Introducing a novel use of technology
SMOS will have only one instrument: an L-band radiometer named MIRAS. MIRAS is not the first L-band radiometer to be flown in space, but its truly novel approach sets it apart.
MIRAS’s job is to detect moisture in soil and salinity in water. The theory behind being able to do this is based on the contrast between the electromagnetic properties of liquid water and dry soil, and pure water and saline water.
Increases in the proportion of water in the soil-water mixture and salt in the saline mixture are detected by microwave sensors based on the amount of energy emitted. Detecting this microwave brightness temperature on the surface requires a long wavelength.
This presented a challenge because a long wavelength requires a big antenna, and big antennas are not easily operated in space.
Radio astronomers, which search for celestial objects that are not detectable in optical astronomy, also faced the challenge of needing to detect small signals from point sources in space at a long wavelength, requiring a big antenna.
Since signals are detected as waves, signals from different telescopes can be added to synthesise the pinpointing of a much larger telescope. To achieve this, radio astronomers combined 27 radio telescopes, each 25 m in diameter, and deployed them on a Y-shaped track that can be extended up to 35 km.
SMOS borrowed these techniques – called ‘aperture synthesis’ or ‘interferometry’– to mimic a much bigger antenna by placing 69 small antennas along three arms that together form a Y-shape.
The interferometric measurements will result in images from within a hexagon-like field of view about 1000 km across, enabling total coverage of Earth in under three days.