ACAS X: Whole Airspace Protection from NASA

Very quietly this December, the Radio Technical Commission for Aeronautics (RTCA) published the new DO-386 standard.  However, for those of us serving up situational awareness for unmanned aerial vehicles (UAVs), we are loudly celebrating this achievement. For the first time, DO-386 defines the Minimum Operational Performance Standards (MOPS) for unmanned aircraft flying with ACAS Xu, setting the stage for true detect and avoid (DAA) system development and deployment and integration of UAS in the NAS.

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Sagetech sits on the two committees responsible for this landmark publication, the SC-147 ACAS committee and the SC-228 unmanned aerial vehicle committee. Rather than toss more acronyms your way, David Day, our Chief Compliance Officer, outlined these key takeaways to help the industry understand this truly historic event.

1.       First MOPS ever for ACAS Xu.

In addition to the traditional Airborne Collision Avoidance System (ACAS) Xa for manned aircraft, the industry created a few new ACAS classes for unmanned aircraft. ACAS Xu is specifically for UAS, and defined well by NASA:

“ACAS Xu is designed for UAS and allows for new surveillance technologies and tailored logic for platforms with different performance characteristics. In addition to Collision Avoidance (CA) alerting and guidance, ACAS Xu is being tuned to provide RWC (remain well clear) alerting and guidance in compliance with the SC 228 DAA MOPS. With a single logic performing both RWC and CA functions, ACAS Xu will provide the industry with an integrated DAA solution that addresses many of the interoperability shortcomings of Phase I systems.”

— NASA Conference Paper, March 17, 2017.

DO-386 is the first standard to outline the base set of requirements for the industry to use. For the first time, UAV OEMs and systems manufacturers, like Sagetech, have a roadmap of requirements for creating the next generation of FAA certifiable detect-and-avoid systems.

2.       What is special about ACAS Xu?

These MOPS define the parameters for detecting and avoiding collisions and include algorithms not only for ascend/descend avoidance maneuvers but also horizontal avoidance maneuvers due to the smaller size and slower speeds of unmanned aircraft.

From the RTCA website, here is the DO-386 MOPS official description:

  • Document Title:  DO-386 Vol I Minimum Operational Performance Standards for Airborne Collision Avoidance System Xu (ACAS Xu) (Vol I), and DO-386 Vol II Minimum Operational Performance Standards for Airborne Collision Avoidance System Xu (ACAS Xu) (Vol II: Algorithm Design)
  • Description: This set of documents define the minimum operational performance standards (Vol I) and Algorithm Design Descriptions (Vol II) for the Airborne Collision Avoidance System Xu (ACAS Xu) equipment, designed for platforms with a wide range of surveillance technologies and performance characteristics such as Unmanned Aircraft Systems (UAS). Volume I contains system characteristics that should be of value to users, designers, manufacturers, and installers. These characteristics are intended to accommodate the requirements of various users. Vol II provides the Algorithm Design Description (ADD) for the Surveillance and Tracking Module (STM) and the Threat Resolution Module (TRM) of the next generation Airborne Collision Avoidance System (ACAS X). The algorithms are described at a sufficiently high level to allow for implementation in a variety of software languages and hardware platforms, thereby providing maximum freedom to manufacturers while ensuring the intended output from the system.

Unmanned systems that are expected to require ACAS Xu equipment include those flying >400 AGL and weighing more than 55 pounds. This is not expected to include future air taxis, which will fly in compliance with a separate ACAS designation – Xr.

3.       Flying BVLOS for unmanned aircraft just got real.

Until now, UAS operators were required to fly in special areas, or fly tethered, or hire dedicated spotters to fly BVLOS (beyond visual line of sight). FAA waivers for BVLOS have been available, but difficult to obtain and requiring significant safety case justifications. DO-386 eliminates this hassle by creating new requirements for logic-based, FAA-certifiable detect and avoid systems, requiring transponders and interrogators at the core, that enable UAS to autonomously detect and avoid other aircraft without requiring pilot sight. These certifiable systems will eliminate the need for FAA waivers for unmanned aircraft in the national air space. Much like manned aircraft, operators of UAVs equipped with DAA and ACAS Xu equipment, and possessing an aircraft type certification, will be able to file a flight plan and take off.

4.       A major step for integrating UAS into the NAS.

The new MOPS set the stage for near-term DAA systems to be developed and certified for use on unmanned aircraft, same as manned aircraft. At Sagetech, we’re pleased to see the industry agree with us that unmanned aircraft must be equipped with surveillance and collision avoidance technology like manned aircraft operating in Part 91 national airspace.

So, with this important step complete, what is next? “There will be another set of MOPS for how DAA systems integrate with ACAS and TCAS, DO-365,” explains Day. “Right now, we’re still in Phase 2 of the 3-phase DAA plan put together by FAA and RTCA, where we focus on extending the UAS operation environment to controlled airspace. This is an incredibly exciting time for unmanned aircraft, and at Sagetech we’re proud to support the integration of UAS in the NAS through situational awareness solutions including DAA.”

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Sagetech recently announced a new partnership and new technology approach to creating certifiable detect-and-avoid systems. See our approach to a DAA solution here.

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