FMCW signal and its characteristic in balanced heterodyne detecting were described. The difference in instantaneous slant range between SAIL and conventional SAR were analyzed.A Doppler shift term was induced by the continuous motion of the platform that was ignored by conventional SAR in “stop-and-go” model.A refined frequency scaling algorithm was deduced,which was suitable for airborne SAIL in stripmap model and was taking the Doppler shift term into account.The simulation result shows that the given algorithm can compensate for the effect of the continuous motion of the platform efficiently.