Topographic effects on the antenna gain pattern correction

Analyses and quantifies the topographic effects on the antenna gain pattern: correction of existing spaceborne synthetic aperture radar systems, namely ERS-1, JERS-1, SIR-C, and X-SAR. Simulations and real SAR data of a test site are used. The corrections are carried out taking into account the local surface topography and compared with the standard method based on a reference ellipsoid. Results show that elevation variations in the ERS-1 and JERS-1 cases do not affect significantly the antenna gain pattern correction. For extreme topographic differences, greater than 3000 m, a reference altitude or the radiometric calibration is suggested. On the other hand, for the low-orbit SRL-1/2 terrain information is strongly recommended, particularly, if relief differences within the image are significant, namely greater than 1000 m. Furthermore, it is shown that in the SIR-C case, even if the polarizations of the antenna gain patterns are slightly different, polarimetric calibration errors due to relief variations can be neglected. Finally, implications for forthcoming spaceborne SAR systems, i.e. ERS-2 and RADARSAT, are discussed.