Civil aircraft may be subject to interception by military aircraft, particularly if they have failed to maintain two way communications on the designated ATC frequency. Interception is especially likely if they have then entered national airspace without explicit or implied permission. Unauthorised - and usually inadvertent - entry into Prohibited or Restricted Airspace or into an active Danger Area may also result in interception (or other signalling methods). The probable close approach by intercepting aircraft may trigger a Airborne Collision Avoidance System (ACAS) RA; however, if the circumstances are appreciated when an RA occurs, the normal response is inadvisable for two reasons: