When a small scatterer which is relatively isolated and strong can be found in the imaging target

the motion compensation in ISAR imaging can be transfered into two steps: the range alignment for each return and the phase compensation of each return with the phases of the isolated scatterer. In this paper

we estimate the return Doppler frequencies from the return phases and propose a new way to resolve the Doppler ambiguities. Then the unambiguous Doppler frequencies are used to reconstruct the target trajectory so as to conduct range alignment for each return.