The psychotelemetric circuit, or psychotelemeter, was a special component of the TARDIS used to "home in" on a particular destination based on a provided sample, such as hair, by determining the origin of that sample.
Without repairing it, the Third Doctor claimed he would never get to Parakon, since he only knew its whereabouts "within a few thousand lightyears". (AUDIO: The Paradise of Death [+]Loading...["The Paradise of Death (audio story)"])