Validating Nominal Bias Error Limits Using 4 years of WAAS Signal Quality Monitoring Data

R. Eric Phelts, Eric Altshuler, Todd Walter and Per Enge

Abstract:	Nominal signal deformations are small imperfections of GNSS signals that cause them to differ from one another. While generally small, they exist on all SV signals and potentially lead to user range biases that are present all the time. These subtle effects have been measured in the past using specialized equipment and processing techniques and also using more conventional receivers as well. Augmentation systems such as SBAS, GBAS and ARAIM rely on assumptions about the magnitudes and the stability of these biases to ensure safe, high-integrity navigation for aviation users at all times. However, little is known about how these distortions evolve over time, how they are affected by satellite configuration and system changes, or how well the existing continuously-operating, signal monitors can measure these nominal biases. To this end, this paper analyzes approximately 4.5 years of continuous WAAS signal deformation monitoring (SDM) measurements that have been collected. The results are used to assess the evolution of nominal signal distortions of the L1 C/A coed signals from all the GPS satellites in operation over this time period. Old and new satellites are compared and changes due to potential anomalies or system changes are discussed. It is shown that the WAAS signal deformation monitor measurements agree with past high-resolution measurements of nominal signal deformations, and that they are very sensitive to changes in the signal deformations. And, while the signals are quite stable and significant signal deformation events tend to be rare, unexpected changes can occur. Accordingly, the maximum nominal range biases on the C/A codes of the current GPS constellation may be quite close to the range error limit required to guarantee integrity of future dual-frequency WAAS aviation users.
Published in:	Proceedings of the ION 2015 Pacific PNT Meeting April 20 - 23, 2015 Marriott Waikiki Beach Resort & Spa Honolulu, Hawaii
Pages:	956 - 963
Cite this article:	Phelts, R. Eric, Altshuler, Eric, Walter, Todd, Enge, Per, "Validating Nominal Bias Error Limits Using 4 years of WAAS Signal Quality Monitoring Data," Proceedings of the ION 2015 Pacific PNT Meeting, Honolulu, Hawaii, April 2015, pp. 956-963.
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