Water Vapor Radiometry in Geodetic Applications

The technology that is being used now in geodesy offers the potential to measure distances as large as several thousand kilometers with accuracies of 2 to 5 cm as described by Shapiro and Knight (1970) and Resch (1980). The geophysical information content at this level of accuracy is enormous. The measurement techniques divide according to the wavelength sensitivity of the detection apparatus. At optical wavelengths, the principal technique is satellite laser ranging. At microwave frequencies there are ranging systems and interferometric systems. One of the principal advantages of the microwave systems is the “all-weather” capability. The principal disadvantage of the microwave systems is the sensitivity to propagation path effects that are induced by atmospheric water vapor. Water vapor is not a well mixed atmospheric constituent and it is impossible to model with high accuracy. If our accuracy goal in a position or baseline measurement exceeds 2 to 5 cm, then we must measure the line-of-sight vapor delay. The most promising technique available for making the vapor measurement is the use of passive microwave radiometry.