Airborne collision avoidance systems (ACAS) are a crucial component of modern aviation. The Convention on International Civil Aviation, one of the founding documents of the United Nations, established the international rules of airspace, aircraft registration, and the safety and security of aircraft. Within these rules is a mandate requiring all aircraft – both manned and unmanned – to use ACAS and/or other types of detect and avoid technology.
Generally, ACAS systems enhance the situational awareness of pilots by actively monitoring the airspace around an aircraft, assessing potential collision threats, and providing guidance to avoid those threats.
There are several different types of ACAS, but all such systems share a few major characteristics. ACAS systems monitor the airspace around an aircraft and warn pilots of any other aircraft or terrain features in the immediate vicinity. ACAS systems make use of the transponders that are required in all aircraft. Transponders are devices that produce a response when receiving a radio-frequency interrogation and are used by air traffic control radar to identify aircraft. ACAS systems utilize these transponders and provide collision avoidance protection for a broad spectrum of aircraft types. Another of the major characteristics shared by ACAS systems is that they operate independently of ground-based air traffic control.
ACAS systems utilize transponder signals to assess the potential for in-air collisions. In practice, ACAS systems actively send out transponder interrogation signals to nearby aircraft. When a transponder receives an interrogation signal, it responds by sending back information about its altitude, identity, and other relevant data. ACAS systems collect this information and asses if there is a threat of a potential collision. If the ACAS determines that a potential collision threat exists, it issues a Resolution Advisory (RA) to the flight crew. An RA provides the crew with maneuver recommendations to avoid potential collisions.
While ACAS systems drastically reduce the risk of mid-air collisions, they do not protect against aircraft that do not have an operating transponder. The use of transponders is a well-established aviation safety practice throughout the world, and intentionally disabling a transponder is a serious violation of aviation regulations and can result in severe consequences. That said, there are scenarios in which transponders are deactivated. These scenarios range from the innocuous, such as during maintenance or testing, or a transponder malfunction, to more deliberate or aggressive, such as during combat missions, or in the process of illegal activities. However, in cases where a transponder has been disabled or has malfunctioned, some emerging technologies seek to perform the same tasks as ACAS without the use of transponders.
Automatic Dependent Surveillance-Broadcast
Aircraft equipped with ADS-B rely on satellite communications to track and broadcast the same types of information utilized by ACAS systems. Within ADS-B systems, there are two modes of communication – ADS-B in and ADS-B out. ADS-B out devices function very much like a normal transponder. Each ADS-B out signal broadcasts information about an aircraft’s current position, altitude, velocity, and identity. The ADS-B in, however, is responsible for the reception and processing of ADS-B out signals.
There are several benefits to ADS-B systems. It is significantly cheaper to install and maintain ADS-B systems as the ground stations do not require the vast radar systems that characterize standard ground traffic control stations. Further, ADS-B is technology agnostic. In ACAS systems, aircraft can only see and/or gather information from other aircraft utilizing the same technology. Additionally, ADS-B systems have a dramatically larger range and provide a much more robust data set than transponder-based systems.
Automatic Dependent Surveillance-Contract (ADS-C)
The main differences between ADS-B and ADS-C are primarily associated with the types of connections, and how data is transmitted. Whereas ADS-B provides data through periodic broadcasts, ADS-C provides data through a direct connection. The relationship between ADS-B and ADS-C systems is analogous to the relationship between two-way radios and telephones. Essentially, ADS-C is a direct link between an aircraft and air traffic control.
FLARM and Portable Collision Avoidance Systems (PCAS)
FLARM and PCAS devices are lower-cost, lower-tech solutions for ACAS. Generally, FLARM systems obtain position and altitude readings from an internal GPS and barometer and broadcast this data together with an aircraft’s flight track. At the same time, its receiver listens for other FLARM devices within range and processes the information received. PCAS systems monitor and notify pilots of the nearest transponder-equipped aircraft, and their relative height, distance, and whether that distance is decreasing or increasing. Both systems offer significantly smaller and more narrow situational awareness data than any of the above but are exponentially less expensive.
Radar and Sensor Systems
Most large modern aircraft are equipped with advanced radar and sensor systems that can provide collision avoidance information independently of transponder or ADS-type signals. These systems can fuse data from various sensors, such as radar, lidar, and cameras, to assess the airspace and detect potential threats. Like FLARM and PCAS systems, various modern radar and sensor systems do not provide anywhere near the holistic view of the local situation as either ADS systems or transponder-based systems, but they are better than nothing at all.
The Future of ACAS for Crewed and Uncrewed Aircraft
As aerospace technologies become increasingly complex, the successful deployment of new equipment requires careful attention to SWaP-C2. In unmanned technology specifically, there is a delicate balancing act between more complex components and systems, the weight requirements, and costs. Therefore, it is crucial that ACAS systems are designed with the smallest possible SWaP-C2 footprint.
Sealevel has decades of experience in SWaP-C2 optimization. By leveraging PCIe/104 and COM Express technology, Sealevel’s team can engineer application-specific functionality to meet defined requirements. For more information about our experience, see our Autonomous & Unmanned and Military & Aerospace capabilities.
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