Flight Testing GBAS for UAV Operations

Abstract

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-ofsight 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.