Assessment of the corrected CMOD6 GMF using scatterometer data

An assessment of the agreement between the ERS scatterometers (ERS-1 and ERS-2) and the Metop scatterometers (ASCAT-A and ASCAT-B) is essential for the consistency of the C-band scatterometry dataset. ERS-1, ERS-2, ASCAT-A and ASCAT-B are C-band fan-beam radar scatterometers covering a range of common incidence angles. During these C-band scatterometry missions, different calibration campaigns have been carried out mainly relying on active ground transponders and natural distributed targets such as the rainforest. Additionally, these missions differ in time with some overlapping periods. Therefore, an assessment of the agreement between ERS and ASCAT measurements is an important and challenging task. This assessment is usually performed over the rainforest but only considering the common incidence angles. In order to perform the comparison over the whole incidence angle range of both radars, a Geophysical Model Function (GMF) is needed. An empirical correction of the CMOD5.n GMF has been suggested recently by KNMI resulting in a new GMF called CMOD6. This correction was derived from the comparison of the ASCAT backscatter measurements and the CMOD5.n model. Taking ASCAT's measurements as reference, the differences between the CMOD5.n and ASCAT measurements were attributed to GMF errors. Additionally, an overview of the existing C-band models is given. The comparison of these models shows relatively large differences. The aim of this paper is the assessment of the CMOD6 GMF using ERS-1 and ERS-2 ocean backscatter measurements and the validation of the applicability of the corrected GMF to the whole C-band scatterometry dataset. Finally, a method is suggested to calibrate the residual bias of all the C-band scatterometers w.r.t CMOD6. It is shown that after calibration a consistent scatterometer data model is obtained.