Installed only on the Secure Recon TARDISes (Type 65), the SID is designed to screen out unauthorized personnel to prevent theft, hijacking, and stowaways. This system is independent of the main computer banks and detects the biological rhythms of anyone coming aboard and compare them with the Approved Entrance File. To repel intruders the SID is equipped with non-lethal gas, the ability to control the Architectural Configuration System, and weapon systems capable of killing a Time Lord. The system could be set up so that an intruder could remain alive as long as they didn't move. The system was over-sensitive and often triggered (sometimes violently) for no readily apparent reason. Heavy modifications were required for field use.