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).
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
