noun: (aviation) The approach course formed by the rear lobe of the signal pattern of a localizer antenna array (the portion of the localizer signal emitted from the rear face of the antenna array), oriented in the reciprocal direction to the primary approach course (i.e., the back course - if a usable back course exists at all - will be rotated 180° from the primary course). Not all localizers produce a usable back-course signal; newer equipment radiates very little signal into the rear and side lobes, not enough to allow for a back-course approach.
