SMOS measurement principle

SMOS mission overview

For optimum results, SMOS will measure microwave radiation emitted from Earth's surface within the L-band (1.4 GHz) using an interferometric radiometer.

Measurement principles

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.

Interferometry principle
Interferometry principle

The SMOS radiometer will exploit 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 will achieve global coverage every three days.

Mission approach

Mission SMOS
Launched 2 November 2009
Duration Minimum 3 years
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
Receiver spacing 0.875 lambda = 18.37 cm
Polarisation H & V (polarimetric mode optional)
Spatial resolution 35 km at centre of field of view
Tilt angle 32.5 degrees
Radiometric resolution 0.8 - 2.2 K
Angular range 0-55 degrees
Temporal resolution 3 days revisit at Equator
Instrument data rate 89 kbps H & V pol.
Mass Total 658 kg launch mass comprising: platform 275 kg, payload 355 kg, fuel 28 kg
Orbit Sun-synchronous, dawn/dusk, quasi-circular orbit at altitude 758 km. 06.00 hrs local solar time at ascending node.
Launcher Rockot, KM-Breeze upper stage
Bus Proteus (1 m cube)
Power Up to 1065 W (511 W available for payload; 78 AH Li-ion battery.
Spacecraft Operations Control Centre CNES,Toulouse, France
S-Band TTC link 4 kbps uplink, 722 kbps downlink
Payload Mission and Data Centre ESAC, Villafranca, Spain
X-Band data downlink 16.8 Mbps

Last update: 6 November 2013

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