Abstract: The invention describes a sensor mechanism for measuring hitch articulation angle. A configuration of the contact face between hitch ball and receiver eliminates the effect of the unwanted trailer motions, such as roll and pitch while accurately registering trailer yaw motion. Also, there is no yaw slippage at the contact that was a major problem during the successive acceleration and brake maneuvers. With the freely rotating ball and shaft, the angle is measured using an angle sensor, such as an optical encoder, potentiometer, or other means. For the case of a relative angle sensor, automatic zero angle calibration can be used to allow an absolute angle measurement.