“Sense and Avoid” - What's required for aircraft safety?

The safe integration of unmanned aircraft into the national airspace is a topic receiving considerable attention. The US Congress has mandated that the Federal Aviation Administration (FAA) engage in rulemaking that will allow for civil operation of unmanned aircraft in the national airspace system (NAS), with a release of those rules in 2016*. Preparations are occurring simultaneously for the implementation of the FAA's NextGEN System on 1 January 2020, changing Air Traffic Control (ATC) in the US from one that relies upon radar to a GPS-based system. The introduction of Unmanned Aerial Systems (UAS) into the NAS is expected to be integrated with NextGEN. The availability of small, light ADS-B Out equipment (Autonomous Dependent System-Broadcast) is anticipated to facilitate that, and is discussed. Further, control of UASs will likely adhere to the procedural rules present in today's ATC System and in NextGEN, rules that require a pilot in the loop, making fully autonomous operation forbidden. UASs are therefore expected to employ additional systems and equipment to enhance their sense & avoid capability. Collision avoidance in manned aircraft was improved in 2005 when certain aircraft were required to use the Traffic Collision Avoidance System, TCAS II. The sense & avoid performance of UASs within the NextGEN-controlled airspace should similarly be enhanced by a new system called the Airborne Collision Avoidance System, ACAS. This TCAS replacement will have several variants for various aircraft types including UASs, and it will be capable of accepting data from various types of onboard sensors, making “optimal” use of that data when available. Thus collision avoidance on future aircraft should benefit from enhanced sensing devices such as LIDAR, Mode-S transponder interrogation, IR sensors, and possibly from that provided by the “Due Regard” radar system currently in test on the Predator UAS.