Motion compensation is the key of ISAR imaging

and is usually carried out in two steps. First

compensate the time delay of complex envelope

which is a roughly alignment. Second

search for a reference point to precisely adjust the initial phase

which is usually referred to as autofocusing. The paper presents two improved autofocusing techniques called weighted multiple scatterer algorithrn （WMSA）and improved scatter centroid tracking （ISCT）. They are analyzed and compared with the dominant scatterer algorithm （DSA）. Finally

the outdoor measured ISAR data of aircrafts are processed and the imaging results are compared with each other.