Uncrewed Systems Technology 046

104 and some conductive metals can be prohibitively heavy for an airframe, one approach is to use a high-impedance surface ground plane with the antenna. That helps keep the overall system profile quite narrow, as a normal ground plane would be comparatively large. That is because a quarter-wavelength separation would be needed between the antenna and its ground plane, whereas the high-impedance system can function with a much smaller separation. The solution also uses a split-band high-impedance surface topology, which enables the antenna to operate over two frequency bands that are near to each other. That helps to achieve the multi-band, multi-constellation functionality that is so important for uncrewed vehicle navigation. INS While improvements in hardware, software and testing have been key to maintaining safe navigation in GNSS- denied conditions, the development of a robust inertial navigation solution remains one of the backbones of a well- integrated and efficiently used GNSS, for dead reckoning during satellite outages as much as accurate acceleration and attitude data for platform stabilisation. Key to any INS is a high-quality Kalman filter, a complex algorithm that typically combines 10 to 12 core equations in order to output an estimate of a vehicle’s state – that is, a function of its position and velocity. Implementing such a filter poses major challenges during the development process, however, and some low-end manufacturers choose to circumvent this by using an open- source Kalman filter without optimising it for the INS hardware, vehicle platform or expected mission parameters. Neglecting any one of these can present problems in how the INS functions. For instance, accelerometers and gyroscopes aren’t always outputting data at similar rates or to comparable levels of precision and accuracy, so any differentials have to be handled by the algorithm in a smart manner. Tuning the Kalman filter for noise coming from measurements, processing and other onboard sources is essential as well. Naturally, an IMU will be subject to some degree of drift over time. Developers wanting to account for that can mitigate it by using methods such as modelling the drift and using runtime bias estimations to correct for them regularly over the course of a mission. Of course, a high-quality IMU is key to this as well, with low bias instability and angular random walk being two of the main parameters regarding the performance of the inertial sensor before it is selected and integrated. MEMS sensors have advanced to the point that a bias instability of less than 1 º /hour is achievable in SWaP-optimised, low-cost devices. Transponders High-quality GNSS also plays a critical role for transponders, as a source of global positioning information and of timing for synchronisation purposes. For instance, a precise PPS (pulse per second) GNSS signal output for Mode A, C and S transponders allows for the use of multi- lateration (MLAT), a process in which the location of a signal source – in this case, an uncrewed vehicle – can be derived based on data from multiple receivers such as base stations on the ground. MLAT thereby allows the tracking of many more aircraft than air traffic managers would otherwise be able to track, so it could be critical to future UTM efforts, and it also protects against the spoofing of ADS-B signals. PPS signals are also used by FLARM, which like ADS-B is a potential contender as a future standard for UTM systems, particularly in some parts of Europe and North America. And while transponders are increasingly acknowledged as an absolute necessity for certified operations (particularly BVLOS) in the near future, they also mean adding even more weight and power consumption alongside all the other systems UAVs already need to carry. Further miniaturisation of GNSS is therefore essential, along with the miniaturisation of transponder electronics and housings. Any GNSS chipset capable of fitting October/November 2022 | Uncrewed Systems Technology Designing antennas with meandering spirals, and substrates with a relatively high dielectric constant, lowers each antenna’s profile while maintaining bandwidth and gain (Courtesy of Arralis)