Abstract:The frequency variance of single-frequency signal with rectangular envelope in time domain equals to the effective bandwidth in frequency domain, and the frequency variance is a function of Signal to Noise Ratio (SNR). When the SNR drops, the calculated frequency variance error of azimuth of target line spectrum enhances. As a result, the Variance of frequency detector which weights bearing spectrum using the variance of peak frequency of each azimuth losts its target. The gain of traditional Alpha-beta filter is constant, thus it fails to correct frequency estimates effectively. The paper puts forward a post-processing method which sums the power spectrum of each azimuth several times and puts it as a gain corresponding to each frequency. When the gain corresponding to the frequency observation is large, the new information in the recursive process counts more, whereas the gain corresponding to the frequency observation is small, then the predicted value counts more. At last the bearing spectrums are rectified by using the modified variance of estimated frequency of each azimuth as weight. Simulation and experiment results show that under circumstance of low SNR, the modified post-processing method can extract weak line spectrum target and suppress interference background.