Artistic impression of SMOS in orbit (ESA)
Artistic impression
of SMOS in orbit (ESA).
SMOS measures microwave radiation emitted from the Earth's surface at L-band (1.4 GHz) using an interferometric radiometer.

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.


The SMOS radiometer exploits 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 763 km, the antenna views 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.

Mission Characteristics
Launch 02/11/2009
Duration Nominally 3 years (including a 6 month comissioning phase) with an optional 2 year extension.
Instrument Microwave Imaging Radiometer using Aperture Synthesis - MIRAS
Instrument concept Passive microwave 2D-interferometer
Frequency L-band (21 cm-1.4 GHz)
Number of receivers 69
Spatial resolution 35 km at centre of field of view
Radiometric resolution 0.8 - 2.2 K
Temporal resolution 3 days revisit at Equator
Mass Total 683 kg launch mass comprising:317 kg for platform, 366 kg for payload
Orbit Sun-synchronous, dawn/dusk, circular orbit at an altitude of 763 km.