HADS

The HADS (Hostile Alien Displacement System) is the primary defence mechanism of a Tardis. Upon detecting imminent danger to the physical structure of the Ship, the unit briefly dematerialises the Tardis, and rematerialises it a short distance away, in a safer locale. The Doctor's Tardis would appear to have a HADS with an intermittent fault, as it has, on occasions, remained in dangerous locations from which the HADS ought to have removed it.