The Soil Moisture and Ocean Salinity mission

Surface soil moisture is a key variable of water and energy exchanges at the land surface/atmosphere interface. But currently there are no means to assess it on a global and timely fashion. Similarly, our current knowledge of sea surface salinity is very reduced. One way to overcome this issue would be to use an adequate space-borne instrument. The most promising instrument would then be an L-band microwave remote sensing sensors as they are able to provide estimates of surface soil moisture and sea surface salinity, on spatial and temporal scales compatible with applications in the fields of climatology, meteorology and large scale hydrology. The ESA Earth Explorer Opportunity mission SMOS is the first to attempt to fulfill such a gap. SMOS is based upon an L-band 2-D interferometer. It is thus an innovative concept of bi-dimensional aperture synthesis method to obtain surface measurement with an appropriate resolution from a tractable (in terms of dimensions) space-borne instrument. Moreover, the sensor has new and very significant capabilities especially in terms of multi-angular view configuration. This paper will describe the SMOS concept in terms of instrument (characteristics) and will investigate the main aspects of the retrieval capabilities of the 2-D microwave interferometer for monitoring soil moisture, vegetation biomass and sea surface salinity. The analysis is based on model inversion taking into account the instrument characteristics. The standard error of estimate of the surface variables is computed as a function of the sensor configuration system and of the uncertainties associated with the spatial measurements. The inversion process is based on a standard minimisation routine that computes both retrieved variables and standard error associated with the retrievals. Nevertheless, retrieving surface variables from such as instrument is not necessarily straightforward. Over the oceans, a very high sensitivity and accuracy are acquired. Over the land the main issues are linked to mixed pixels and topography. Using other sensors/mission (such as Aquarius over the oceans) and assimilation techniques will be used to address these issues. The potential of SMOS, depending on the view angle configuration and the use of the sole 1.4 GHz is thus investigated. These questions are key issues to define the observation configuration of SMOS that meets the scientific requirements and the technical constraints of the spatial missions.