Responding to the changing market environment, while addressing the challenges associated with traditional approaches, calls for a novel solution in maritime cargo transport. KAIST proposed Mobile Harbor (MH) as a next generation maritime container transport solution. Since the MH system operates under harsh at-sea condition, controlling the undesirable swing motion of a spreader requires a robust and accurate sensing system for the measurement of a swing motion. In this paper, we proposed a robust sensing system which can provide information on all 6 DOF motions (surge, sway, heave, trim, list, and skew) of a spreader for at-sea cargo transfer.