A Single Frequency Approach to Mitigation of Ionospheric Depletion Events for SBAS in Equatorial Regions

S. Wu, S. Peck, T. Schempp, P. Shloss, H. Wan, P. Buckner, P. Doherty, J. Angus

Abstract:	Currently, Satellite Based Augmentation Systems (SBAS) can provide a level of precision guidance for the single frequency users in the mid (magnetic) latitudes. In particular, the Wide-Area Augmentation System (WAAS) is certified for use in commercial aviation with Lateral Precision with Vertical guidance (LPV) approaches within the continental US and Alaska. The certification process ensures that this precision guidance meets the strict safety-of-life integrity requirements. Operational feasibility demands that LPV service be available nearly all the time except during a limited number of times during severe ionospheric activity. This paper addresses the question “Can an SBAS provide an operationally feasible level of precision guidance for the single frequency users in the equatorial latitudes?” The ionospheric phenomena typically found at the equatorial latitudes would significantly challenge current SBAS approaches to precision guidance. It is quite common to have large gradients in the Total Electron Content (TEC) between +/- 10 to 20 degrees magnetic latitude, especially during the peak of the solar cycle. This is often referred to as the equatorial anomaly. This macroscopic ionosphere feature is not approximated very well by the 5 x 5 degree grid thin shell model that current SBAS use. Also, depletion features often form during the evening hours (sunset to midnight). These microscopic phenomena (sometimes referred to as bubbles) cause sharp gradients in the isomorphic delay over fairly small baselines. Finally, the scintillation environment at equatorial latitudes could cause problems for the user and reference receivers. This paper addresses a backwards compatible approach that adds one ionospheric message (with less than double the current Ionospheric information content) and some additional user avionics processing to overcome these challenges. The performance of a distributed approach that uses a new ionospheric message and user integrity monitoring is analyzed using a combination of simulated and real data. The simulated data is obtained from a theoretical physics-based low-latitude ionospheric model (LowLat) model which captures the large scale features of the equatorial anomaly. This is used to evaluate the performance of the depletion free delay and Grid Ionospheric Vertical Error (GIVE) performance. Data collected from Brazilian receivers is used to model the effects of scintillation on the user and for preliminary analysis of a depletion detector algorithm. Results show promising LPV availability with such a system. The main conclusion of this paper is that an SBAS can provide an operationally feasible level of precision guidance for the single frequency users in the magnetic equatorial regions. The new approach requires changes in the Minimum Operational Performance Standards (MOPS) and the changes can be made in a backward compatible fashion. It further requires some additional user processing, which necessitates a software update on the existing avionic GPS receivers. This is a significant finding for both users and civil aviation authorities contemplating SBAS usage in these regions.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	939 - 952
Cite this article:	Wu, S., Peck, S., Schempp, T., Shloss, P., Wan, H., Buckner, P., Doherty, P., Angus, J., "A Single Frequency Approach to Mitigation of Ionospheric Depletion Events for SBAS in Equatorial Regions," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 939-952.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In