Assessment of EGNOS Tropospheric Correction Model

A.H. Dodson, W. Chen, H.C. Baker, N.T. Penna, G.W. Roberts, R. J. Jeans and J. Westbrook

Abstract:	Tropospheric delay is one of the major error sources in satellite navigation. The signal transmission delay caused by the troposphere can be over 2 m at the zenith and 20 m at lower elevation angles (e.g. below 10 degrees). If the tropospheric delay is not properly modelled, positioning errors in excess of 10 m can result. From the EGNOS implementation, a significant residual error in positioning is due to tropospheric delay effects. The ICAO SARPS recommend the application of an em-pirical correction algorithm, based on a receiver’s height and estimates of five meteorological parameters using av-erage and seasonal variation data (Collins and Langley, 1997). However, such a simple average and seasonal variation model is unlikely to model the temporal weather changes exactly. Although this model is globally appli-cable, most of meteorological data used to generate the model was from North American. This paper aims to evaluate the potential error involved in the application of the recommended algorithm and the consequent effects on the positioning errors, under typical UK weather condi-tions. Two locations in the UK have been selected for the study. In order to assess the seasonal variation of weather condi-tions, data from four weeks in different seasons have been sampled over a period of one year. The tropospheric de-lays from the EGNOS recommended tropospheric model were compared with the measured delays using static ground-based GPS, to evaluate the accuracy of the model. We believe that this is the first time that a tropospheric delay model has been tested by such a direct measure-ment. The position errors caused by the use of this model were analysed using a GPS simulation package. It is shown that with this model, the maximum vertical tropo-spheric delay error is about 16 cm, which may result in horizontal and vertical positional errors of up to 43 cm and 155 cm respectively.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	1401 - 1408
Cite this article:	Dodson, A.H., Chen, W., Baker, H.C., Penna, N.T., Roberts, G.W., Jeans, R. J., Westbrook, J., "Assessment of EGNOS Tropospheric Correction Model," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 1401-1408.
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