Time path indicator

The time path indicator was a component of a TARDIS used to detect time machines in the Time Vortex which were on the same flight path. It could also be removed from the console and used to detect time machines.

The time path indicator in the First Doctor's TARDIS signalled its crew that the Daleks were following them in the Dalek time machine following their escape from Aridius. (TV: The Chase) It later detected the Monk's TARDIS following them. (TV: The Daleks' Master Plan)

The time path indicator also registered when it detected the Master's TARDIS near the Darkheart system. (PROSE: The Dark Path)

The Seventh Doctor, having received a warning about the Timewyrm, removed the time path indicator from the TARDIS so he could detect its coming. (PROSE: Timewyrm: Genesys) He later used it to track her down after she escaped. (PROSE: Timewyrm: Exodus)

Behind the scenes
In The Chase, the component is called the time path detector, but is renamed the time-path indicator in the novelisation and all subsequent stories.