Nominal GPS Signal Deformations: 10 years of WAAS Signal Quality Monitoring

Approximately 10 years of continuous WAAS signal quality monitoring (SQM) measurements have been collected. Numerous changes to the GPS satellite constellation, WAAS ground system, and user receiver configurations have occurred during that time. A number of anomalous signal deformation events have also been cataloged on L1 since 2009. Despite these various changes, WAAS SQM data can be used to assess the stability and evolution of nominal signal deformations of the L1 C/A code signals from the GPS satellites. In addition, the more recent availability of L5 SQM data and the addition of GPS Block III SVs allows comparisons to the original L1 algorithm and predictions for future nominal L5 characteristics. It is shown that WAAS SQM measurements remain sensitive to changes in nominal signal deformations. Also, while the limits of nominal deformations appear quite stable and signal anomalies tend to be rare, unexpected events can and do occur. Finally, though GPS Block III SVs have dissimilar signal characteristics to previous block types, the maximum nominal range biases on L1 continue to be well-bounded by the threat model and minimum range error limits required to guarantee integrity for single-frequency WAAS aviation users. It is anticipated this will hold true for future dual-frequency WAAS users as well.