Calibration of space-borne scatterometers : towards a consistent climate data record for soil moisture retrieval

Abstract

Higher level scatterometer products, such as surface soil moisture and wind vector fields became an important input dataset for climate change research. For this reason an accurate radiometric calibration of space-borne scatterometers is needed by the scientific community to establish a long-term consistent climate data record. Accordingly a novel radiometric calibration methodology is developed with the objective to achieve a consistent calibration of the European scatterometer mission onboard of the European Remote-Sensing Satellite (ERS) and the Meteorological Operational Platform (MetOp). The introduced calibration method is a stepwise relative calibration approach by making use of applicable extended area land-targets. A set of appropriate extended area calibration targets is determined by a threshold based decision scheme using three parameters to characterise backscatter attributes of individual land-targets. Consistent calibrated backscatter coefficients throughout a specific scatterometer mission are achieved by conducting sensor intra-calibration. Radiometric calibration deficiencies are explored in individual antenna beams of ERS-2 AMI-WS and MetOp-A ASCAT revealing deviations to a defined calibration reference in the order of 0.2 dB for AMI-WS and 0.1 dB for ASCAT respectively. Similarities in the instrument technical design of AMI-WS and ASCAT encourage a merging of these European scatterometer missions. As a consequence, a second calibration methodology is introduced on top of sensor intra-calibration, with the objective to identify possible differences in the radiometric calibration levels of the two scatterometer missions, referred to as sensor inter-calibration. Differences in the calibration levels of AMI-WS and ASCAT are found to reveal biases ranging from 0.39 dB to -0.08 dB. The capability of the developed stepwise calibration methodology to correct for such calibration related deficiencies is demonstrated by the use of independent verification targets. Taking advantage of the developed stepwise calibration methodology will result in a long-term consistent calibrated European scatterometer data archive comprising more than 30 years of global data.