SHADO Space Intruder Detector (SID)
Web site by: Phil Broad

In the series "UFO" the earth defense forces at SHADO utilized a deep space tracking network to alert them to the presence of the Alien invaders. This system was known as the "Space Intruder Detector" or "SID" for short. It formed some of the first hardware put into space by SHADO and represented their first line of defense. Although only one "SID" satellite was ever shown in the TV series, in reality there would have to have been a whole constellation of them to provide total coverage of the approaches to earth. SID provided warning of alien craft approaches to earth and targeting data as well as tracking of earth probes and debris in orbit. It could also provide a back-up up/down communications link to the SHADO forces on the moon if the primary systems failed.