Focusing on the problem that the effective rotation vector is unknown in advance for ISAR imaging, this paper brings forward two estimation methods for effective rotation vector (ERV). The one estimates the effective rotation vector by using the linear relation of chirp rate to the range of scattering point, and the other one proposes the estimation method of effective rotation vector based on the image entropy, by compensating the quadratic phase term of scattering unit signals. By virture of these two estimation methods of effective rotation vector, the selection of range units in application is discussed. Simulation result and the processing result of test data show that this proposed estimation of effective rotation vector is available.% 针对ISAR成像中有效转动向量事先未知的问题，提出了两种估计有效转动向量的方法。一种利用调频率与散射点距离成线性关系估计有效转动向量；另一种通过补偿散射点单元信号二次相位项，提出基于图像熵的有效转动向量估计方法。利用这两种估计有效转动向量方法，对实际应用中距离单元的选择进行了探讨。仿真和实测数据处理结果表明，所提出的估计有效转动向量方法是有效的。
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