Azimuth-based antenna group delay variation modeling for dual-frequency multi-constellation GBAS

Antenna Group Delay Variation (AGDV) is a hardware error source that affects the performance of Dual-Frequency Multi-Constellation (DFMC) Ground-based Augmentation System (GBAS), and these errors are difficult to distinguish from multipath errors. Currently, AGDV is usually modeled as a part of the multipath error, which is called the multipath-AGDV model. However, because of the inconsistency of AGDV and multipath when switching among different positioning modes of GBAS, and because the traditional model does not consider the impact of the azimuth on AGDV, using the traditional multipath-AGDV model will cause the protection levels to be inaccurately calculated. In this paper, azimuth-based modeling of AGDV is conducted by using anechoic chamber measurements. The biases and standard deviations of AGDV based on azimuths are analyzed and modeled, and the calculation method for the DFMC GBAS protection level is optimized. The results show that the azimuth-based AGDV model and protection level optimization algorithm can better avoid the error exceeding the protection level than the multipath-AGDV model. Compared with AGDV elevation model, the VPLs of the B1C signal are increased by 0.24 m and 0.06 m, and the VPLs of the B2a signal are reduced by 0.01 m and 0.16 m using the 100 s and 600 s DFree filtering positioning modes, respectively. The changes in the B1C and B2a protection levels reflect the changes in AGDV corresponding to the azimuth for the respective frequencies, further ensuring the integrity of airborne users, especially when they turn near the airport.