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80 and-avoid range with each other while reducing the impact of unmanned aircraft transponders on ATC and manned aircraft operations. That would be coupled with managing UAV traffic density, particularly around ADS-B ground radio stations, further reducing the impact of unmanned operations on airspace safety. Adjustable power outputs have previously been tested in formation flight of manned military aircraft. While close together, aircraft might lower the power to their transponders while increasing their update rate. Each aircraft therefore continues receiving enough information to ensure collision avoidance with those in the immediate vicinity, while reducing the range of their signals and the chance of overwhelming any air vehicles further afield. A rule by which this technique could be used may be included in future civilian aircraft regulations, potentially enabling swarms of UAVs to operate in congested airspace without having an impact on ATC operations with manned aircraft. Future innovations aimed at combating frequency congestion could also involve combining transponders with 4G LTE, in order to use the signal that is most appropriate in the context of the area a UAV is operating in, whether that is a function of altitude or geography. Future regulation Despite the various technology considerations, regulations are still – and are likely to remain for some time – the most significant driver in terms of defining UAV transponder systems. That applies to their proliferation, use and evolution. Around the world, there is a loose collection of regulations with inconsistencies in them, their languages and formats, making them difficult to decipher and abide by. The challenge therefore is to produce systems that are designed to conform to the international requirements expected in the future, especially for systems such as those looking to incorporate certified GNSS, altitude encoders, ADS-B and Mode S in a single enclosure. Given the low accident rates in manned aviation, it is likely that UAV regulations will follow largely similar lines in safety procedures and technology specifications. The procedures will be painstaking, with literally thousands of requirements, and requirements-based testing will become increasingly standard practice as regulators insist on them during the engineering and production phases. In terms of UAV-specific regulations, however, nothing coherent has emerged yet. They have yet to define performance standards for subsystems, opting instead to focus on operational constraints, the conduct of a user, licensing mechanisms and general advisory guidelines. The debate over future UAV regulations has generated many ideas about the performance standards they will resemble. The general view is that the more risky a UAV’s mission to itself and others, the more significant the safety demands of the onboard equipment need to be, but there is nothing to enforce those yet. Ideally, part of the regulations would identify sufficient power requirements to ensure that UAVs maintain a functional detectability range (of perhaps a few kilometres or less) to enable air-to-air sense and avoid to prevent accidents above populated areas. They should also help with ground- to-air visibility for ATC management, to maintain separation services by avoiding overloading the transponder comms spectrum, as well as keeping power requirements realistic for smaller, battery-powered UAVs that lack the energy density of MALE-class unmanned systems. ADS-B, by way of ES, is widely tipped to be at the forefront of such requirements. In 2016 for example, the UK’s Civil Aviation Authority identified this protocol in CAP 1391 as the most practical solution to low- power, low-cost, low-hazard interplay and high interoperability electronic conspicuousness of air vehicles. December/January 2018 | Unmanned Systems Technology To avoid congestion in the ADS-B spectrum, military aircraft may opt to lower their transponders’ power outputs using devices such as this (Courtesy of Leonardo)