Many measurements to satellites, whether of direct scientific interest or merely for tracking purposes, require a knowledge of the atmospheric propagation delay which affects the observation, The delay has to be precisely determined for such purposes as geodetic or navigation positioning using the TRANSIT system or GPS (Global Positioning system) and gravity-field determination by precise orbit determination using satellite laser ranging. In particular, the high accuracy of satellite altimetry can be severely degraded by propagation delays not only in the altimeter measurement itself but also in the tracking-system observations.