Since the mid 1980s, aperture synthesis interferometric radiometers have received increased attention to monitor the Earth at low microwave frequencies (L-band), where there is a maximum sensitivity to soil moisture and ocean salinity. At L-band, classic radiometers require large steerable antennas to meet the spatial resolution requirements (30-50 km at most, 10-20 km wished for), from a low polar orbit platform. During the 1990s, technological studies were conducted by the ESA with an eye to design a 2D synthetic aperture L-Band radiometer (the Microwave Imaging Radiometer by Aperture Synthesis project: MIRAS). In 1998, in answer to a call for Earth Explorer Opportunity Missions issued by ESA, the Soil Moisture and Ocean Salinity Mission proposal (SMOS), based upon a radiometer concept derived from the MIRAS studies, was submitted, In 1999, following a selection procedure, ESA approved the SMOS mission for an extended phase. This paper summarize part of the work carried out on the interferometric radiometry concept and the optimization of the instrument configuration.

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