Computing SBAS Protection Levels with Consideration of All Active Messages

T. Sakai, K. Matsunaga, K. Hoshinoo, T. Walter

Abstract:	The integrity of SBAS is implemented as a protection level concept. SBAS broadcasts integrity information regarding uncertainty of corrected pseudorange. Based on the information, user receivers compute horizontal protection level (HPL) and vertical protection level (VPL) by applying the procedure defined in the SBAS standard. The SBAS provider must ensure that the broadcast information meets required integrity level; The actual user position error does not exceed the associate protection level. The receiver compares the protection level with the alert limit (formerly called as containment limit) defined for each phase of flight; In case that the protection level exceeds the alert limit, SBAS does not serve to the receiver for the associate phase of flight. If the protection level does not exceed the alert limit, the user position error also does not exceed the alert limit. In terms of ensuring integrity, the SBAS must ensure that any combination of visible ranging source satellites (GPS and SBAS) in the view of a user receiver does not cause a situation that the associate user position error exceeds the associate protection level because the SBAS could not specify the actual combination of satellites used in the user receiver. Additionally, the SARPS acutually specifies that the SBAS shall ensure required integrity for any combination of all active messages. Each SBAS message used for protection level computation is active for 6 to 1800 seconds until timed out except fast corrections overrode with the integrity alert. This means we should consider protection levels computed for all combination of broadcast messages not timed out, in addition to all combination of ranging source satellites. Such an approach is essential to ensure integrity but requires heavy computational load. Binding and Pruning Processes and Hypercube Method were developed to reduce computational load down to the realistic level. Employing these methods, it is possible to compute protection levels for all combination of all active messages which perfectly covers all possible situations for any kind of user receivers. Looking from another angle, we can choose applied messages so to compute the smallest possible protection level. By implementing this strategy into the user receiver, the system performance is improved a little. In summary, the authors propose an approach to verify integrity performance of the SBAS by computing protection levels for all combination of all active messages. The proposal includes some methods to reduce computational load and resulted chart perfectly covers all possible situations. On the contrary, computing the smallest possible protection level with consideration of all active messages gives a little performance improvement.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	2042 - 2050
Cite this article:	Sakai, T., Matsunaga, K., Hoshinoo, K., Walter, T., "Computing SBAS Protection Levels with Consideration of All Active Messages," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 2042-2050.
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