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Synthetic aperture sonar (SAS) is used to produce high-quality side-scan imagery of the sea floor in which the along-track resolution is constant with range. SAS sensor technology is transitioning from towed vehicles toward more versatile autonomous underwater vehicles (AUVs). Experience to date has shown that the 21-inch AUVs tend to be less stable than the longer towed vehicle of the same diameter used during the U.S. Navy's initial SAS technology demo efforts. The corresponding increase in undesirable motion of the AUV platform carrying the sonar has motivated the development of a new generation of motion estimation and compensation schemes. This paper describes the results obtained from one such motion estimation method applied to the U.S. Navy's SAS21/Reliant SAS system. The data set used for evaluating the image improvement consists of 31 km of track taken at the 2004 Combined Joint Task Force Exercise (CJTFEX-04) off the North Carolina coast. The motion estimation combines the angular velocity information provided from an inertial navigation unit with the traditional redundant phase center approach in order to accurately estimate the ping-to-ping translation of the SAS array. This approach consistently yields better imagery compared to the previously-used technique.