Phase Gradient Matrix Autofocus for ISAL Space-Time-Varied Phase Error Correction

A new autofocus method of the space-time-varied phase error (PE) correction is presented for inverse synthetic aperture ladar (ISAL). The PE is corrected by a two-dimension matrix that is estimated in the form of phase gradient both in frequency and time domains. The autofocus method we named as PGMA is capable of compensating non-parametrical motion errors varying from pulse to pulse and is robust for widespread scene contents where any isolated strong scatterer is not necessary. The effectiveness is verified by comparing the imaging results of extended targets between PGMA and the conventional phase gradient autofocus (PGA) both in simulations and experiments. When the motion of targets contains both translational and angular motion errors, PGMA produces well-focused images while PGA merely corrects translational motion error, leaving the formatted images smear by angular motion error, especially to the edges.