Global Soil Moisture from C-Band Scatterometers (GLOBSCAT)


2005-02-01 – 2008-08-31

Soil moisture is a key state variable of the global energy and water cycles. Due to the lack of affordable in-situ measurement techniques, soil moisture has long been a research focus in the field of remote sensing. Particularly, sensors operated in the lower part of the microwave spectrum (1-10 cm wavelength) hold a large potential due to their high sensitivity to soil moisture induced changes of the soil dielectric constant. Retrieval of soil moisture data is however aggravated by the influence of vegetation and surface roughness on the microwave measurements which so far hampered the operational use of this source of information for operational global application. Only recently a methodological breakthrough has been achieved by our team at the Institute of Photogrammetry and Remote Sensing (I.P.F.) of the Vienna University of Technology which has lead to the first global remotely sensed soil moisture data set. The data set has been found to be of comparable quality as state-of-the-art models. Validation with ground observations has shown that the retrieval error is about 5 vol. % volumetric soil moisture. The multi-year data set (1992-2000) has been freely distributed to over 60 research groups worldwide. The experience collected in first pilot projects and the encouraging feedback of first users has shown that the scientific algorithms have reached a level of maturity which makes scatterometer derived soil moisture products attractive for a number of applications. Beside these methodological advances scatterometry also experienced a technological breakthrough. An advanced scatterometer will be part of the operational METOP satellite series which starts in 2005 and guarantees a flow of high quality data for the next 14 years.It is the goal of the GLOBESCAT project to demonstrate the operational utility of C-band scatterometers by 1) producing a global, long-term (1991-2007) soil moisture data set, and 2) establishing an automated Internet service to deliver global soil moisture products in near-real time. The pilot users of the GLOBESCAT soil moisture products are first-rank research institutions and operational organizations. After the completion of the GLOBESCAT project, the I.P.F. is committed to maintain the Internet service for another year. It is expected that over the course of this period an agreement will be reached with an external private or institutional partner, who is willing to continue the operation of the system. Since ASCAT is more advanced than the ERS scatterometer, it is necessary to adapt the existing algorithms and software. It will therefore be a task of GLOBESCAT to study the impact of the changing sensor characteristics on the retrieval technique and to adapt the existing methods for the ASCAT sensor. Further methodological limitations of the current technique will be identified, remodeled and a comprehensive validation of the derived products will be carried outThe success of GLOBESCAT will significantly contribute to current priorities of the global community in the fields of meteorology, climatology, hydrology and agronomy. With this respect GLOBESCAT is seen in context of the SMOS and HYDROS satellite missions currently planed by the European Space Agency ESA and the National Aeronautics and Space Administration NASA. Both are dedicated soil moisture missions with a foreseen launch date within this decade and evince the relevance of global soil moisture remote sensing for the global community.

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