Toward Real-Time SQM for WAAS: Improved Detection Techniques

R.E. Phelts, T. Walter, P. Enge

Abstract:	Signal Quality Monitoring (SQM) algorithms propose to detect anomalous signal distortions primarily through the use of SQM receivers. These receivers employ anywhere from three or more correlator pairs per channel-each slaved to the tracking pair. The measurements from each correlator output can be used to form detection metrics, which are, in general, simple algebraic combinations of the measurements. However, previously such metrics have been derived based primarily on observations of the correlation peak distortions or based on modeled characteristics of the measurement noise. This approach has traditionally led to a relatively large number of recommended detection metrics, and hence threshold tests. SQM2b (the suite of detection metrics validated for Cat I LAAS SQM), for example, calls for 11 tests. For high-integrity applications, such as WAAS (and LAAS), large numbers of threshold tests pose several problems. The first is a continuity problem, since each test effectively poses an additional risk of false alarm. Another concern is the amount of energy and resources (i.e., manpower) required to validate the tests. For each, data must be collected, histograms must be analyzed, satisfactory overbounding must be performed for the integrity documentation. Given three SQM receivers at each of 25 WAAS reference stations, this could pose a significant workload. A third issue is one of efficiency and simplicity: fewer, more-effective tests should replace redundant and/or ineffective ones. A more-efficient metric would combine the measurements from all correlators in a way that accounts for the anomalous signal distortion and that due to thermal noise and multipath. This paper describes an approach for formulating such metrics based on finding the eigenvectors of the signal-plus-noise system. It then introduces one such metric based on the measurements from an SQM receiver having a 16MHz bandwidth and Early-minus-Late correlator spacings of 0.05, 0.1, 0.15, 0.2 chips, respectively. (The tracking pair was the narrowest pair of 0.05 chips.) This paper assesses the performance of these tests in addition to several phenomenological or heuristic ones-including those analogous to the ones used to validate Cat I LAAS. In addition, it provides and compares the alpha test specific to this SQM receivers, which is very similar to the ones expected for WAAS Offline Monitoring facilities.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	2739 - 2749
Cite this article:	Phelts, R.E., Walter, T., Enge, P., "Toward Real-Time SQM for WAAS: Improved Detection Techniques," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 2739-2749.
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