alignment reference frame (ARF)

This term is defined in ECSS-E-ST-60-20C Rev. 1 as
reference frame fixed with respect to the sensor external optical cube where the origin of the ARF is defined unambiguously with reference to the sensor external optical cube

This term is defined in ECSS-E-ST-60-21C as
frame that is fixed with respect to the gyro external optical cube where and whose origin is defined unambiguously with reference to the gyro external optical cube