The MIRAS instrument exploits the interferometry principle, which by way of 69 LICEF (Lightweight Cost-Effective Front-end) receivers measures 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.

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.

Three of these antennas are also able to operate as accurate, highly stable Noise Injection Radiometers (NIR). They are equally distributed over the three arms and central structure. The overall antenna structure extends up to 8 metres. By taking advantage of the interferometric technique, the receivers combine to give a spatial resolution similar to that of a filled-aperture antenna with the same overall dimensions. 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.

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.

Learn more about MIRAS instrument design in the multimedia book:

MIRAS is designed for three operational modes:

• Dual-polarisation mode, in which all receivers are switched synchronously to either Horizontal (H) or Vertical (V) polarisation.
• Full polarimetric mode, in which segments of the array are switched according to a predefined sequence between H and V.
• Calibration modes, in which measurements of the internal load, the noise diodes, or the so-called "fringe washing function" are determined.

The full polarisation mode has been the default operational mode since June 2010.