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The purpose of the Soil Moisture and Ocean Salinity (SMOS) mission is to measure soil moisture and sea surface salinity (SSS). The measurement of SSS using microwave radiometry requires a very sensitive instrument. In SMOS, the image is formed using the interferometric technique complemented by the average brightness temperature, or zero baseline, to set the absolute level of the image. Therefore, the measurement of the zero baseline is very critical for the success of the mission. In this paper, the radiometric resolution and stability of the radiometers dedicated to the measurement of zero baseline on SMOS are estimated. The results of a measurement campaign carried out in an anechoic electromagnetic compatibility chamber are used. The results show that the zero-baseline radiometers have the potential for relative accuracy better than 20 mK, depending on the integration scenario, satisfying the mission requirement for SSS retrieval.