Validation of Coarse Resolution Microwave Soil Moisture Products

The strong relationship between soil moisture content and the soil dielectric constant offers a direct way of measuring soil moisture with microwave sensors. Global coarse-resolution soil moisture datasets are currently retrieved from active microwave spaceborne instruments (AMI onboard ERS and ASCAT on Metop-A) and in the near future from dedicated passive microwave satellite sensors (SMOS). This article summarizes recent soil moisture research activities and briefly discusses the strategies for validation and cross-comparison of remotely sensed soil moisture datasets. Special attention is given to the first validation of the soil moisture data from the ASCAT instrument on Metop-A in relation to the absolute and relative calibrations of the instrument. A first assessment of the quality of the ASCAT surface soil moisture is given by studies of stable targets and the spatial and temporal extent of recent extreme drought and rainfall events.