Elimination of tropospheric path delays in GPS observations with the ECMWF numerical weather model

Abstract

Recent comparisons have shown that zenith total delays (ZTD's) derived from Global Positioning System (GPS) observations and ZTD's calculated from analysis fields of numerical weather prediction (NWP) models agree within 1–2 cm (rms). Thus NWP models describe the tropospheric delay quite well, even though no GPS data are assimilated into the model. In this work we investigate how a NWP model can be used to calculate and eliminate the influence of the troposphere on GPS measurements. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) global weather model we calculate the GPS path delays for each single GPS observation using a high precision 3D ray tracing procedure and subtract it from the GPS observations.

We discuss two methods to process the corrected GPS observations. We find a reduction of the rms value of the phase residuals at low elevations, if the dry delay is eliminated and the wet delay is modeled in the adjustment. If the total delay is eliminated, tropospheric modeling of the remaining delay, whose correlation matrix will be determined, is still necessary. The analysis of the eigenvectors of the correlation matrix gives a set of mapping functions and weights which can be used to process the corrected GPS observations.