GNSS Satellite Autonomous Integrity Monitoring (SAIM) using inter-satellite measurements Original Research Article

Integrity is the ability of Global Navigation Satellite Systems (GNSS) to detect faults in measurements and provide timely warnings to users and operators when the navigation system cannot meet the defined performance standards, which is of great importance for safety of life critical applications. Compared with both Receiver Autonomous Integrity Monitoring (RAIM) and ground based GNSS Integrity Channel (GIC) methods which are widely adopted nowadays, the Satellite Autonomous Integrity Monitoring (SAIM) method can be used to monitor orbit/ephemeris and clock errors, and has advantages in monitoring orbit and clock quality and providing instantaneous responses when faults happen. This paper presents a SAIM algorithm, which utilizes inter-satellite measurements (ISM) for satellite orbit/ephemeris integrity monitoring. In the proposed algorithm, ISMs are obtained by the onboard ranging hardware. After removing the clock error component, ranging residuals are determined as the differences of the actual distance measurements and the ephemeris predictions between satellites. The test statistic is defined as the sum of the squared residuals. Under both Null Hypothesis (H0) and Alternate Hypothesis (Ha), the expressions and numerical results of the proposed test statistic are provided, including the threshold and minimal detectable bias (MDB). For the errors in radial direction, the MDB ranges from 7.35 m to 8.98 m, depending on the number of observations. The lower bound is proved to be close to the performance limit. A test criterion used for identifying a faulty satellite is provided. The performance of the proposed SAIM algorithm depends on the a priori knowledge of orbit errors, as well as on the inter-satellite observation geometry. By comparing all four global navigation systems (GPS/GLONASS/Galileo/Compass), it is noted that GPS provides rather good geometry because of abundant satellites in the constellation. The Compass system which incorporates additional five GEO satellites also provides sound performance.