Assessment of EGNOS tropospheric correction model

Within the implementation of the European Gee-stationary Navigation Overlay System (EGNOS), a significant residual error in positioning is due to trop ospheric delay effects. The EGNOS guidelines recommend that tropospheric de lay is modelled using an empirical correction algorithm based on a receiver 's height and estimates of meteorological parameters developed from average and seasonal variation data. However, such se simple average and seasonal variation model is unlikely to emulate temporal weather changes exactly. Th e potential errors involved in the application of the recommended algorithm and the consequent effects on the positioning errors, under typical UK wea ther conditions, are detailed in this paper. This was achieved by comparing tropospheric delays produced by the EGNOS model, with tropospheric delays estimated from high precision carrier phase GPS, over a one-year period for five UK stations. The RMS EGNOS model zenith tropospheric delay errors ran ged from 4.0 to 4.7 cm, with maximum errors ranging from 13.2 to 17.8 cm. T he errors were also shown to be spatially correlated. The subsequent effect on position error is shown to be dependent on the satellite elevation cut- off angle adopted and on whether or not the observations are weighted accor ding to the satellite elevation angle.