GBAS for UAV Operations – The Positioning Service, Vertical Integrity and Operational Lessons Learned

In continuation of previous work, this paper addresses three challenges regarding the GBAS positioning service for UAV navigation. Firstly, the concept of ensuring the position integrity not only in the horizontal but also vertical domain is explored. As the GBAS positioning service as it is currently defined does not foresee vertical integrity, a vertical protection level, similar to the horizontal one, is developed. Secondly, the architecture and implementation of the system is shown such that real-time navigation is enabled. For this, the GBAS messages are received close to the GBAS ground station. They are then forwarded to a server via a TCP socket, stored into databases, and used for position calculation and error bounding. To ensure the timely message transmission and application we monitored the latency introduced by the architecture. Thirdly, during a dynamic flight the attitude of a drone may change frequently resulting in loss of track of satellites leading to a degradation of the navigation and integrity performance. This challenge is addressed by using a gimballed antenna mount. A test flight was carried out in May 2023 using a UAV at the glider airfield in Winterthur. For positioning and integrity evaluations corrections from the GBAS ground station at Zurich Airport (18 km away from Winterthur) were used. Results showed that the accuracy of the navigation solution was generally well below 1 m, horizontally and vertically. The protection level remained below 3.5 m horizontally and 9.5 m vertically. In both cases, the protection level was driven by the ephemeris error bounds. The monitoring of the latency indicated that the calculation lasts 0.09 s on average. This means that the calculation is sufficiently fast to prevent latency. Finally, using a gimballed antenna mount showed no significant improvements, as also for the static mount no significant number of satellites were lost.