Unmanned Systems Technology 038 l Skyeton Raybird-3 l Data storage l Sea-Kit X-Class USV l USVs insight l Spectronik PEM fuel cells l Blue White Robotics UVIO l Antennas l AUVSI Xponential Virtual 2021 report

82 high positional accuracies), and integrate sufficient elements and design knowhow for compatibility across all the GNSS satellite constellations. GNSS constellations have been regularly modernised in recent times, particularly over the past few years with new satellites and signals for Galileo and BeiDou. As more signals means more redundancy and consistency in keeping accurate GNSS position estimates amid buildings or tree canopies, GNSS antenna and receiver companies are working rapidly to keep up to date with such developments. While the exact process for doing that is complex, a critical part is accessing the interface control documents (ICDs) for new signals as soon as they are released. ICDs contain records of all the necessary information for compatibility with a new interface protocol, from connector plugs and signal voltage levels to instructions on correctly programming a positioning engine to track and use the new GNSS information as effectively as possible, in addition to engineering and tuning elements for resonance in line with a new signal’s frequency band. GPS L5 for example is one of the newest bands, offering position accuracies of within 30 cm with updates coming over a centre frequency of 1176.45 MHz. Another, Galileo E5b, transmits potentially higher accuracies still over a carrier of 1207.14 MHz (with a 20.46 MHz bandwidth). In general, new generations of GPS (as an example) have used gradually lower frequencies, going down from 1575.42 MHz for L1 to 1227.6 MHz for L2 and 1176.45 MHz for L5. Tuning an antenna to resonate to these new bandwidths, while retaining resonance with the older ones as well as filtering out interference from multi-path and in- or near-band transmissions, takes considerable simulation and engineering rigour. The result though is that some high-end modern antenna designs now enable millimetre-accurate performance. New correction services from both satellite-based augmentation systems (SBASs) and global networks of terrestrial base stations or ground- based augmentation systems (GBASs) also bear consideration when designing new antennas for compatibility with different frequencies. SBAS constellations are highly useful but are often restricted to a specific constellation (matching the country that developed them). Some GBAS systems such as Real-Time Kinematic and Precise Point Positioning correction are well- known and use measurements from base stations to achieve accuracies down to 2-3 cm, but they can be limited by the range of the nearest base station – which can occasionally drop out of service. Newer systems such as CenterPoint RTX provide more consistent coverage around the world, across all working constellations to provide 1-2 cm accuracies without relying on base stations. Real-time satellite measurements are taken from a global network of tracking stations, and are combined with models June/July 2021 | Unmanned Systems Technology Focus | Antennas Tight packaging of GNSS antennas with their receivers and inertial systems enables easier installation for unmanned system designers and integrators (Courtesy of Trimble) Radionor’s CRE2-179-UAV uses DSPs to allow Ethernet- and IP-based network comms with Robot Aviation’s SkyRobot FX450 UAVs from a phased-array antenna (Courtesy of Robot Aviation)