88 Product focus | Transponders to signals or interrogations within extremely short timespans, to opt for CPUs that run software would be too slow or risk losing the signal in an iterative loop, so microprocessors and field-programmable gate arrays (FPGAs) typically form the high-speed, low-complexity ‘brain’ of a transponder. Additionally, some power-conditioning systems must be installed around these components to minimise variations from the input voltage reaching the transmitter, receiver or microprocessor and damaging them. For instance, some high-end transponders integrate capacitors to store power over time that exceeds the troughs of the transmit cycle, so when such cycles peak – meaning an ADS-B signal must be amplified to full; perhaps 500 W for a fraction of a second – the stored power can be released to satisfy the transmitter’s load. A growing number of transpondertype solutions made for the UAV space now integrate GNSS receivers to create a more complete solution, which can be particularly crucial in ensuring both a UAV’s transponder and its GNSS source fit TSO-specified minimum standards on precision, accuracy and integrity. Simpler than these are interrogators (which ping out requests for ADS-B information from in-range aircraft) and ADS-B In receivers. While not technically transponders, transponder manufacturers are increasingly producing both. The former are key for military users in distinguishing friendly from nonfriendly aircraft, and will likely constitute part of future ACAS infrastructure. Meanwhile, the latter are anticipated by some to be used even more prolifically than transponders among UAVs over the next few years, because ADS-B Out still lacks the breathing room of a dedicated frequency band. As certified transponders will increasingly need to transmit 200-300 km away, it would greatly alleviate band congestion to require that some UAVs – particularly smaller, Part 107-sized aircraft such as delivery drones working at lower altitudes, where crewed aircraft will be rarer – simply receive ADS-B data from other aircraft (specifically bigger, more expensive and more imposing aircraft that genuinely need to be tracked) and thus give way to them as and when they come near, rather than have every single UAV fill up the airwaves. Specific advancements For those certified UAVs in need of robust, SWaP-efficient transponders, a number of improvements across these components are being targeted and leveraged by system makers today. Much of this revolves around making the microprocessor’s job easier, and ensuring the transponder works in harmony with other systems. For instance, selecting or designing a transmitter with very robust masking, such that it transmits with extreme precision on the 1090 MHz frequency, and with minimal (if not zero) out-of-band transmissions, is critical. One reason is that spilling out of band could cause transmissions to not be recognised by receivers. Another is that as the transmissions slip closer to, or even past, 1100 MHz (1.1 GHz), the secondary harmonics encroach on the 2.4 GHz ISM band. As crowded as 1090 MHz is, such harmonics could then interfere with and smother uncrewed system C2 links, among countless other systems. Conversely, high-end transponder manufacturers are discretely managing all of the filters across their receivers, such that out-of-band signals can be rejected and relevant signals can be recognised for the precise timing of their pulses, even when multiple transmissions are coming in. Not doing so risks copious interference for transponders from unwanted signals, or the device failing to recognise when and how it is being interrogated for its ID and position data (and failing to respond in good order), along with other issues that make it harder for the microprocessor to do its job. Fortunately, chips are coming in smaller packages with higher processing power, enabling more internal space for lengthier and more complex transmit/ receive arrays. These include highly power-efficient system-on-a-chip (SoC) devices that integrate microprocessors and FPGAs directly (rather than having the two sit as separate components inside a transponder), eliminating the need for a comms bus between them that could August/September 2024 | Uncrewed Systems Technology Transponder manufacturers are taking care to choose efficient microprocessors, transmitters with robust masking, certifiable GNSSs and other high-end parts (Image courtesy of Trig Avionics)
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