Mysterious Black Knight Satellite has been monitoring human civilisation, perhaps from the beginning

Apparently, earth has had such an alien satellite in orbit for perhaps several thousand years. The Black Knight Satellite, which it is referred to, is a mysterious large piece of equipment that has been in earth's orbit long before human civilization became scientifically advanced enough to place satellites in space.

Nicola Tesla was first to pick up unknown repeating radio signals from space in 1899 and later, publicly announced it at a conference.

Some 20 years later in the 1920s, amateur HAM radio operators were able to receive this same signal. 

From UFO Digest