Troposphere parameters are part of the parameter space of space-geodetic techniques and thus they present a basis for the combination of microwave techniques. They can provide valuable quality information about the consistency of other parameters due to correlations and they can be analysed modelling the water vapor content of the atmosphere in the vicinity of the geodetic observatories.
Combining space-geodetic techniques, usually station coordinates are linked by terrestrial difference vectors (local ties) at co-location sites. The combination of the troposphere parameters of microwave techniques provides a further link between the techniques. Therefore, it is necessary, that a consistent parametrization is used by all contributing techniques. In order to combine the troposphere zenith delays of different techniques, the effect of the height difference on the delay (tropospheric tie) must be considered. Tropospheric gradients are assumed to be identical parameters and are thus combined directly. The potential of different refraction models and troposphere models for the determination of tropospheric ties is under investigation.
While zenith hydrostatic delays are modelled considering equilibrium conditions of the dry atmosphere constituents, the wet constitutents, mainly water vapor, heavily vary in time and space due to insufficient mixing. Thus, wet delays are commonly estimated within the adjustment of microwave techniques together with other parameters. Therein, the most significant correlations occur among troposphere and station position parameters (see figure). Consequently, if station positions have to be solved with highest accuracy, highly-accurate troposphere parameters are required. Vice versa, it is possible to infer the consistency of station coordinates and velocities by investigation of estimated troposphere delays.
Homogenized and consistently re-processed long time-series of troposphere parameters are suitable for the study of water vapor climatology. The low number of geodetic observatories and their inhomogeneous global distribution enable local studies, but do not allow for the determination of regional or global water vapor models. The inclusion of radio occultation measurements, e.g. of the satellite CHAMP, would allow for a densification. However, the time series of the terrestrial observing stations, which cover periods of up to 30 years, allow for a validation of radiosonde measurements or of the water vapor in re-analysis models, e.g. ECMWF ERA-40, which are used for climatology.