Additional SQM metrics for generic TM detection

Abstract

The ICAO Threat Model describes three cases of distortions that can occur on GPS L1 C/A signals. It defines digital and analog failures that may occur on the satellite payload. To cover a wider area of possible distortions, a Generic Threat Model (TM) based on the Fourier Transform of the signal distortions is proposed. Then, the distortion effects have to be detected and mitigated through Signal Quality Monitoring (SQM) algorithm, which is implemented on SBAS and GBAS ground infrastructures. The basic form of SQM uses simple metrics based on correlator outputs that are normalized by the correlation function prompt. Those normalized outputs can be used directly as a simple ratio metrics. They can also be combined to compute difference or sum metrics between symmetric correlator outputs pairs (with respect to the prompt) [1]. In this paper, an extended set of metrics is tested. It includes simple ratio, sum and difference between all monitored correlator outputs. The influence of several parameters related to correlator output positions is studied. The benefits provided by these new metrics compared to a baseline SQM are assessed and conclusions on the performed SQMs are made. The carried out tests are applied to Galileo E1 OS signals.