where M(y) is the mapping function and y is the zenith angle of ray in the air, Dr1w;;2p;f is the DD WZD mainly caused by water vapor in the air, and Drl1;;p2;f is the DD liquid zenith delay caused by propagation through volume filled with liquid droplets. For a cumulus congestus, Drl1;;p2;f can be up to 5.6 mm, but it is less than 1 mm under usual atmospheric circumstances (Hanssen, 2001). Hence, for the atmospheric delays observed in the interferogram water vapor will be the main driving force. In many space geodetic techniques, e.g. GPS, the total wet delay is measured.