Within the implementation of the European Geo-stationary Navigation Overlay System
(EGNOS), a significant residual error in positioning is due to tropospheric delay effects. The
EGNOS guidelines recommend that tropospheric delay 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 a simple average and
seasonal variation model is unlikely to emulate temporal weather changes exactly. The
potential errors involved in the application of the recommended algorithm and the
consequent effects on the positioning errors, under typical UK weather 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 ranged from 4·0 to 4·7 cm, with maximum errors ranging from 13·2 to 17·8 cm.
The 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 according to the satellite elevation angle.