A global ERS-1/2 SAR wave mode imagettes calibration method

In absence of specific information for SAR imagette radiometric precision analysis, the authors' method is to derive a relative calibration from global direct comparisons between co-located SAR and scatterometer central antenna measurements. Such a method led the authors to refine the exact incidence angle used to obtain the ERS-1/2 SAR wave mode products. The study clearly shows the evidence of the change of imagettes incidence angle on ERS-1 data. Moreover, it is shown that ERS-2 data are almost not affected by the analog-digital converter saturation because of a reduction in on-board gains. The issue of the calibration constant K retrieval is addressed and K values are given for both configurations of ERS-1 SAR and for ERS-2. Finally, the consistency of this empirical calibration method is discussed in comparison with scattering model CMOD IFREMER.