An end-to-end simulator for the assessment of the performances of a spaceborne interferometric radiometer measuring the brightness temperature of Earth’s surface (ground and sea) is presented. The tool covers the complete simulation of the input brightness temperature map, the full instrument modeling, the instrumental error correction, and the inversion algorithm to recover the brightness temperature. The output maps are presented in a graphical format including most of the standard cartographic projections. The simulator has been developed with two main objectives: (1) to provide the engineers a tool to predict system performance, and (2) to provide the scientific and end users a tool to help them in the implementation and improvement of algorithms for the Soil Moisture and Ocean Salinity (SMOS) retrieval for the SMOS Earth Explorer Opportunity Mission of the European Space Agency.