PERFORMANCE OF AN ADAPTIVE NOTCH FILTER FOR SPECTRAL-ANALYSIS OF COHERENT LIDAR SIGNALS

An adaptive notch filter (ANF) is proposed for range-resolved frequenc y estimates of Doppler lidar atmospheric returns. The ANF is based on the spectral filtering of lidar return to remove the atmospheric contr ibution from noise. An adaptive algorithm is used to retrieve the filt er parameters at a time k knowing both the input signal and filter out put at times k - i, where i = [1, k]. It is shown that ANF performs we ll at low SNR(-5 dB) compared to the poly-pulse-pair (PPP) estimator c urrently used for Doppler lidar signal processing. The standard deviat ion of frequency estimates is 0.01F(s)-0.02F(s) (F-s is the sampling f requency) at SNR = -5 dB, depending on the signal spectral width. It c orresponds to a wind velocity uncertainty of 2-4 m s(-1) for F-s = 40 MHz and a laser wavelength lambda = 10/mu m. The ANF also proved to pe rform better than PPP in tracking a time-varying frequency, and in the presence of a colored noise.