An unambiguous heading direction is calculated to determine the forward/reverse state of a vehicle. A heading alignment error is determined at step 100, being the difference between a GNSS direction of motion and the unresolved IMU heading of the vehicle. The heading alignment error is adjusted by 180° to be within a predetermined range at step 200. The unresolved IMU heading of the vehicle 10 is adjusted using the heading alignment error to determine an ambiguous error corrected IMU heading at step 300. Step 400 determines whether the ambiguous error corrected IMU heading is substantially in the true direction of the nose of the vehicle. The unambiguous heading direction is calculated at step 500 by offsetting the ambiguous error corrected IMU heading by 180 degrees if the ambiguous error corrected IMU heading is substantially opposite the true direction of the nose the vehicle of the vehicle. The forward/reverse state is determined by comparing the unambiguous heading direction with the GNSS direction of motion of the vehicle.