When inverse synthetic aperture radar (ISAR) imaging the target with complex motion

the time-varying rotation vector will induce phase errors depending on the scatter positions.Such errors can not be eliminated by traditional motion compensation algorithm

and the radar images obtained by the conventional range Doppler (RD) algorithm will smear.The intrinsic image-smearing reasons are analyzed when the rotation vector varies with time

and the radar received signal expression in presence of three dimension rotation is determined.An ISAR imaging algorithm based on the property of the scatter signals is proposed for the complex motion targets.This algorithm is effective for imaging the target which has constant rotation orientation but time-varying rotation velocity.Furthermore

when the rotation orientation varies slowly

the algorithm can also effectively improve the quality of ISAR images.Finally

several simulation results demonstrate the performance of the algorithm.