This paper focuses on the optimization of the image quality for the ENVISAT ASAR ScanSAR modes. The best tradeoff between Noise Equivalent 0 (NE0) and Equivalent Number of Looks (ENL) is achieved by optimizing both system and processing parameters within the instrument constraints. An end-to-end model to validate the study has been used. Thereafter a novel phase preserving focusing technique is presented. Conventional approaches (that exploit the SAR matched filter) introduce a space-variant distortion when focusing very low resolution ScanSAR data. The focusing technique here proposed uses a set of space-variant focusing kernels properly computed by means of Wiener pseudoinversion. In this way focusing is achieved with no distortion, thus getting the finer resolution and the lower sidelobes.