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Tipping calibration is an important technique for the absolute calibration of ground-based microwave radiometers. The standard tipping calibration is usually used in clear atmosphere. The spatial variations of liquid water in cloudy atmosphere may lead to great calibration errors in the process of standard tipping calibration. A new method of tipping calibration for ground-based microwave radiometers is proposed, which uses an accurate channel to calibrate another inaccurate channel in cloudy atmosphere. The simulation and measurements show that the method proposed here can eliminate the radiation of cloud liquid water and calibrate the radiometer under cloudy-sky conditions effectively. A relative tipping calibration is also introduced to retrieve the parameters in cloudy atmosphere. It has been proven that the relative tipping calibration can reduce the retrieval error significantly in cloudy atmosphere even though neither of the two channels is accurate. [ABSTRACT FROM PUBLISHER]

A prerequisite for the successful retrieval of geophysical parameters from remote sensing measurements is the development of an accurate forward model. The European Space Agency Soil Moisture and Ocean Salinity (SMOS), carrying onboard an L-band interferometric radiometer (Microwave Interferometric Radiometer using Aperture Synthesis), was launched on November 2009. Due to the lack of L-band passive ocean measurements from space, several prelaunch forward models were developed and initially used in the SMOS ocean salinity operational processor. In this paper, an update of the prelaunch semi-empirical forward model is presented, using for the first time, real SMOS data. In particular, the ocean surface emissivity modulation at L-band due to rough sea surface is reviewed and reanalyzed. A new model definition is provided with the help of a simple neural network. The improvement is quantified in terms of retrieved salinity accuracy compared with the climatology and concerns essentially the range of wind speeds higher than 12 m·s-1. [ABSTRACT FROM AUTHOR]