Abstract-Sea surface salinity can be measured by passive microwave remote sensing at L-band. In May 1999, the European Space Agency (ESA) selected the Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission to provide global coverage of soil moisture and ocean salinity. To determine the effect of wind on the sea surface emissivity, ESA sponsored the Wind and Salinity Experiment (WISE 2000). This paper describes the field campaign, the measurements acquired with emphasis in the radiometric measurements at L-band, their comparison with numerical models, and the implications for the remote sensing of sea salinity.

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Aperture synthesis radiometry is becoming a feasible concept for imaging applications, especially at low microwave frequencies where it takes clear advantage of the absence of mechanical antenna motion. A 2D interferometric radiometer consists of a large number of receivers with small antennas distributed along a 2D structure, and the brightness temperature image is formed by inversion of the measured cross-correlation between all pairs of antennas. This is the concept of MIRAS (MIcrowave Radiometer by Aperture Synthesis), the core instrument of the SMOS (Soil Moisture and Ocean Salinity) mission selected by the European Space Agency (ESA) and planned to be launched in 2005. In its preliminary design, MIRAS receivers are uniformly distributed along a Y-shape structure and work at L-band. This approach, however, poses a challenge in the specifications required for the receivers: a) The short integration time due to the platform motion strongly limits the achievable sensitivity, b) the spatial resolution is determined by the structure dimensions which cannot be made arbitrarily large and c) the radiometric accuracy depends on the non ideal behavior of the receivers, although, to some extent can be corrected by internal calibration. This paper contributes to define the main trade-off between hardware requirements and system performance of this complex instrument.Peer ReviewedPostprint (published version

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.Peer ReviewedPostprint (published version