Adequacy of the SBAS Ionospheric Grid Concept for Precision Approach in the Equatorial Region

R. Lejeune, M. Bakry El-Arini, J. A. Klobuchar and P. H. Doherty

Abstract:	Satellite-Based Augmentation Systems (SBASs) to the Global Positioning System (GPS) are being developed to facilitate aircraft navigation from en route navigation through Category I Precision Approaches (PAs) in several regions of the world. In mid-latitude regions, SBASs are expected to provide a high availability of Lateral approach Procedure with Vertical guidance (LPV) service, a PA service associated with a minimum decision height (DH) of 250 ft. SBASs use a standardized grid to communicate ionospheric delay information to single- frequency users. This approach has been found to work well in mid-latitude regions where spatial and temporal changes in the structure of the ionosphere are fairly smooth during magnetically quiet conditions (i.e., most of the time) and causes relatively small decorrelation errors and slant-to-vertical conversion errors. In contrast, the equatorial ionosphere is known to exhibit much higher range delays, as well as a much greater spatial and temporal variability, even during quiet magnetic conditions. These features of the equatorial ionosphere will noticeably complicate the task of modeling and communicating ionospheric delays. In fact, the adequacy of the current SBAS ionospheric grid concept for supporting a high availability of PA operations is not assured in this region. In a first look into this issue, techniques developed and used during the development of the U.S. Wide Area Augmentation System (WAAS) have been applied to simulated equatorial ionospheric data generated by the LowLat model for various levels of solar activity. The results of these simulations show large spatial decorrelation errors that can be more than an order of magnitude larger than those observed in the conterminus United States (CONUS) during periods of high solar conditions. Errors of the order of several meters due to the standard slant-to-vertical conversion have also been obtained. With errors of such magnitudes, designing a GIVE algorithm that protects the single- frequency user with the required level of integrity (i.e., with a probability of Hazardously Misleading Information of 10-7 or less) and, at the same time, generates GIVEs that are small enough to provide a high availability of any PA service during periods of high solar conditions appears to be a high risk endeavor.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	1330 - 1340
Cite this article:	Lejeune, R., El-Arini, M. Bakry, Klobuchar, J. A., Doherty, P. H., "Adequacy of the SBAS Ionospheric Grid Concept for Precision Approach in the Equatorial Region," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 1330-1340.
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