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

90 Focus | Navigation systems There are different approaches to accommodating the rising number of receiver bandwidths, signals and amount of noise that comes with the proliferation of satellite frequencies and unmanned vehicles. One is increasing the number of channels, which many suppliers have done by making receivers with upwards of 300 channels, in anticipation of there being many hundreds of satellites transmitting navigation signals before long. Also, as processing units become faster, companies can now run artificial neural networks on their GNSS boards. These can develop intelligent training sequences that could allow receivers to be programmed to more accurately recognise and reject unwanted signals among incoming streams, to better mitigate and resolve multi-path errors. Antennas The issue of where to mount a multi- band GNSS antenna on an unmanned vehicle should be considered early in the design process. The needs here are to ensure a consistently clear signal via an unobstructed view of the sky, without affecting the centre of gravity in the case of heavier systems. For modern microstrip and helical antennas though (which are lighter than typical geodetic and ceramic-based antennas) that is less of a concern. Once the antenna is installed in an appropriate location, the GNSS receiver can be installed and moved around relatively freely, as it is far more lightweight. Naturally, any areas of the vehicle that are sources of significant heat or vibration should be avoided. Avoiding sources of EMI and other noise is also critical. While receivers can filter it out, power and bandwidth can be saved by avoiding it in the first place. In the case of multi-rotor UASs, it is imperative to avoid placing GNSS antennas near any of the propellers, given the noise and obstruction they might create. The growing number of electronic systems on an unmanned vehicle also makes it useful to have access to some form of spectrum analyser functionality in the GNSS receiver. This can detect the power being transmitted at different frequencies, and thus help identify which subsystem is generating any excess RF noise. The ability to perform ‘GNSS compassing’ is also increasingly critical for some unmanned systems. This refers to the concept of using two receivers (or a single ‘heading receiver’) connected to two antennas integrated onto a vehicle at some distance apart from each other, to take measurements from both and calculate a heading angle from the differences between their received signals. For a multi-rotor aircraft, it can be problematic but exceedingly useful to know the heading angle when sitting still on the ground with no airflow, or hovering in the air. In congested environments such as forests or industrial complexes for example, it is critical to know on take- off which direction the UAV is facing – as a multi-copter’s coordinates often stay the same during launch even as altitude increases, and a wrong heading might mean a collision or wasted journey time. GNSS compassing can be even more troublesome for helicopter UAVs, which provide little structural space for integrating antennas with an open view of the sky, except for complex integrations atop the main rotor mast. The second antenna will typically be installed atop the tail, ideally with enough space from the tail rotor to prevent its noise interfering with satellite tracking. PPS In addition, many GNSS manufacturers continue to rely on using serial comms with their receiver boards, with the time stamps coming from their serial ports in orders of every few milliseconds. However, the use of the PPS (pulse- per-second) comms standard can give accuracy to a few nanoseconds, time stamping the position data with far higher reliability. While that may seem a negligible improvement, one must keep in mind that UAVs increasingly travel at speeds greater than 70-80 kph. This means that receiving GNSS data 50 ms late could lead to significant discrepancies when time stamping camera or Lidar images collected by the aircraft. It could even mean a full metre of error in position reporting. For urban or swarm operations of unmanned systems, that could lead to collisions with buildings, people or other vehicles. October/November 2018 | Unmanned Systems Technology Dual-antenna configurations aimed at ‘GNSS compassing’ could be crucial for estimating headings (Courtesy of UAV Navigation)