SMOS-Earth's water monitoring mission

Abstract

The SMOS (Soil Moisture and Ocean Salinity) project is the second Earth Explorer Mission of Opportunity within the European Space Agency's (ESA) Living Planet Program. The purpose of the SMOS mission is to provide soil moisture and ocean salinity maps from space. These two geophysical parameters are of key importance in improving climatological forecasting, increasing the understanding of the water cycle, providing new approaches to acquiring knowledge of the phenomenon of climate change and monitoring the planet's fresh water reserves. The mission employs a satellite in low-earth sun-synchronous orbit with an altitude of 755 km and a revisit time of 3 days. SMOS measures the thermal noise generated by the earth at L-Band (1.4 GHz) with a spatial resolution of 50 km and radiometric sensitivity of 3.5 K per snapshot at boresight. The thermal radiation detected by SMOS at L-Band is where microwave theorists have devised a direct relationship between Soil Moisture (SM) and Ocean Salinity (OS) with earth emissivity. The SMOS single-instrument MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) is an innovative 2-D aperture synthesis radiometer. Aperture synthesis, or, interferometry, is an alternative to real aperture instruments that permits the synthesis of a theoretical antenna of very large aperture using a diverse collection of small antenna/receivers which achieves a greatly improved instrument weight/geometric resolution ratio. The fundamental theory behind this technique is the same as that used for decades in radio astronomy. The instrument measures the cross correlations between all pairs of receivers to derive the visibility function. In a first-order approximation, the brightness temperature of the source is computed as the inverse Fourier transform of this function. However, the large field of view present in earth observation applications introduces non-negligible effects of individual antenna patterns, obliquity factors and spatial decorrelation effects. Experimental work on SMOS has shown that mutual effects of closely spaced antennas, as well as their individual matching, become important to fully understand the measurements. For SMOS, a new formulation of the visibility function, including full antenna characteristics and interactions between receivers, was developed. These effects, never taken into account in previous approaches, have an important impact on inversion techniques and also on instrument specifications and performance. MIRAS consists of an array of separate radiometric receivers. The system acts as a radio camera, and as the satellite moves forward, a wide swath is covered without mechanical movement to create a larger synthetic antenna in order to increase the image resolution. The raw data output of MIRAS consists of one-bit digital correlations that are transmitted to ground to the Data Processing Ground Segment (DPGS) via an X-Band communications link. Calibration of any earth observation sensor is a key stage which encompasses those tasks necessary to convert the raw measurement data into science data. Calibration is an important prerequisite to performance verification (which demonstrates the instrument meets its requirements) and the validation of geophysical parameters produced as higher level products. The flight model satellite of SMOS, developed by European space industry, is scheduled for launch within the last quarter of 2008 with a planned lifetime of 3 to 5 years. A second generation of SMOS satellites (SMOS Ops) is under study to continue the supply of soil moisture and ocean salinity maps with improvements in pixel resolution and revisit time. Following the successful deployment of SMOS in orbit and a satisfactory demonstration of its capabilities, it is hoped that the SMOS concept and design will form the basis of future soil moisture and ocean salinity missions for earth observation purposes and as a major contributor to operational meteorology and climate change awareness. © 2009 Springer Netherlands.