92 The route taken to get to that point can vary between companies. Some will start with a ‘breadboard’ design, which is a PCB several times larger than the deck of cards-sized transponder commonly made for the UAV space. With so much room on a large board, the functional pathways and lines of code needed to meet performance and safety requirements are defined and then shrunk down to fit the desired form factor. However, breadboard development yields a high chance of hitting a wall in how far the prototype can be miniaturised, as the software and architecture have been constructed using a certain measure of processing power, hard-drive space and bandwidth, and other parameters across the board. So, transponder developers increasingly impose a hard and tight limit on the form factor of their first prototype, possibly allowing a few millimetres of expansion when unavoidable. Going forwards, developing and certifying a GNSS device small enough to fit within such limits will become an expected duty of manufacturers to guarantee that UAV operators can document in their aircraft when seeking certification to fly routinely and beyond visual line of sight (BVLOS). Also being engineered into some transponder architectures are mutual suppression buses. These are a common system in crewed commercial and military aircraft, by which multiple onboard L-band transmitters can arbitrate and take turns to transmit (during which the other systems’ receivers can stop listening for a measure of milliseconds). Naturally, as UAVs integrate mounting numbers of L-band radios for ADS-B, GNSS, mobile networks, multimedia broadcasting and even C2 data links, all of these could interfere with each other while operating near-simultaneously onboard. So, including mutual suppression buses in transponder designs (guided by published standards on such systems) could become as important to UAV certification and safety as correct placements of antennas and ground planes. Rigorous testing Once a prototype has been iterated enough times that the transponder manufacturer is confident in its compliance and performance, it can enter rigorous functional tests to unearth bugs through hardware-in-the-loop testing, and ensure the basic data processing and that transmit/receive tasks can be performed correctly. On top of this, environmental testing (compliant with DO-160 standards) is critical for revealing that further bugs remain hidden during operations in Goldilocks conditions and lets engineers program or calibrate them out. While the development of software often outpaces that of hardware early in the engineering process, the last hundred or so tweaks tend to be software changes. This includes extensive shock and vibration testing to ensure the physical exertions of UAVs at high speeds, catapult launches, dynamic manoeuvring and so on will not cause capacitors to crack or housing lids to come loose. Environmental chambers will simulate extremes of pressure altitude, humidity and temperature, with the standard laying out many further tests, including waterproofing, salt and fog resistance, dust ingress and lightning susceptibility (the latter can involve linking a transponder to a copper plate and running 600 V into it to simulate lightning). More transponder-specific testing equipment may naturally include anechoic chambers to precisely gauge transmission and reception accuracy and integrity when combined with an antenna. On top of this, GNSS simulators can pose a great expense to manufacturers (hence not all transponder makers have one), but effectively emulate satellites from the GPS, Galileo and other constellations (with signals either conducted to the transponder via a cable or radiated throughout a test facility). These simulators enable scenarios for satellite movement and visibility (relative to the transponder) to be programmed and played out, thereby showing how well a transponder continues performing in the face of GNSS variations, and allowing for calibrations to compensate for such changes. Rarer still are KIV emulators for IFF transponder testing. These cannot be purchased and can only effectively be made by reverse-engineering the interfaces for a KIV, although non-NSA encryption would still have to be used to measure how a transponder performs amid cryptography duties. As unusual as these testing and simulation systems may sound (among avionics testing), tests for transponders must become even more wide-reaching to ensure their consistent functioning in all environments that UAVs may fly in. August/September 2024 | Uncrewed Systems Technology It pays for transponder suppliers to broaden the range of tests performed to work bugs out of their products before engaging in certification or manufacturing (Image courtesy of Aerobits)
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