Time sensor

A time sensor, which Jo Grant called a "TARDIS sniffer-outer", was a device she explained that "detect[ed] disturbances in the time field". It was used by the Third Doctor to detect the presence of the Master's TARDIS, calibrated to provide distances to a disturbance in Venusian feet and miles. (TV: The Time Monster)

It measured temporal disturbances down to six decimal places on the Bocca Scale. The Sixth Doctor explained that, while he could use his TARDIS to locate the Master's, it would require a regeneration to recalibrate the systems as the Master had altered his TARDIS so much that it bared little resemblance to the Type 40. (PROSE: The Quantum Archangel)