Temporal Monitor

Numerous Temporal Monitors were present in the Temporal Control Room, which the Time Lords used to observe the time streams of the universe. (PROSE: The Three Doctors, TV: The Three Doctors)

As a Time Lord messenger told the Third Doctor, the Time Lords had used "the Monitor" to track to Earth before losing contact as a result of "some kind of alien interference". As the Doctor discovered, the Master was working with the Nestene Consciousness. (PROSE: Doctor Who and the Terror of the Autons)