Issue 37 Unmanned Systems Technology April/May 2021 Einride next-gen Pod l Battery technology l Dive Technologies AUV-Kit l UGVs insight l Vanguard EFI/ETC vee twins l Icarus Swarms l Transponders l Sonobot 5 l IDEX 2021 report

78 Focus | Transponders It is also increasingly common for new UAV transponders to incorporate ADS-B Out as well as ADS-B In. That means UAVs equipped with such transponders can also receive ADS-B Out information, along with additional information from authorities about air traffic control, weather and flight restrictions. Lastly, tools for simplifying the integration process are being more widely offered. In addition to intuitive software for installation purposes, GCS user interfaces are being updated to reliably process and display ADS-B In data on their navigational maps. In general, more transponders and GCSs are emerging that have been engineered with significant software- defined behaviours. That reduces processing power requirements and enables pathways for patching-in future updates to transponder standards (rather than requiring any hardware changes). These might be the future Mode 5 2B updates, which are expected to integrate ADS-B In and Out into specified IFF requirements, or improved algorithms for calculating UAVs’ positions, headings or intentions, as well as new forms of encryption or other improvements. Civilian equivalents of Mark XIIB- compliant transponders are due later this year, and will come with ADS-B Out as well as similar SWaP footprints, I/Os and interfacing protocols. Other transponder modes and bands continue to be used, however, not only for the applicability of their protocols in different circumstances – in which ADS-B Out may be surplus to safety requirements – but for making better use of the RF spectrum. Radar tracking transponders for example are still used in testing centres for tracking UAS prototypes and airborne target UAVs. Cost-optimised transponders designed for transmissions over the C band are highly useful for such trials. Newer modes are also being put forward. The FLARM (Flight ALARM) protocol for example – particularly in its latest format, PowerFLARM – implements key requirements of the latest FAA, EASA and other countries’ regulations, and is now in use in countries in both hemispheres. Like ADS-B, FLARM encodes and transmits an aircraft’s data on GNSS and altitude position, as well as on near-future 3D flight path planning. However, it also includes motion prediction algorithms for avoiding potential collisions with up to 50 aircraft in its range (using FLARM data received from them), and transmits the flight alarms to the pilot based on the actual risk of collision. Also, while ADS-B Out operates on the same frequency as Modes A, C and S – the increasingly congested 1090 MHz band – FLARM is broadcast over the 868 MHz ISM band. Uptake of FLARM in Europe is accelerating, so it is possible that air-to-air transponder comms could be siphoned off into this long-established but comparatively underused band, to limit the rate at which 1090 MHz is becoming saturated. In addition, some aviation administrations outside the US might start using 978 MHz as a dedicated band for ADS-B in unmanned traffic management (UTM). This frequency is underused by most countries other than the US, where it is used in general aviation, but it still retains the useful properties of the 1090 MHz band for long-range data transmissions. Commercial trials are being conducted in the UK at the moment that use this band for ADS-B combined with electronic conspicuousness signals for identifying UAVs – and 978 MHz is already accessible by transponders worldwide, having been designed in accordance with FAA standards. The aforementioned dynamic RF systems could also reduce the congestion in the 1090 MHz band by persistently ‘listening’ within a certain radius but not always replying. UAVs fitted with such systems could therefore withhold transmitting their position and altitude until the safety case required it, for example if the probability of a collision or proximity of another aircraft passed a certain threshold. While the technology for installing dynamic RF systems on transponders is certainly mature though, and the algorithms needed for UAVs to intelligently operate them on a case-by-case basis are easily achievable by current standards, it is not yet allowed for in US regulations. Transponder chips To make transponders small and lightweight for integrating into UAVs, while also meeting the transmission and reliability requirements for Mark XIIB and similar standards, takes considerable design rigour. These systems must be extremely sensitive to the most minute variations between RF signal inputs down to -82 dBm while simultaneously blasting out their own RF comms at up to 57 dBm. That is easy if the unit’s transmitter and receiver are spaced many centimetres apart, but far less so if they have to be the same size as the latest generations of transponders, so extensive design and testing regimens for PCB and RF optimisation are critical. Integrating high-quality electronics is also imperative. At the heart of a transponder is a chip for algorithmically generating the signals to be April/May 2021 | Unmanned Systems Technology Sagetech’s MX12B is certified to DoD AIMS Mk XIIB as a Mode 5 IFF transponder for UAVs (Courtesy of Sagetech)