Abstract: This paper proposes an improved Motion Information Sensor (MIS) Based on GNSS/SINS for SAR motion compensation. The MIS can provide not only the long term information with absolute accuracy, but also the short term data with high relative accuracy during SAR imaging. System design, error models and navigation algorithms of the MIS are introduced in the paper. The experiment results show that the MIS can provide accurate navigation information (position, velocity and attitude ) to meet the requirements of SAR motion compensation. Especially , the MIS is suitable for the case: the accuracy of airplane master inertial navigation system is too low or not configured.