Staggered Examination of Non-Trusted Receiver Information (SENTRI) Algorithm for Spoofer Detection and Integrity Monitoring in GNSS Receivers

Abstract

Various types of harmful Radio Frequency Interference (RFI) have become more prevalent in recent years in disrupting GNSS operations for commercial aviation resulting in threats to safety and significant economic impact to the industry. These newer RFI threats, which include both jamming and spoofing, were not anticipated when GNSS-based navigation was originally certified. Hence, new mitigation techniques are needed to address the new threat environment. In this paper a technique called Staggered Examination of Non-Trusted Receiver Information (SENTRI) will be presented. SENTRI uses the navigation-grade Inertial Reference System (IRS) units already on the aircraft together with GNSS information to detect if the GNSS receiver is being spoofed. The essential idea behind SENTRI is that the IRS solution is compared to the GNSS solution and if there is sufficient divergence between the two, a spoofer alert is indicated. The SENTRI algorithm uses multiple overlapping inertial-only solutions, which are periodically reset when no spoofer is detected, in order to detect a spoofer attack. Utilizing the statistics of the GNSS position and the IRS-only position solution a SENTRI detection test statistic is derived as well as horizontal and vertical Protection Levels (PLs). The PLs are computed using integrity optimized RAIM principles and the detailed mathematical development will be presented in this paper. Given the pre-detection PL values a world-wide availability analysis will also be presented. The operational and performance tradeoffs of using the SENTRI PLs in conjunction with legacy RAIM PLs and SBAS PLs to protect against spoofer attacks and single satellite faults will also be discussed. The paper will conclude with a summary of results and certification recommendations.