The integral of refractivity along the propagation path introduce a time delay affecting the time of arrival of the GPS signal at the receiver antenna. Most of the observed variations is typically caused by the atmospheric water vapor.
Assuming a horizontally stratified atmosphere the timing of radio signals arriving at different elevation angles allows an estimation of the size of the propagation delay. This effect was in the late eighties and the early nineties often estimated as a constant atmospheric correction in the zenith direction which was valid for a session lasting from some hours up to a day. When the quality of GPS receivers and the satellite constellation improved it was possible to increase the temporal resolution in the analysis. Today we do not only estimate a zenith delay but also local parameters for spatial variations, such as horizontal linear gradients. During the last couple of years estimations based on tomographic methods and using several GPS receivers in dense networks have also been evaluated.
Validation of the GPS estimates is often carried out through comparisons to data from microwave radiometry and/or radiosonde observations. These comparisons results in conclusions concerning the different types of error sources involved which are of fundamental importance when data are to be used in meteorological research, operational weather forecasting, or in climate monitoring.
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