Uncrewed Systems Technology 046

102 Focus | GNSS corrections of out-of-frequency noise. That can be challenging though, and will lead to saturation of the RF front end if there are any errors in the algorithms. For costlier systems, where filtering in the front end successfully mitigates against most local interference, the bulk of the software work goes into multi-path error rejection. Various physics models are used for calculating whether a given received signal has come directly or indirectly from one satellite or another, and hence a receiver’s processor can determine which satellites’ data (assuming several are being tracked at once) should be disregarded because of a lack of confidence in its accuracy or integrity. The data downstream can then be further refined by RTK algorithms for centimetric accuracy, which are comparatively well-understood, and their quality and consistency can generally be taken as a good test of a given receiver’s unaided positioning accuracy. Also, the software must be constructed carefully for simultaneous intelligent and seamless connections with different constellations. Single-band GNSS solutions are rare these days (apart from very low power IoT devices), and adding a constellation opens up considerable opportunities for gains in precision, accuracy, integrity and consistency for navigation and geo-referencing. GNSS software is typically configured to work with the NMEA protocol, which is key to setting the receiver module’s parameters for update rate, baud rate, constellation access, SBAS support and other factors during initialisation. Some developers however aim to switch from NMEA to the UBX protocol in the future, which stands to offer more low-level information such as NED (North, East, and Down) velocities, pDOP (position Dilution of Precision) and estimates of accuracy for both horizontal and vertical positioning and velocity. Aside from the software written directly into GNSS solutions, creating GNSS simulator scenarios to show that aviation authority standards are being met can be a major challenge, and are not something that can be easily acquired as a COTS solution. Simulators from various vendors, each with their own capabilities, can be necessary to fully exercise GNSS equipment during testing. Considerable experience in RF design is also critical for ensuring software resistance to interference. Antennas Naturally, when discussing GNSS integration, the qualities of GNSS antennas must also be considered. Some of the most desirable antenna qualities are largely universal: they should be lightweight, small and in particular low-profile, especially for faster UAVs that have to optimise their aerodynamics. Patch-type and chip antennas have therefore emerged as the most popular configurations, with the ability to surface- mount them to allow seamless integration and reduce the need to manufacture or cut through-holes into airframes. Some other qualities are must-haves for GNSS antennas these days. For instance, the polarisation of the antenna – as in the orientation of its radiated EM waves – should be circular rather than linear, primarily because it means the antenna, transmitter and receiver don’t need to be aligned for localisation data to be consistently available. And in order to have multi-band, multi-constellation coverage, modern antennas must be designed as wide- bandwidth devices. In the absence of a human pilot to work out where the vehicle is from time to time, uncrewed systems must be able to draw navigation signals from as wide a selection of satellites as possible, for redundancy and to aid precision and integrity. That can be particularly frustrating during the design process, since at today’s lower GNSS frequencies (about 1 GHz) the wavelengths are large enough to make it challenging to build very small antennas that are still physically capable of detecting them. That is driving some companies to develop techniques for shrinking their cross-sectional areas. One supplier designs its antennas with meandering spirals to maintain bandwidth and gain while compressing their overall volume; it also makes use of substrates with dielectric constants that are higher than typical. Both of these help to lower its antennas’ profiles. Also, the ground plane is a critical component in the antenna’s operation, as it is the reflective surface used to ‘bounce’ the signal transmission and hence better achieve the desired coverage. As composites and plastics make for poor ground planes, October/November 2022 | Uncrewed Systems Technology New components such as MEMS oscillators, SAW filters and the latest FPGAs and processors are being leveraged in the latest GNSS and INS solutions (Courtesy of Advanced Navigation)