Doppler Centroid Estimation of the Clutter
In our signal model establishment of stripmap SAR, the system is assumed to be with a squint angle of zero. However, in the real flight of an airborne SAR, because of the instability of the platform and the installation error of the antenna, there will be an antenna direction error. The antenna direction error induces a spectrum shift of the clutter, which will affect the azimuth window function and the RWM correction. Therefore, before the SAR imaging algorithm is preceded, the actual Doppler centroid of the clutter must be estimated.
Meanwhile, estimation and correction of the Doppler centroid of the clutter are also the foundation of GMTI in the azimuth spectrum domain in Sect. 3.4. The spectrum shift of the clutter will deteriorate the performance of the clutter suppression. Also, estimation and correction of the Doppler centroid of the clutter is important for the cross-track velocity estimation, which will be introduced in Sect. 3.5.
In a radar system, the Doppler centroid data is recorded by the INS system. However, the precision of the INS system cannot satisfy the imaging resolution. Therefore, Doppler centroid estimation algorithms based on the echo signal are produced . Classical Doppler centroid estimation algorithms include energy balancing method , correlation Doppler estimator, SDE , and MLE . Energy balancing method, which is widely used in airborne SAR imaging, has a promising performance in homogeneous scenes, whereas its estimation precision declines sharply if the homogeneity is destroyed, especially when prominent point targets exist in the scene.