Measurement Noise versus Process Noise in Ionosphere Estimation for WAAS

J. Blanch, T. Walter, P. Enge

Abstract:	One of the parameters driving the performance of the Wide Area Augmentation System (WAAS) is the Grid Ionospheric Vertical Error (GIVE). The GIVE bounds the estimation error of the ionospheric delay at each Ionospheric Grid Point (IGP). The GIVE is generated such that a user interpolating both the vertical ionospheric delays at the IGP and the GIVE, is protected. The GIVE is a function of several parameters: the geometry of the measurements, the measurement noise, and the state of the ionosphere, which yields the process noise. It is very important to distinguish carefully between measurement noise and process noise, as they have a different behavior in the generation of the confidence bound. The measurement noise contribution to the confidence bound tends to zero as the number of measurements increases. The process noise, characterized in the current WAAS algorithm by a standard deviation around the planar trend in nominal conditions, indicates a lower bound for the confidence bound. For each satellite, the measurement noise is well characterized as a function of the elevation and tracking time. However, the process noise, which has a large variability due to the possibility of storms or mild irregularities, is measured or tested only through observations that have measurement noise. For this reason, the observability of the state of the ionosphere is impaired by the measurement noise. Although this is not a problem for the current level of service for WAAS, it could become an issue as we try to increase availability by decreasing the conservatism of the nominal state of the ionosphere. In this study, we develop a set of formulas to evaluate the Probability of Hazardously Misleading Information (PHMI) for two possible algorithms. This analysis takes into account the loss of observability of the state of the ionosphere (which determines the process noise) due to the measurement noise. It can be applied to any vertical ionospheric delay model that has a deterministic trend and a random gaussian component. We will also see in what cases it is essential to distinguish between measurement noise and process noise. This analysis will help maintain integrity for further improvements to the WAAS ionospheric algorithms.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	854 - 860
Cite this article:	Blanch, J., Walter, T., Enge, P., "Measurement Noise versus Process Noise in Ionosphere Estimation for WAAS," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 854-860.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In