Robust Signal Quality Monitoring and Detection of Evil Waveforms

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 6-order Butterworth filter. BACKGROUND Satellite signal anomalies, or “evil waveforms” (EWF’s), result from a failure of the signal generating hardware on one of the GPS space vehicles (SV). These anomalies may cause severe distortions of the autocorrelation peak inside GPS receivers. In local area differential systems, undetected evil waveforms may result in large pseudorange errors, which in general do not cancel. One such failure occurred on SV19 in October of 1993. It caused differential pseudorange errors on the order of 3 to 8 meters [1]. The Local Area Augmentation System (LAAS) requires that a monitoring system be placed at ground reference stations to detect these failures when they occur. The envisioned signal quality monitoring (SQM) scheme would consist of one or more (wideband) GPS receivers having several correlators configured to sample the correlation peak at multiple locations in order to determine its level of distortion. Rather than simply sending corrupted differential corrections to airborne users, if an EWF is detected the SV would then be flagged and its pseudorange subsequently removed from the users position solution. For designing an SQM configuration, the “secondorder step” model was developed [1]. This model assumes the anomalous waveform is some combination of second-order ringing (an analog failure mode) and lead/lag (a digital failure mode) of the pseudorandom noise (PRN) code chips. Threat Models (TM) A, B, and C are used to characterize these failure modes. (See Figures 1,2,3). The model parameter bounds for fd (damped natural frequency) σ, (damping) and ∆ (lead/lag) are provided in Figure 4. An effective ground monitoring implementation would detect any and all EWF’s that would result in unacceptably large differential PRE’s. 0 1 2 3 4 5 6 2 . 5 -2 1 . 5 -1 0 . 5 0 0 . 5 1 1 . 5 2 2 . 5