Bounding Errors Caused by Nominal GNSS Signal Deformations

Nominal signal deformations are present in GNSS-GPS and WAAS-GEO satellite signals. They result in pseudorange errors, which in turn cause navigation errors for GNSS users. These navigation errors depend on the userand reference-receiver configuration parameters as well as satellite geometry and user location. Previous papers by the authors focused on obtaining a better understanding of the pseudorange errors caused by nominal signal deformations. For this paper, we examine the user navigation error effects of nominal signal deformations from many perspectives: based on the pseudorange tracking errors from real GPS data, what would be the resultant worst case position errors? When do they become a significant integrity concern? Which user receiver configurations appear least (or most) sensitive to them? What are the effects in the dualfrequency and/ or depleted constellation configurations? The user errors are also a function of best-case and worstcase satellite geometries at the user receiver, which are in turn dependent on user/ reference receiver locations. Nominal and degraded satellite geometries for different user locations were generated using the MATLAB Algorithm Availability Simulation Tool (MAAST) Matlab toolbox, developed at Stanford, and the best and worst case position errors were determined for these cases. The results presented are for typical WAAS aviation users of a single-frequency, GPS-only constellation, but they can be easily extended to multiple-frequency, multiconstellation systems. These results may be especially important with the upcoming increasing availability of dual-frequency systems – the use of linear combinations of signals from the same satellite to remove ionospheric errors in turn scales the signal-deformation-induced errors. These results indicate that for dual-frequency WAAS users, mitigation is needed to protect against navigation errors from nominal signal deformation. This paper also proposes one such practical method – augmentation of the existing Vertical Protection Level (VPL) equation with a Vertical Error Bounds for Signal Deformation (VEBSD). This method is shown to be an effective protection mechanism.