Contents I. Introduction
Microwave radiometers have been widely used in propagation applications for the experimental characterization of the atmospheric radio-propagation channel. In space geodesy applications, microwave radiometers have been used for the accurate estimation of the path delay due to the water vapour, for model development, and were the reference instruments, together with radiosondes, for the validation of integrated precipitable water vapour (IWV) retrieval methodology by using GNSS receivers [1], [2]. The ability of water vapour radiometers to calibrate changes in tropospheric delay was also demonstrated during very long baseline interferometer (VLBI) observations at Goldstone, California [3], [4]. A study devoted to the development of retrieval algorithms and to the identification of radiometer calibration and stability requirements, is given in [5], in the framework of the Cassini radio science experiment. A specific module of the Deep Space Mission Systems (DSMS) Telecommunications Link Design Handbook [6] is devoted to provide propagation models for the NASA Deep Space Network (DSN). The Ka-band model in [6] is based on actual water vapor radiometer noise temperature measurements made at 31.4 GHz at all three DSN sites (Goldstone, Canberra, and Madrid) and L-/S-band and X-band statistics were created from the Ka-band statistics through frequency scaling. Microwave radiometers can play an important role in the calibration of beacon receivers as demonstrated during the European Program Olympus [7]. The power level available at the receiver output of a beacon receiver station depends on the status of the radio channel.
