Unmanned Systems Technology 022 | XOcean XO-450 l Radar systems l Space vehicles insight l Small Robot l BMPower FCPS l Prismatic HALE UAV l InterDrone 2018 show report l UpVision l Navigation systems

89 While L1 provided just a 2 MHz bandwidth, L5’s is 24 MHz, which leads to greater accuracy in RF-congested environments such as cities. L5 also has a chip rate (similar to a bit rate for signal code) of 10.23 MHz – 10 times that of L1. This provides the higher bandwidth performance and accuracy, while enabling lower signal distortion. L5 signals also undergo better data encoding as well, using parity and cyclic redundancy checks to improve signal and data integrity – that is, the guarantee that the tools giving the readings are working correctly, as opposed to accuracy, which aims to minimise the readings’ margins of error. Dual-frequency systems are starting to be supplanted by triple-frequency GNSS boards, which allow for tracking signals from L1, L2 and L5 simultaneously, for example. Single-frequency GNSS receivers were typically limited to position accuracies ranging from 5 to 30 m, but modern multi-frequency systems can provide accuracies down to 30 cm or even just a few centimetres if they use carrier phase measurements or real-time kinematic (RTK) processing, and have sufficient time to perform the required calculations of position. Hypothetically, the distance between an RTK base station and a single-frequency GNSS receiver on a UAV could be as much as 1 km, beyond which accuracy would be reduced and centimetre-level accuracy would no longer be possible. With dual-frequency GNSS, however, the receiver and the base station can be up to 10 km apart before accuracy starts to be lost. Multi-constellation In addition to the ability to process signals from multiple constellations simultaneously, a wider range of constellations is now accessible via the latest generation of GNSS receivers and their embedded programming. This also applies to some existing receivers through downloaded firmware updates that enable them to demodulate the various waveforms and frequencies. Modifying existing hardware to accommodate additional constellations typically involves increasing the bandwidth of a receiver to handle the additional signals, while also ensuring it can tolerate the higher noise levels this could bring. When integrating new constellations into a receiver’s positioning engine, GNSS engineers must account for an accompanying range of error terms, as these can be consistent across a constellation’s various bands but not across multiple constellations from different countries. These errors can occur across the different receiver RF front-ends’ hardware biases (such as poor synchronisation between a receiver’s clock and that of a satellite), or even across the biases on different groups of satellites that were launched years and continents apart with different technological advances and configurations. Some of the latest receivers and firmware updates can use the newest launches of BeiDou – which at the time of writing consists of 15 active satellites and 18 under test, to comprise 35 satellites eventually – and Galileo, the EU’s GNSS, which has launched 26 of its planned 30 satellites. BeiDou’s new B3 service operates on 1263.52 MHz, and has a 24 MHz bandwidth. The latest Galileo service typically available with the newest GNSS receivers is called E5, and is divided into the sideband carrier frequencies E5a and E5b that together cover the full 51.15 MHz bandwidth of E5. E5a operates on a centre frequency of 1176.45 MHz (which it shares with L5), while E5b operates from a 1207.14 MHz centre, with the two kept separate by E5’s centre frequency of 1191.795 MHz (this forms the upper limit of E5a and the lower limit of E5b). Also, both B3 and E5 offer a chip rate of 10.23 MHz, to match L5. The core benefit of these additional satellite groups for unmanned vehicles comes because of the fact that the GPS and GLONASS constellations and earlier receivers were designed on the assumption that users would always have clear enough skies to maintain constant and accurate position updates via uninterrupted, lossless satellite links. With USVs and medium-to-high altitude UAVs, that assumption remains generally true (the exception being docking or recovery at harbours and landing strips with a lot of buildings, vehicles and RF activity around them). But with UASs and self-driving road vehicles operating in highly congested and reflective urban canyons, for example, the assumption is no longer practical. With dozens of new GNSS satellites now available, however, greater redundancy is possible. For example, an autonomous road or air vehicle tracking four GPS satellites while moving through an urban canyon could momentarily lose sight of them while passing under a bridge or by a skyscraper, say, but if it still has a clear line of sight to four BeiDou satellites, it can continue navigating safely using their signals, providing it has multi- constellation capability. Alternatively, a vehicle operating in a congested city environment could find it easier to maintain an accurate position reading if it can track satellites using different frequencies. Navigation systems | Focus GNSS receivers are being updated to offer developers access to the latest BeiDou and Galileo constellations in addition to GPS and GLONASS (Courtesy of Swift Navigation) Unmanned Systems Technology | October/November 2018