UFO was a science fiction TV series set in the future (1980s) about a secret military organisation that worked undercover to defend Earth from alien invasion.
The organisation is S.H.A.D.O. - Supreme Headquarters Alien Defence Organisation - who operate covertly from a film studio in London but also have a satellite in orbit for advance warning of alien incursion.
The organisation has space Interceptors on the Moon, a submarine/aircraft called Skydiver on Earth and various other vehicles to deal with aliens.
The series was designed by Gerry and Sylvia Anderson and first broadcast in the UK in 1972.