For autolands for real (i.e. low visibility) there is a requirement to protect the integrity of the Instument Landing System. When an airport goes into "low visibility procedures" they place a restriction on the movement of vehicles within a certain section of the ILS beam, to prevent any distortion. Additionally there is a greater spacing between aircraft on approach, to ensure that the previous aircraft has cleared the sensitive area in good time. One of the reasons for delays when the weather conditions deteriorate. A practice autoland is one being carried out in better weather conditions, usually to keep crews current on the procedure. If the beam is distorted by a moving vehicle or aircraft it is not so critical, as the crew can see the deviation, monitor it and, if necessary, land manually.