In order to analyse the effect of the modulated nonlinear error on 1-dimension focusing and accurately correct this error for FMCW(Frequency Modulated Continuous Wave)SAR(Synthetic Aperture Radar)

a high order error model of VCO(Voltage Controlled Oscillator) modulated non-linearity is introduced to deduce and quantitatively analysis in theory the specific effect of the modulated nonlinear error.A nonlinear correction method for FMCW SAR based on open-loop is proposed on the premises of the stability of the VCO and low electricity shift varying with time.Taking the signal's compression index as the optimal convergent rule

the method corrects the signal's modulated nonlinear error first by one-off compensation for harmonic wave error

and second by correcting the high-order swing error caused by the input voltage update

the electronic shift of VCO by iteration and the thermal noise.The simulation of the signal and the process of the real data indicate that the method can solidly and effectively corrects the modulated nonlinearity error of FMCW SAR signal source.Compared with the traditional software correction based upon open-loop

this new method can correct the nonlinear error without complicated error model and also it has the advantages of simpleness

lower cost and less computation.