Homing device
From Tardis Wiki, the free Doctor Who reference
The homing device was a spherical object able to track down the Doctor's TARDIS. Romana gave one to the Fourth Doctor's forthcoming companion Adric. (TV: Full Circle) The Fourth Doctor himself also had one with him; he used it to find precisely where he'd parked his beloved machine. (TV: The Keeper of Traken)
The Fifth Doctor likewise kept an extra in the TARDIS; Tegan Jovanka took it with her and gave it to the Brigadier in 1977. The Brig later gave it to the Doctor who was stuck in 1983, allowing him to locate his TARDIS. (TV: Mawdryn Undead)
