Uncrewed Systems Technology 046

100 Focus | GNSS systems will in all probability depend on the type of application and method of operation, and the calculated risk of the effects of incorrect data about position, velocity or timing. Such requirements will of course be dictated by certification regulations. TSO certification of a GNSS receiver for instance requires a high degree of design assurance for both the hardware and the software, with comprehensive testing to verify its functionality and resilience to various interference and environmental conditions. All of this needs to be achieved through high-quality manufacturing organisations approved by the national aviation authority. After all, one does not simply take a COTS GNSS and add integrity to it – it is a quality that starts from the ground up, particularly if a module is to be certified and kept as lightweight and low-power as possible while including the integrity features required for autonomous and BVLOS operations that the FAA (and other authorities, either in tandem or following suit) will probably mandate in the future. Architectural components Hardware design starts with architecting a system that uses components selected and right-sized for the job. There are many highly integrated FPGAs, processors and radio components available, but while they make development of complex systems easy, they can sometimes be uncertifiable and difficult to maintain. Ensuring certifiable BVLOS (or VLOS) operations therefore needs careful selection of parts designed and constructed in compliance with avionics regulations. Central to a highly effective GNSS is the timing source: everything in the receiver must run off a common clock, so the quality of that clock determines much of the performance of the overall solution. While quartz-based clocks used to be ubiquitous, some high-end developers in the autonomous space are now moving to MEMS-based clocks. Although quartz oscillators are available in much greater quantities, a few companies have made major leaps in MEMS oscillators that have superior stability against vibration, temperature and shock. They make for a much more secure and accurate timing reference – particularly when operating in harsh environments or with heavy-duty powertrains – with faster tracking and time to first fix. High-quality filtering is also becoming more and more important, as a professional uncrewed system will probably have computing and radio equipment such as radar, wi-fi and ISM radios situated close to the GNSS receiver that will interfere with it. As a prime example, more and more high-end receivers are being produced with surface acoustic wave filters, which previously were rare outside highly customised solutions, and exhibit excellent rejection of out-of-band or out- of-frequency signals (interference from transmissions at frequencies next to that of a given satellite). They operate by converting electrical energy into acoustic or mechanical energy on a piezoelectric material, using interdigital transducers that have interleaved metal electrodes to perform the required energy conversions. In addition to electronics, the hardware for ruggedising GNSS is critical to preventing signal integrity from being hampered by EM emissions from electric and electronic interfaces – HDMI is particularly notorious for causing EMI in such tight spaces – as well as other RF systems inside the vehicle. In addition to copious EMI and ESD testing, the latest in small, ruggedised connectors and aluminium and magnesium housings are prized among GNSS developers for their (SWaP- optimised) electrical and mechanical protection. Meeting certification standards outlined in Mil-Std 461, Mil-Std 810 and DO-160 are often a minimum benchmark for such hardware. Overall tests for GNSS integrity and accuracy can be challenging, given that uncrewed vehicles can (for example) be blown about by adverse weather. These and other conditions will never be exactly the same from one test day to the next, making it nearly impossible to correct for them in test results. One company’s novel solution to that though is to test its GNSS receivers on railway vehicles, which of course follow a mechanically fixed set of waypoints. That makes for a very fair test with knowable degrees of multi-path interference from structures, tunnels, mountains and trees. Software considerations Software is a huge point of focus for less expensive GNSS receivers, as such systems will restrain their input costs by seeking to resolve onboard RF interference largely through software-based October/November 2022 | Uncrewed Systems Technology Lab and field tests are key to ensuring that errors and interference are detected and rejected by a GNSS receiver’s embedded algorithms (Courtesy of CubePilot)