Zenith lag

As zenith delay (engl. Zenith delay ) is in the Geodesy and astronomy referred to the influence that the earth's atmosphere on radio waves ( microwaves exerts) the exactly vertical (from the local zenith arrive). In light , the refraction integrated over the entire atmosphere from the sea ​​level (geoid) averages 240 cm, but fluctuates by a few centimeters due to the general weather conditions.

In the case of microwaves, there are additional deviations due to the humidity (vapor pressure), which has a stronger influence on radio waves than light. The zenith delay is therefore also a measure of the total water vapor content of a vertical column of air.

The zenith delay is an important parameter for modeling the high atmosphere , because the lower area (the troposphere ) can be modeled relatively accurately by terrestrial measurements. Its size and temporal or spatial variance is of decisive importance for satellite geodesy (see GPS and GLONASS ) and for astrometry (see VLBI with radio telescopes) in order to be able to evaluate the high-precision measurements to an accuracy of centimeters to millimeters.