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SMOS - Cal/Val
Bundesstra├če 53
D-20146 Hamburg
Tel.: +49 40 42838-2103
Fax: +49 40 42838-7471
E-Mail:  meike-sena.martins(at)

SMOS Instrument

The payload of the SMOS mission represents an innovative concept, employed for the first time in space. The sensor is a passive radiometer in 1.4 GHz (wavelength 21 cm) Lband range with a synthetic aperture, in which 69 small antennas along three arms (Figure 1, left) interact to simulate a large conventional microwave antenna. The individual antennas measure the phase difference of the incident electromagnetic waves (Figure 1 right, interferometry) by correlating all antenna signals with each other. As long as we are able to derive from the phase difference that the waves were emitted simultaneously, we are able to pinpoint their source. If the time difference of individual waves coming from the earth to the receivers are larger than a wavelength the phase difference is ambiguous (alias effect), which limits the field of view of the sensors.

The device MIRAS (Microwave Imaging Radiomter using Aperture Synthesis) is manufactured by EADSCASA. MIRAS can measure in two different polarisation modes. In DualPolarisation mode the sensor can switch between the measurement of the horizontal and the vertical component of the microwave radiation. In the polarimetric mode both components are measured simultaneously thus doubling the amount of data sent to the ground station. MIRAS  finally operates in the full  polarization mode maximizing the measuring accuracy.

Fig. 1: MIRAS Instrument
Fig. 2: Interferometry principle

After the lift off SMOS  unfolded its three arms, each four meters long (Figure 2). The two solar collectors consist of four elements of 0.8 x 1.5 m┬▓, so that each collector arm has an approximate length of 4 meters. Before the unfolding of all arms, SMOS is 2.4 m high with a diameter of 2.3 m.