One of the biggest challenges in developing the SMOS mission was to fly and demonstrate an instrument that would measure microwave radiation emitted from Earth’s surface within the ‘L-band’, around a frequency of 1.4 GHz. This frequency provides the best sensitivity to variations of moisture in the soil and changes in the salinity of the ocean, coupled with minimal disturbance from weather, atmosphere and vegetation cover.
In order to achieve the spatial resolution required for observing soil moisture and ocean salinity, the laws of physics mean that to take measurements in L-band, a huge antenna would have been required – too big for a satellite to carry. To overcome this challenge, the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument was developed. Here the size of the antenna needed has been simulated through 69 small antennas, distributed over the three arms and central hub of the instrument.
The three deployable arms are folded up for launch, but once SMOS is in orbit each of the arms fold out into an unusual three-pointed star shape. Hence, with a diameter of eight metres, MIRAS is often dubbed a ‘star in the sky’.
The 69 antenna elements, called LICEFs, are antenna-receiver integrated units, each measure radiation emitted from Earth’s surface at L-band. One LICEF antenna weighs 190 g, is 165 mm in diameter and 19 mm high.
MIRAS was built by a consortium of over 20 European companies led by EADS-CASA Espacio in Spain.