Signal quality monitoring of SBAS for satellite-induced elevation-dependent anomaly

GNSS users may suffer satellite-induced elevation-dependent (SIED) ranging errors when some faults occur in the signal generation hardware onboard, as in the cases of GPS SVN-49 and BDS-II satellites. The wide-area differential corrections of SBAS will be invalidated in face of a SIED anomaly due to the various elevations of the widely distributed reference stations; thus, the integrity for safety–critical users may be damaged. Signal quality monitor (SQM) is utilized to detect potential hazardous deformations in GNSS signals and protect the integrity. However, in face of a SIED anomaly, the reference-averaging process of current SQM architecture is invalidated as well because the SQM measurements of reference stations will be elevation-dependent. We propose two new approaches to augment and enhance the current SQM method, namely reference-station-voting and metric-differencing processes, and develop a methodology of SQM algorithm design and evaluation as a support for validation of the improved SQM method. By applying GPS L1 C/A signals, we verify that the proposed new approaches of the improved SQM method as well as the designed SQM algorithms are effective. Furthermore, we prove that a hybrid SQM algorithm based on both multi-correlator and chip domain observables is able to protect the SBAS users against SIED anomalies with a performance margin of about 4 dB under the requirements of Category-I precision approach of civil aviation.