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A variety of approaches have been proposed in the literature to detect spooing of Global Navigation Satellite Systems (GNSS). These approaches vary widely based upon the assumed capabilities and a priori knowledge of the spoofer. This paper considers a method to detect spoofing based on comparing the relative (not absolute) platform trajectory estimated by the GNSS receiver to the relative trajectory developed from IMU measurements (specifically pitch and roll from a gyro compass). The primary contribution of this paper is the development and analysis of a GNSS spoofing detection algorithm that exploits the unknown (to the spoofer) “high” frequency pitch/roll motion of the ship as seen by a commercial-off-the-shelf (COTS) receiver and an inertial measurement unit (IMU) that may already be in use onboard ships. We focus on generalized likelihood ratio tests using simple models of the GNSS and gyro measurements. Further, we avoid using a navigation filter, such as the extended Kalman filter, on the measurements; instead, the algorithm directly employs the instantaneous trajectories. Experimental results are shown using a commercial GNSS receiver with data from a GNSS simulator with IMU capability. The length of time and amount of motion required to achieve low probabilities of false alarm and missed detection are analyzed.