What inputs are required to maintain a North-up, True Motion display?

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To keep a North-up, True Motion display, the radar needs data that describe the vessel’s own movement so it can remove that motion from the scene. The gyro course provides a stable true-heading reference, and the log speed gives how fast the ship is moving through the water. With both heading and speed, the system can translate the radar scene into true motion and orient it to true north, so stationary targets stay put relative to the ground while the screen remains north-up. The other inputs don’t provide the necessary combination of heading in a true reference and speed needed for this compensation: compass heading can be magnetic and less stable, wind speed isn’t used for motion compensation, GPS position alone doesn’t give instantaneous own-motion data, and rudder angle plus engine RPM don’t directly supply the heading and speed needed to render true motion.