Analysis and Verification of RAIM Techniques Implemented in GNSS Receivers
Cemil Kesik, Gazi University & Turkish Aerospace Industries, Inc., Turkey
Location: Pavilion Ballroom West
Alternate Number 2
GNSS (Global Navigation Satellite System) receivers mostly encounter with unpredictable events like jamming, spoofing, GNSS signal weakness etc. During the last few years, RAIM (Receiver Autonomous Integrity Monitoring) have been becoming increasingly important to avoid these kind of threats. Moreover, most of the civil aviation organizations require RAIM capable GNSS receivers and enforce users to use RAIM availability monitoring or RAIM prediction tools. Integrity monitoring technique is based on identification of faulty satellite and exclusion of it from the navigation solution. By this way receivers guarantee the integrity of the navigation output according to the MOPS (Minimum Operational Performance Standards) of aviation authorities. Recently developed GNSS receivers have ARAIM (Advanced Receiver Autonomous Integrity Monitoring) capability to make vertical guidance in addition to traditional RAIM which indicates only lateral integrity status. Furthermore, these types of receivers can eliminate multiple faults by using the multi-constellation feature in contrast with the traditional RAIM. It means that RAIM techniques such as range comparison method can also be used for comparison between different positioning and augmentation systems like GPS, GLONASS, Galileo, Beidou, WAAS, EGNOS etc. This paper focuses on verification and validation of FDE (Fault Detection and Exclusion) algorithms implemented in two different GNSS receiver models manufactured by different companies. One of the receivers uses “weighted least square estimation” for fault detection and “maximum residual method” for fault isolation; however, the algorithm implemented on the other receiver is not known. On the other hand, constellation capabilities of the receivers are different. One of them uses GPS, GLONASS, Galileo, Beidou, and QZSS while the other one uses GPS and GALILEO. To make a reliable verification, same flight route scenarios including injected faults were applied by using a GNSS simulator. These scenarios are in accordance with RTCA DO-229E “Minimum Operational Performance Standards for Global Positioning System/Satellite-Based Augmentation System Airborne Equipment”. By making such an experimental verification, this paper aims to reveal whether the GNSS receivers’ achieved performances are convincing or not. HPL (Horizontal Protection Level) and VPL (Vertical Protection Level) values of the receivers are decisive to make a proper comparison. This study also allows to assure fault detection and exclusion capability of the GNSS receivers by implementing several complicated flight scenarios using GNSS simulator. The second phase of this study continued as follows: the GNSS signals applied using GNSS simulator have been manipulated until the protection levels exceed the limits identified by aviation standards. Comparison of the position, velocity, and DOP (Dilution of Precision) information provided by the receivers have been also performed in order to observe the effects of the GNSS signal degradation on the navigation solution.
The obtained results clarify that how algorithms differ from each other, and provide the user to estimate the unknown algorithm within the receiver. Test have been performed for only GPS (Global Positioning System) constellation because GNSS simulator is only able to simulate GPS signals despite the receivers have multi-constellation capability; and hence, this study is a preparation work for evaluation of multi-constellation RAIM approach.
The full paper will present the flight scenarios test results in details via related figures and charts.