Robust Signal Quality Monitoring and Detection of Evil Waveforms

TLDR: This report evaluates the best current evil waveform (EWF) signal quality monitoring configuration, SQM2b, using the “secondorder step” threat model and compares the maximum regional pseudorange errors (PRE’s), computed at varying elevation angles, to the maximum allowable pseudorange (MERR) errors.

Abstract: The ability to monitor and detect problematic distortions in the received GPS-SPS signal is a task of critical importance. Detection of these satellite signal anomalies or “evil waveforms” can be accomplished using detailed monitoring of the correlation peak. Using the “Second- Order Step” threat model for evil waveforms, previous analysis has attempted to show that monitoring sufficient to satisfy GBAS and SBAS requirements for Category I precision approaches may be obtained from a receiver design that requires minimal modifications to existing GPS hardware. A “robust” signal quality monitor (SQM) design methodology has been developed for this purpose. This report evaluates the best current evil waveform (EWF) signal quality monitoring configuration, SQM2b, using this methodology. First, it compares the maximum regional pseudorange errors (PRE’s), computed at varying elevation angles, to the maximum allowable pseudorange (MERR) errors. Second, it examines the sensitivity of these results to differential group delay variations of up to 150ns. In addition, it examines the effects of user filter magnitude response. Using SQM2b, neither group delay variations nor changing MERR’s were found to pose a threat to standard E-L correlators in any of the four protected regions. However, they did induce some small, unacceptable ∆∆ receiver errors. To account for this, a “notch” was removed from the critical protected region to exclude those unprotectable ∆∆ receiver configurations. It was also shown that user PRE’s were sensitive to transition bandwidth variations. To mitigate this sensitivity it was recommended that the minimum airborne filter transition bandwidth be upper-bounded by that of the 6th-order Butterworth filter.