Impact of antenna group delay variations on protection levels

Requirements for single frequency airborne antenna group delays were developed after the completion of ground and airborne specifications for aircraft precision approach operations using the Global Positioning System (GPS). Airborne antenna group delays were not included in the aircraft position protection level calculations as they were assumed to be part of the allocation for aircraft pseudorange noise and multipath. Unfortunately, the dual-frequency antennas used for the characterization of aircraft pseudorange noise and multipath exhibit much smaller group delay variations as a function of signal arrival angle than the allowable errors for single frequency antennas. Therefore, antenna group delay variations must be treated as a new error source for high-accuracy aviation applications such as the Ground Based Augmentation System (GBAS), the Space Based Augmentation System (SBAS) and future Advanced Receiver Autonomous Integrity Monitoring (ARAIM) concepts. This paper provides an analysis of the impact of antenna group delay variations on aircraft position protection levels for single frequency users. In addition, it is shown that dual-frequency users experience similar group delay errors as single frequency users when an ionosphere-free pseudorange solution is used.