimes, it is used in coordination with the previously mentioned methods to confirm if and when a maneuver needs to be executed. Depending on the rate of closure and position of the converging aircraft, that maneuver can be very time-sensitive and aggressive in execution, especially when prior notification is not available (from systems, pilots or controllers) and visual acquisition of the converging aircraft occurs late. Even though technology has matured enough to execute avoidance maneuvers in the layers before see and avoid needs to be executed, in manned aircraft it still remains a viable method in case those other layers fail (TAS, TCAS or ADS-B).

Figure 2: Traffic Separation Layers. NASA Access 5. 2008. Collision Avoidance Functional Requirements for Step 1. Retrieved from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080017111.pdf

Current & Future Implementation of DSA Technology For Collision Avoidance

Current testing has centered on using manned aircraft and Next Gen technologies to execute Detect/Sense and Avoid (DSA) actions. Detection and sensing is more appropriate for UAS operations because sensors will need to denote if something is there and if it presents a threat, either to a remote pilot or the autopilot in a fully automated UAS (FAA, 2009). Non-cooperative traffic detection aims to replace the pilot seeing a traffic conflict, while cooperative sensors provide an additional capability (NASA Access 5, 2008 & Rosenkrang, 2008). Together, the combination of systems are comprised of radar, TCAS and ADS-B these sensors represent active systems to detect cooperative and non-cooperative traffic (FAA, 2009). These sensors already have certification from the FAA, which will speed up the process for NAS integration.

Future systems and specifically, smaller UASs, may see an emergence of more passive systems like electro-optical and infrared devices to define the presence of uncooperative traffic in lieu of radar (FAA, 2009). While early DSA efforts focused on single systems, more recent efforts have focused on multiple sensor that are capable of cooperative and uncooperative detection/sensing. This synergy provides a fuller spectrum to cover gaps and provide a redundant/cross-referencing capability for some attributes of DSA, see Figure 3 (FAA, 2009).

Figure 3: Technology Attributes for DSA FAA. 2009. Literature Review on Detect, Sense, and Avoid Technology for Unmanned Aircraft Systems. Retrieved from http://www.tc.faa.gov/its/worldpac/techrpt/ar0841.pdf

It will represent the new norm for medium and high-altitude long endurance UASs, but small UASs might not be able to carry the same amount or type equipment due to its smaller size and lower power generation (FAA, 2009). Thus, a solution for small UASs might be to remove the system from the UAS itself and provide more technologies (applications in GCS, ground radar or other methods) that are capable of facilitating collision avoidance to meet the detect/sense and avoid requirement. Active systems that can be further miniaturized (like ADS-B) provide an additional alternative or additive capability (FAA, 2009). Utilizing ground systems (radar and cellular towers) and ADS-B is NASA’s focus for testing and providing a complete UAS Traffic Management (UTM) system (NASA, 2017).

References:

Electronic Code of Federal Regulations. 2017. Title 14, Chapter I, Subchapter F, Part 91 – General Operating and Flight Rules. Government Publishing Office. Retrieved from https://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&sid=3efaad1b0a259d4e48f1150a34d1aa77&rgn=div5&view=text&node =14:2.0.1.3.10&idno=14

FAA. 2009. Literature Review on Detect, Sense, and Avoid Technology for Unmanned Aircraft Systems. Retrieved from http://www.tc.faa.gov/its/worldpac/techrpt/ar0841.pdf

NASA Access 5. 2008. Collision Avoidance Functional Requirements for Step 1. Retrieved from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080017111.pdf

NASA. 2017. Unmanned Aircraft System (UAS) Traffic Management (UTM). Retrieved from https://utm.arc.nasa.gov/index.shtml

Rosenkrang, Wayne. 2008. Flight Tech: Detect, Sense and Avoid. Aviation Safety World Magazine. Retrieved from http://flightsafety.org/asw/july08/asw_july08_p34-39.pdf?dl=1

Skybrary. 2016. See and Avoid. Retrieved from http://www.skybrary.aero/index.php/See_and_Avoid