Detect and Avoid (DAA) Requirements Explained According to Airspace and ACAS Classifications

Figure 1: ATM, UTM and ETM Airspace (Source:

Detect and Avoid (DAA) requirements for aircraft are continuously emerging and right now, there is no “one size fits all” requirement for equipment on unmanned aircraft. Instead, manufacturers and operators should choose equipment based on their concept of operations (CONOPS) and the operating rules under which they will fly.  This can be approximately aligned with the airspace management scheme:

  • Under 400 ft AGL will be managed under the Unmanned Traffic Management (UTM) system and generally operate under the new remote ID rules of 14 CFR Part 89
  • Between 400 ft AGL and 60,000 ft MSL will be coordinated through Air Traffic Control (ATC) in the Air Traffic Management (ATM) system and will usually operate under Part 91 or Part 135.
  • High altitude operations above 60,000 feet MSL are managed per the ATM, although work is being done to more clearly define airspace management processes for the Upper Class E Traffic Management (ETM) system as technology evolves to create more traffic in this area.

Detect and Avoid (DAA) and Airborne Collision Avoidance Systems (ACAS)

With the lack of a pilot onboard to “see and avoid” other aircraft, UAS will depend on certifiable DAA solutions to ensure airspace safety.  RTCA is currently generating standards for the FAA which define the Detect and Avoid requirements based on the Airborne Collision Avoidance Systems (ACAS).  For UAS, three version of ACAS are defined or in development:

  • ACAS-Xu – Minimum Operational Performance Standards (MOPS) DO-386 were released in December, specifying logic-based hardware requirements and algorithms for recognizing and avoiding potential collisions for fixed wing UAS operating under Part 91 or Part 135 rules. Certifiable systems will need to adhere to DO-386.
  • ACAS-Xr – ACAS Xu will be adapted for rotorcraft.
  • ACAS-sXu – will define requirements used in DAA systems of small UAS getting type certifications flying under Part 89.


Figure 2: ACAS Xu (red), sXu (blue) and Xr (green) (Source: FAA CONOPS for ACAS-sXu, 2020)

DAA Systems Maximize Airspace Safety

In the language of Detect and Avoid there are two types of traffic: cooperative and non-cooperative.  Cooperative traffic is similarly equipped, and ACAS-based DAA systems minimize the risk of collision by providing for two-way avoidance coordination between cooperative aircraft.  As traffic approaches a situation where “well clear” may be violated, evasive action is coordinated, i.e. one aircraft climbs and one descends.  With non-cooperative traffic, there is a higher risk of conflicting avoidance maneuvers, so a larger well-clear buffer must be maintained.  By recognizing and coordinating with cooperative traffic, DAA systems greatly reduce the risk of collision.


Figure 3: ACAS-Xu and ACAS-Xr based DAA systems

DAA Equipment – What’s Needed and What’s Certifiable

For UAS operating under Part 91 or Part 135, which will generally be 55 pounds or greater, flying beyond visual line of sight (BVLOS) in the National Airspace System greater than 400 ft AGL, visual or simple sensor-based systems will not be enough. Rather, the FAA will require a logic-based, certifiable ACAS-based DAA system that includes the following components:

  • Radar for long-range all-weather detection of non-cooperative traffic
  • An ADS-B receiver with active surveillance interrogator for validation of ADS-B tracks (required due to the possibility of ADS-B spoofing)
  • A transponder for coordination of deconfliction maneuvers
  • A DAA computer, containing a surveillance tracking module for traffic sensor fusion from the radar and validated ADS-B sensors, and a threat resolution module to determine conflict and provide warnings and avoidance commands to either the RPAS operator or flight control computer.

Meanwhile, smaller UAS flying at low altitude will operate under Part 89 and may equip with ACAS-sXu based DAA systems, which will likely utilize different technologies including the emerging remote ID technologies and dual-band ADS-B receivers for traffic sensing and coordination.

Current Status of Detect and Avoid Solutions

Sagetech currently sits on the standards committees to help shape the Detect and Avoid requirements for certification and technology standards, which are all currently emerging.  The FAA and EUROCAE recently approved the MOPS for ACAS-Xu, which provides standards for unmanned aircraft to achieve type certification and avoid the costly and time-consuming waiver process for BVLOS flight.  Standards for ACAS-sXu (small UAS) are expected in mid-2022, with standards for ACAS-Xr (rotorcraft) to follow.  Each of these will form a critical part of DAA solutions.  Standards for certified DAA (MOPS DO-365) which leverage ACAS solutions as part of a full system are complete for operations in Class A airspace.  Extensions for transiting Class B airspace and operations in Class D, E and G airspace are expected soon.

Sagetech's Detect and Avoid SolutionMeanwhile, Sagetech is working with numerous fixed and rotary wing UAS customers who are incorporating certifiable DAA systems in their design and type certification projects right now. Sagetech is helping these customers with regulatory guidance, component technology solutions including transponders and interrogators, and system design to ensure these systems in development will satisfy the anticipated certification requirements. Sagetech itself, through technology development and partnerships with companies including Hover Inc., expects to offer a complete DAA prototype system by the end of this year.


View Sagetech’s DAA Solutions.