SISE Prediction and Iono/Tropo Corrections in a Local Element Augmentation System

Riccardo Di Corato, Stefano Falzini, Andrea Beccarini, Alessandro Caporali, Carlotta Morini, and Tommaso Pepi

Abstract:	This paper describes principles and algorithms to predict the Signal in Space Error (SISE) in a local scenario, and to compute user position dependent ionospheric and tropospheric corrections. This work has been performed under the GALILEA (Galileo Local Elements Augmentation) project, co-funded by GJU (Galileo Joint Undertaking). SISE prediction and iono/tropo corrections can be considered as additional features, with respect to the baseline services presently planned for Galileo local elements. We assume an application scenario where a user integrates the Signal in Space (SIS) with data generated locally. The basic concept is to provide, to the users in the proximity of reference stations, additional information to improve both the accuracy and the safety of the navigation. This approach may lead to a Galileo generated business application, in the sense of local providers of integrity data, system performance and differential corrections. We address here the question on whether the SISE for each satellite in view can be predicted in the near future. The practical relevance of this problem rents on the opportunity for users to be alerted with some time in advance about a change of the integrity flag for each satellite in view. To overcome the lack of globality that could affect the SISE estimation accuracy and that is inherent to an approach based on few local stations, we opted for the use of the IGS orbital and clock high accuracy predictions. The selected data fusion approach consists in correcting the IGS SISE prediction with realtime error measurements (SISE corrections) in order to take in account effects not included in the IGS prediction (e.g. unpredicted failure). SISE corrections are computed applying a recursive digital filter to the pseudorange residuals fitted with an RBTB model and averaged on a number of reference stations located in the local area. Concerning the ionospheric and tropospheric corrections, algorithms have been developed after a trade-off between different models, taking in account the specific Galileo features and the local architecture. The ionospheric correction module is based on an analytical threefrequency algorithm, which allows the second order terms of the ionospheric error to be taken into account. The tropospheric correction module makes use of local measures of temperature, pressure and humidity collected by a network of meteorological stations; this can be considered as an important additional service for a local user, since the tropospheric effects tend to rapidly change with space and time. Ionospheric and tropospheric corrections are output in the form of coefficients: in this way, the corrections are dependent on the user position. The coefficients are computed interpolating the iono/tropo delays resulting by the network of local stations.
Published in:	Proceedings of the 2007 National Technical Meeting of The Institute of Navigation January 22 - 24, 2007 The Catamaran Resort Hotel San Diego, CA
Pages:	979 - 989
Cite this article:	Di Corato, Riccardo, Falzini, Stefano, Beccarini, Andrea, Caporali, Alessandro, Morini, Carlotta, Pepi, Tommaso, "SISE Prediction and Iono/Tropo Corrections in a Local Element Augmentation System," Proceedings of the 2007 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2007, pp. 979-989.
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