lntegrity and Continuity of Service Analysis of LADGPS and WADGPS Architectures with SNAPSIS Simulator

B. Christophe, J. L. Jonquiere, D. Flament and N. Marchal

Abstract:	Since few years, the so-called WADGPS (Wide Area DGPS) and LADGPS (Local Area DGPS) systems are extensively studied and now considered as possible means for Civil Aircraft landing for phases of flight until Precision Approach CAT 1 (WADGPS) and even further (until CAT 3) for LADGPS. Operational requirements have been defined for each given phase of flight independantly of the navigation sensor. They have been expressed in the so called Required Navigation Performances (RNP) and Tunnel concept. The RNP are translated in Accuracy, Availability, Integrity and Continuity of Service. Under French Civil Aviation (DGACYSTNA) contract, Thomson-CSF has conducted theoretical analyses on the RNP requirements and then proposed both LADGPS and WADGPS architectures compliant with Precision Approach RNP requirements. More particularly, Thomson-CSF proposed an Integrity Monitoring System working at different levels in the navigation system. The more recent analyses were dealing with the difficult Integrity and Continuity Of Service performance assessments offered by the proposed system in various scenarii. These analyses have been carried on with the ONERA simulator. ONERA has developped, under CNES contract then under STNA contract, a simulation tool called SNAPSIS ‘Satellite NAvigation systems Performance analySIs simulation Software’. This simulator has been designed to modelise different types of differential system architectures and to analyse their performance ‘in term of RNP. At a previous congress, preliminary results with LADGPS and WADGPS architectures have been presented. Since, new modelisatiqns, both for the spatial segment and for the user segment, have been developped and their influence on the performances has been studied. Moreover, new methodologies have been used for the statistical evaluations of performances. At first, this paper will present the main Thomson- CSF conclusions about the RNP analysis and the resulting LADGPS and WADGPS solutions mainly from the Integrity and COS points of view. More particularly, we will expose the different elements of the Integrity Monitoring System. Then the different modules which compose SNAPSIS and which have allowed to simulate finely and accurately the different architectures with a great choice of possible components will be presented (ground segments, integrity monitoring algorithms, pseudolites, etc. .). Some illustrative results showing the great sensitivity of the RNP performances to some system parameter (SA dither characteristics, for instance) will be given. Finally, we will insist on the methodology applied to perform extensive Integrity and COS analyses based on a preliminary theoretical approch of these RNP parameters and the use of probability scaling in the simulations in order to achieve high confidence (99%).
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	797 - 806
Cite this article:	Christophe, B., Jonquiere, J. L., Flament, D., Marchal, N., "lntegrity and Continuity of Service Analysis of LADGPS and WADGPS Architectures with SNAPSIS Simulator," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 797-806.
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