Soil Moisture Ocean Salinity (SMOS) mission of European Space Agency (ESA) is the first ever satellite mission designated for soil moisture observation. SMOS was launched on November 2, 2009 and carries the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS). The MIRAS senses L-band microwave emission (1.400 - 1.427 GHz) that could penetrate soil depth to about 5cm and vegetation cover with vegetation water content up to 5 kg/m2 (Kerr et al., 2000). 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 around 758 km, the antenna will view 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 achieves global coverage every three days. More details of the SMOS mission can be found at www.esa.int/esaLP/ESAL3B2VMOC_LPsmos_0.html.