Uncrewed Systems Technology 046

99 accurate heading, with some additional consideration of pitch and roll accuracy from the INS. But while UAVs need subsystems that are optimised for size, weight and power consumption – and indeed they are now far smaller and lighter than 5 years ago – USVs are far less restricted in these respects. The latter can therefore focus more on how agile the performance and data feedback from the GNSS solution is, to ensure efficient and effective manoeuvres in challenging environments. Self-driving road vehicles meanwhile need the highest possible accuracy of attitude data, with a focus on pitch, roll, velocity and other INS qualities derived from the integration of and collaboration between an IMU and a GNSS board. The centimetric accuracy of high-end RTK-GNSS solutions common among UAVs and USVs in mapping applications might not be totally necessary in the automotive space, although saving weight is still as welcome in a road vehicle as in an aircraft. As autonomous systems start carrying more expensive and higher-risk payloads such as freight, machinery and people, access to redundancy and corrective services via multi-constellation operation, support for SBAS-type satellites or even Receiver Autonomous Integrity Monitoring for GPS become every bit as important as the precision of the data and timing information, so they are becoming increasingly standard in commercial equipment. Also, a high input sensitivity is becoming more important for enabling a quick ‘time to first fix’. There is a difference between the precision, accuracy and integrity of GNSS receivers. Precision is how exact a location the device claims to provide, accuracy is how close the receiver believes that position is to reality as a statistical measure, and integrity is the degree of trust that can be placed in the ‘correctness’ of the information supplied. For instance, a GNSS receiver might state that its accuracy, in real time, is 5 m (95%), which means it believes that 95% of the time the positioning error will be less than 5 m. Integrity however requires that the user is assured of being contained within a given radius. To provide integrity requires a detailed understanding of the navigation system and receiver. Specifically, it means being able to detect and exclude errors in satellite data that can be caused by interference, such as from solar storms, the ionic disturbances in the atmosphere, multi-path effects near ground level or EMI from other devices on the vehicle. Such detection equipment must be engineered and verified to meet safety standards – aviation certification standards in the case of UAVs, for example. That involves an array of lab and field tests to ensure that adverse conditions are reported, and that proper actions are taken when confronted with injected errors and interference conditions. Many RTK-processed GNSS solutions will work well in the middle of a field of crops, but without high integrity their measurement precision, accuracy and integrity will deteriorate if they are used in a city full of metal and concrete structures that reflect and absorb signals. Crewed aviation has been subject to mandatory requirements for high- integrity GNSS in navigation instruments and air traffic control applications for many years. However, autonomous and uncrewed applications have no specific requirement yet, despite integrity being essential for safe navigation in the absence of a pilot or GCS operator to perform navigation functions. That could change in the years to come though. The level of integrity required for airborne autonomous GNSS | Focus To provide integrity means being able to exclude errors in satellite data that can be caused by interference such as solar storms or multi-path effects Uncrewed Systems Technology | October/November 2022 A number of powerful new correction services, inertial technologies and antenna advances are helping to provide continuous improvements in GNSS receivers (Courtesy of Trimble)