SMOS Calibration and Validation over the Salar de Uyuni
Escorihuela, Maria Jose1; Richaume, Philippe2; Kerr, Yann2

The Soil Moisture and Ocean Salinity (SMOS) mission has been designed to observe soil moisture over the Earth’s land- masses and salinity over the oceans. Launched on 2 November 2009, SMOS is the second Earth Explorer Opportunity mission to be developed as part of ESA’s Living Planet Programme. An important aspect of this mission is that it will demonstrate a new measuring technique by adopting a completely different approach in the field of observing the Earth from space. The SMOS mission carries the first polar-orbiting 2-D interferometric radiometer (MIRAS- Microwave Imaging Radiometer using Aperture Synthesis) acquiring data of emitted microwave radiation at the frequency of 1.4 GHz (L-band). Consequently, with both completely new sensor technology and retrieval approaches, there is the need to carefully assess the quality and validity of the generated data products.

The Salar de Uyuni is the largest salt flat in the world. It is located in the Bolivian altiplane at a height of about 3700 m between latitudes 19◦45 S and 20◦40 S and between longitudes 68◦17 W and 66◦45 W. The Salar is covered with a solid salt crust with a thickness varying between tens of centimeters to a few meters. Underneath its surface is a lake of brine 2 to 20 meters deep. The Salar’s surface is about 9600 km2 (several tenths the SMOS footprint). It is located in a rather uninhabited area with no RFI (Radio Frequency Interferences).

Salar’s climate is cold and dry, being characterized by low temperatures, low relative humidity levels and low precipitation. The rainfall is very low and concentrated from Decem- ber to March. During the austral summer (from December to March), the surface can be covered by a thin water layer. This water layer disappears in the dry season, from April to November, leaving the Salar surface extremely flat and smooth. The large area, clear skies and exceptional surface flatness make the Salar an ideal object for calibrating Earth observation satellites. Consequently, the Salar has been used to calibrate radar and laser altimeters as well as spectral reflectances.

The radiometric temporal and spatial signature of the Salar was characterized at microwave frequencies using data from the AMSR-E on-board Aqua previously to SMOS launch. AMSR-E is a multichannel passive microwave instrument launched in 2002, which measures brightness temperatures at five frequencies in the range of 6.9 to 89 GHz (incidence angle 55◦). Analysis of AMSR-E data at 6.9, 10 and 18 GHz showed that microwave emissivity over the Salar is spatially homogeneous. Temporal analysis of brightness temperature shows that at vertical polarization is basically dependent on soil temperature and that emissivity remains high and constant during the dry period (emissivity 0.93-0.94). Vertically polarised brightness temperature at 6.9GHz was simulated with annual rmse of 1.1K.

In this context, the aim of this study is to use the Salar for SMOS L1c brightness temperature vicarious calibration and for the validation of the SMOS L2 retrieved dielectric constant.

It was found that SMOS BT brightness temperature was homogeneous over the Salar. The impact of pure and mixed measurements in Full Polarisation mode on the angular signature was assessed. It is shown that ’mixed’ measurements are more noisy than ’pure’ measurements. This is expected because of the lower integration time for ’mixed’ measurements. However, ’mixed’ measurements do not seem to induce any bias in the angular signature.

Both SMOS L1c BT_browse and L2 BT_TOA are very similar during the dry season, which validates the inversion algorithm approach. The relationship between emissivity and dielectric constant is very similar to the theoretical relationship calculated from Fresnel reflections coefficients on dry season. However, during the wet season the algorithm is not able to estimate the dielectric constant.