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Inverse synthetic aperture radar (ISAR) imaging of sea vessels is a challenging task because their 3-D rotational motion over the coherent processing interval (CPI) often leads to blurred images. The selection of the duration of the CPI, also known as the coherent processing time window length (CPTWL), is critical because it should be short enough to limit the blurring caused by the 3-D rotational motion and long enough to ensure that the desired cross-range resolution is obtained. This paper proposes an algorithm, referred to as the motion-aided CPTWL selector (MACS) algorithm, which selects suitable CPTWLs for ISAR imaging of cooperative sea vessels. The suggested CPTWLs may be used to obtain motion-compensated ISAR images that have the desired medium cross-range resolution and limited blurring due to 3-D rotational motion. The proposed algorithm is applied to measured motion data of three different classes of sea vessels: a yacht, a fishing trawler, and a survey vessel. Results show that longer CPTWLs are needed for larger vessels in order to obtain ISAR images with the desired cross-range resolution. The effectiveness of the CPTWLs, suggested by the MACS algorithm, is shown using measured radar data. The suggested CPTWLs may also be used to select an effective initial CPTWL for Martorella/Berizzi's optimum imaging selection algorithm when it is applied to measured radar data of small vessels. Lastly, the proposed technique offers significant computational savings for radar cross section measurement applications where a few high-quality ISAR images are desired from long radar recordings.