A new approach to mapping the atmospheric effect for GPS observations

In space to earth geodetic techniques the atmosphere affects the radio wave propagation. In order to estimate the total non-zenith path delay several mapping functions have been proposed to scale the zenith delay to other elevations. For all of them the basic idea is that the ultimate direction of the ray path is exactly that of the vacuum elevation of the radio source, and this is used to estimate the delay. This is true when we consider VLBI observations from a distant radio source but not the case for satellites or other near-earth radio sources. In this case the radio source distance is no longer infinite, and we have to think of a slightly different radio ray direction. This may insert a reasonable error in delay estimation. Thus, for satellite observations, the source elevation angle must be corrected for the ray bending effect before we use it with a mapping function to scale the delay. This means that either we have to estimate this correction of the angle or we have to develop a model that includes the correction. In this report we evaluate the magnitude of the correction and discuss the way we can approach and map it. Another purpose of this report was to study different mapping functions used to predict the hydrostatic delay at low elevation angles. Also a study of models with constant terms shows that such a model optimized for a site or region can give satisfactory results.