Abstract: In this paper,the properties of scanning coverage for a satellite borne ScanSAR have been analyzed in detail.It is shown that the maximun width of observing zone for a satellite borne ScanSAR is independent of radar wavelength and real shape of antenna aperture if the restraint conditions for far range ambiguity are met in design.The antenna design shall be trade off between allowable size and beam number.The average and peak transmitting power of satellite borne ScanSAR and quantitative relationship of restraint condition of ambiguity are explained.The numerical examples about performance computation of a satelliee borne ScanSAR are given finally.