This paper focuses on the optimization of the image quality for the ENVISAT ASAR ScanSAR modes. The best trade-off between Noise Equivalent σ0 (NEσ0) and Equivalent Number of Looks (ENL) is achieved by optimizing the 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 (focusing by SAR matched reference and scalloping compensation by inverse antenna weighting) introduce a space-variant distortion in the focused impulse response (FIRF). The focusing technique here proposed exploits a set of space-variant focusing kernels computed by means of Wiener deconvolution. In this way focusing and descalloping are obtained at one time, achieving a finer resolution and lower distortion of impulse response.