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The distress signal receiver was supposedly a component in the Doctor's TARDIS that let the Doctor pick up distress signals from anyone, anywhere. He always answered them despite not knowing who had sent them. (TV: The Sontaran Experiment, Planet of Evil, The Creature from the Pit, Battlefield, Attack of the Cybermen, Terror of the Vervoids, Dalek, 42, The Curse of the Black Spot, GAME: Blood of the Cybermen)
The distress signal receiver was located on the TARDIS console. (TV: Planet of Evil) On at least three occasions, it was the psychic paper that received the distress signals. (TV: New Earth, Silence in the Library, Night Terrors)