Space Intruder Detector

Also known as SID, the Space Intruder Detector is a satellite in Earth orbit which detects and tracks UFOs.

Function
As well as providing the first warning of a UFO approaching from deep space, SID can also track UFO's flying in the Earth's atmosphere. It provides targeting coordinates for the Moonbase Interceptors and Sky fighters, and communicates with Moonbase and SHADO HQ using a monotone voice.

S.I.D. is capable of detecting UFOs and other spacecraft in deep space as well as on Earth; it can also calculate the trajectory of objects based on visual observation. The S.I.D. control computer has a speech recognition capability.

Behind the scenes
SID was voiced by Mel Oxley, and featured in all episodes except:
 * Survival
 * Conflict
 * Court Martial
 * Confetti Check A-OK
 * The Psychobombs
 * Timelash
 * Mindbender
 * The Long Sleep.