Frequency Modulation Nonlinearity Correction for FMCW SAL Based on WVD With Gradient Rotation Enhancement

Frequency modulation continuous wave (FMCW) synthetic aperture ladar (SAL) system can support the generation and processing of large bandwidth signals, which has strong capabilities in target detection, description, and identification. However, frequency modulation nonlinearity of the transmitted signal has become a bottleneck problem that restricts the high-resolution ranging of FMCW SAL. This letter establishes an echo model with nonlinearity and proposes a nonlinear phase estimation and compensation method based on Wigner-Ville distribution (WVD) with gradient rotation enhancement. First, we establish the nonlinearity model of the internal calibration signal in the time-frequency (TF) domain and use WVD to calculate the TF distribution (TFD). Second, the TFD is enhanced by rotating gradient vector, which can also eliminate noises and false components in the TFD. Then, the nonlinear phase of the transmitted signal is estimated by establishing a nonlinearity model in the TF domain. Finally, the residual video phase (RVP) filtering is introduced to eliminate the range-dependent nonlinearity and achieve nonlinearity correction of echoes at any range. Experiments and real data tests show that the method proposed in this letter can accurately estimate and correct the nonlinearity of the dechirp signal, which effectively improves the range resolution of FMCW SAL.