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A novel instrument was especially developed to provide the observations for both soil moisture and ocean salinity by capturing images of emitted microwave radiation around the frequency of 1.4 GHz (L-band) in the required accuracy. SMOS carries the first-ever, polar-orbiting, space-borne, 2-D interferometric radiometer measuring in L-Band: the MIRAS (Microwave Imaging Radiometer using Aperture Synthesis) instrument.

In order to achieve the required spatial resolution for observing soil moisture and ocean salinity in L-Band a huge antenna would normally be necessary. For the SMOS mission, however, the antenna aperture has been cleverly synthesized through a multitude of small antennae. MIRAS consists of a central structure and three deployable arms, each of which has three segments. During launch, these arms were folded-up, but soon after separation from the launch vehicle they were gently deployed via a system of spring-operated motors and speed regulators.

There are 69 antenna elements – the so-called LICEF receivers, which are equally distributed over the three arms and the central structure. Each LICEF is an antenna-received integrated unit that measures the radiation emitted from the Earth at L-band. The acquired signal is then transmitted to a central correlator unit, which performs interferometry cross-correlations of the signals between all possible combinations of receiver pairs. By pre-processing the calculations on-board, the amount of data that has to be transmitted to the ground is significantly reduced.

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Measurement principles

Moisture and salinity decrease the emissivity of soil and seawater respectively, and thereby affect microwave radiation emitted from the surface of the Earth. Interferometry measures the phase difference between electromagnetic waves at two or more receivers, which are a known distance apart – the baseline.

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The SMOS radiometer will exploit the interferometry principle, which by way of 69 small receivers will measure the phase difference of incident radiation. The technique is based on cross-correlation of observations from all possible combinations of receiver pairs. A two-dimensional 'measurement image' is taken every 1.2 seconds. As the satellite moves along its orbital path each observed area is seen under various viewing angles.

From an altitude of 758 km, the antenna will view an area of almost 3000 km in diameter. However, due to the interferometry principle and the Y-shaped antenna, the field of view is limited to a hexagon-like shape about 1000 km across called the 'alias-free zone'. This area corresponds to observations where there is no ambiguity in the phase-difference.

SMOS will achieve global coverage every three days.