Concept for New SBAS Message Types to Improve GIVE Resolution

Dan Cormier, Thomas L. McKendree and Timothy R. Schempp

Abstract:	The Satellite Based Augmentation System (SBAS) ionospheric correction parameters, including the Grid Ionospheric Vertical Error (GIVE), convey the degree of uncertainty in the broadcast ionospheric delay estimate applied by the user. The ionospheric uncertainty is given by the ó2 i,GIVE value indicated by each of the 16 possible GIVEIi values. These values were originally developed as part of RTCA efforts to develop a table that would be effective for the United States of America. A number of other locations in the world experience more dynamic ionospheric conditions, including a significant density of ionospheric delay uncertainties at levels where the ó2 i,GIVE table has low resolution. An expansion of the SBAS message set to allow greater resolution in describing ionospheric delay uncertainty may thus be useful for many countries. Greater resolution would also improve availability and continuity of integrity operations by reducing quantization losses in calculating the ionospheric integrity bound. One concept to achieve this goal would be to have a new message type, analogous to Message Type 26, but with more GIVEIi bits per Ionospheric Grid Point (IGP). A second concept to achieve this goal would be to have a new message type that augmented Message Type 26 with additional GIVEIi bits per IGP. The easiest concept to describe is to define a new message type analogous to Message Type (MT) 26 with more GIVEIi bits per IGP and fewer IGPs per message. Backward compatible operations on L1 could be achieved only by also broadcasting MT 26, which would result in a total bandwidth requirement over twice as large as that of MT 26 alone, and include a great deal of redundant information with this new message type. Since there is no current standard for an SBAS L5 signal, there is no requirement for backward compatibility on broadcast message types. Thus, on an L5 SBAS this new message type could be used instead of MT 26. To avoid using excessive bandwidth on L1 broadcasting a new message heavily redundant with MT 26, a second approach would be to broadcast a new message that merely adds interpolation bits to the GIVEIi for IGPs. If each interpolation is taken to reduce the corresponding ó2 i,GIVE by some amount, then full backward compatibility will be retained, as MT 26 will be bit-for-bit the exact same message with and without this additional message in the data stream. Timing for this second approach requires careful design. The user should receive the new interpolation message before receiving the MT 26 to which it will be applied. Otherwise, it is necessary for the user to apply the appropriate ó2 i,GIVE value as provided by MT 26, losing the benefit of the finer resolution. These two message types are alternative means of solving the same issue. The replacement for MT 26 appears more attractive in situations where backwards compatibility with RTCA/DO-229C users is not required, and the supplement for MT 26 appears more attractive in situations where backwards compatibility with RTCA/DO-229C users is required. Thus, these two concepts could be definitions on the same message type, with a data ID included to distinguish them. Here, we provide the details of a conceptual design for these new SBAS Message Types. We begin by discussing the need for improving the resolution of ionospheric uncertainty values available for broadcast in order to support Vertical Guidance operations in lowlatitude regions and to improve availability and continuity of such operations in mid-latitude regions. We describe the advantages and disadvantages of several options for providing the improved resolution, which will lead us to the two preferred message concepts. For these preferred approaches, we describe the message structure, paying particular attention to the discussion of bandwidth and timing issues in order that the new messages can be successfully incorporated into SBAS operations. Finally, we provide examples for both mid- and low-latitude regions of the expected improvement in performance due to the implementation of the new messages into SBAS operations.
Published in:	Proceedings of the 2005 National Technical Meeting of The Institute of Navigation January 24 - 26, 2005 The Catamaran Resort Hotel San Diego, CA
Pages:	1140 - 1147
Cite this article:	Cormier, Dan, McKendree, Thomas L., Schempp, Timothy R., "Concept for New SBAS Message Types to Improve GIVE Resolution," Proceedings of the 2005 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2005, pp. 1140-1147.
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