Developed by the European Space Agency (ESA) in cooperation with the space agencies of France (CNES) and Spain (CDTI), the SMOS mission (Figure 2) was launched in November 2009.
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.
As the name implies, SMOS is a dual science mission, with the engineering design driven primarily by acquiring high spatial resolution over land, where the signal strength of surface TB is much greater than over the ocean. The radiometric signals associated with SSS variability are small relative to the SMOS radiometer sensitivity, and the data will require careful calibration and considerable spatio-temporal averaging to reduce measurement noise. Nevertheless, SMOS is the first satellite to provide exploratory global SSS observations.
Upper Panel: The European Space Agency Soil Moisture Ocean Salinity (SMOS) mission. The three radial arms contain small microwave (1.413 GHz) detectors that form a phased array that is about six meters in diameter. Lower panel: The SMOS field of view covers a swath about 1000 km wide, with the average pixel size ~43km. The maximum revisit time is 3 days.
The SMOS satellite flies in a near polar sun-synchronous orbit, crossing the equator at 6 am (ascending or northward) and 6 pm (descending or southward) local time. As shown in the Figure, the sensor consists of three radial arms with 69 small microwave (1.413 GHz) detectors that form a phased array that is about six meters in diameter. From the intercorrelations and considerable ground processing, a twodimensional image is reconstructed with the pattern shown in Figure above, with an average pixel size of 43 km.
The field of view is about 1000km wide, and the maximum revisit time interval at the equator is about 3 days. A surface location is observed multiple times at various angles as the satellite moves along the trajectory. Each viewing angle has different horizontal and vertical polarized surface TB responses to SSS, SST and wind.
This information is exploited with a maximum likelihood estimate algorithm to derive SSS, SST and a wind parameter simultaneously , , ). There is also a parallel effort to develop an alternative algorithm based on neural networks. The retrieval accuracy of these methods remains quite sensitive to initial constraints, radiative transfer model, choice ofancillary data and the range of model functions for wind speed effects. Additional error sources due to instrument biases, image reconstruction processing, and other prominent geophysical correction terms continue to be studied. The actual on-orbit SSS retrieval accuracy is a subject of ongoing refinement of the algorithm, and the retrieval errors per pixel will be reduced by spatio-temporal averaging to 200 km by 30 day scales. The official SMOS ESA mission will deliver data up to level 2 (along swath geo-located retrieved salinity per orbit), with an additional Near Real Time processing chain implemented for operational applications by meteorological centers (mainly for soil moisture). The spatio-temporal averaged and analyzed products (level 3), as well as other value added products including external information (level 4), are generated and distributed by the dedicated processing centers CATDS in France and CP3/4 in Spain.