Effects of Weighting Functions for Reference Receiver Clock Adjustment on GBAS Performance

Various GPS augmentation systems such as GBAS (Ground Based Augmentation System) and SBAS (Space Based Augmentation System) have been proposed and developed to provide airborne users with more accurate and reliable position solutions for precision approach. They broadcast correction information and the corresponding error level for users to eliminate common measurement errors for high accuracy position and estimate integrity protection level. As for GBAS, the reference station provides pseudorange corrections (PRC) and range rate corrections (RRC) for visible satellites along with standard deviation of their range errors and B-values of reference receivers. When GBAS generates the PRCs, it needs to adjust the reference receiver clock offset to limit the PRC values within a certain range specified by the international standards and perform cross-comparison to check the consistency of the PRCs from different reference receivers. RTCA and EUROCAE have described the clock adjustment algorithms in their GBAS standard documents differently.[1][2] EUROCAE introduces a weighted mean of PRCs to apply different weighting factors according to the satellite elevation angles to smooth the generated PRCs, while RTCA uses an average of PRCs as the clock offset. Since the clock-adjusted PRCs are directly used to generate broadcast PRCs, RRCs and B-values, different weighting functions cause different correction information. Consequently, they could have different impacts on their airborne user performance. This paper investigates the effect of the clock adjustment procedure and the weighting functions on sigma(pr _gnd) estimation and GBAS performance. It formulates the GBAS correction values with weighting functions mathematically and predicts the results of different weighting functions. Then, it analyzes their effects on GBAS integrity, continuity and availability performance with simulation results. Based on the analysis, it proposes strategies for sigma(pr_gnd) estimation considering reference receiver clock adjustment to prevent system integrity and continuity risk.