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. 

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

Detection and Localization of Radio Frequency Interference Sources

This paper investigates the conditions under which critical satellites can occur in the potential future GAST-D+, which processes ranging signals from one frequency and uses satellites from the GPS and the Galileo constellations. For this purpose, VPLs are simulated using GAST-D+ and GAST-D processing for 612 locations organized in a grid for latitudes from -80° to +80° and longitudes from -170° to +180°, containing cells spaced by 10° in each direction. The simulations are conducted over 24 hours and are based on broadcast ephemerides for the GPS and Galileo satellites. Throughout the simulations, varying numbers of highest-weighted satellites are deselected in every second of the simulation and at each location. This is done to account for unhealthy highest-weighted satellites in the constellations due to failure or maintenance as well as to simulate the sudden loss of a highest-weighted satellite. Occurrences of critical satellites are identified based on the simulated VPLs. Detailed analyses of the grid and VPLs are carried out for cases in which three and four highest-weighted satellites are deselected. The results show that critical satellites can occur in GAST-D+ processing when one highest-weighted satellite of each constellation is unhealthy, and a further highest-weighted satellite is lost. Increasing the number of unavailable satellites in the constellations increased the number of occurrences of critical satellites. Therefore, it cannot be ruled out that critical satellites occur in GAST-D+ processing at latitudes relevant for GBAS users when one highest-weighted satellite of each constellation is unhealthy, and a further highest-weighted satellite is lost. 

Analysis of the Occurrence of Critical Satellites in GAST-D+ Processing

In this study the concept of using GBAS corrections and integrity parameters for UAV operations is explored. The GBAS messages are received and decoded at the airport to overcome the issue of VDB coverage for alternative applications that do not have line-of-sight to the GBAS. A flight test at a distance of about 15 km from an operational GBAS ground station was conducted using a quadrotor UAV as test vehicle. The recorded GBAS messages and recorded pseudoranges and carrier phases were subsequently used to evaluate the performance that can be obtained. One main issue that was identified is the number of satellites lost when maneuvering the drone. Loss of tracking requires a re-initialization of the smoothing filter and thus entails a wait time before a satellite can be used for positioning again. The GBAS corrections are generated based on 100 s smoothing, requiring a user to use the same smoothing time constant do avoid potential differential error buildups due to different filters. Hence, the time until a satellite can be used again after reacquisition may be substantial. When losing several satellites simultaneously or shortly after another this decreases the number of available satellites significantly and negatively impacts accuracy and protection level performance of this proposed mode of operation. The paper also studies the multipath effect at different altitudes and for different smoothing time constants compared to the standardized error models that were defined for large transport aircraft. When using 100 s smoothing the results indicate that the models are suitable also for the use case of UAV operations. 

Flight Testing GBAS for UAV Operations

A Cost-Efficient RFI Localization Approach to Detect GNSS Jamming and Spoofing

Navigating Interference - Examining In-Flight GNSS Spoofing Patterns and Signal Disruptions

Galileo HAS Availability Analysis Using the Internet Data Distribution Service

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