The focused and non-focused image-forming methods for airborne synthetic aperture radar (SAR) are investigated. The more precise receiving signal model which includes not only traditional additive noise but the multiplicative noise which is the consequence of the aircraft's velocity and height fluctuations is developed. Results of numerical calculations of the root-mean-square (rms) errors of the set of principal image parameters as functions of the additive noise variance and multiplicative noise standard deviation for several weighting windows and the histograms of the distributions of some image parameters are presented.