Just a few thoughts on this;
If this is in a polar orbit(i.e. circling the earth pole to pole) and not in a geo stationary orbit,then it would be the perfect spy satellite as it would cover the whole of the planet during the course of many orbits.

Who's is it? I don't think it is one of ours unless from a previous civilization,have read a lot of stories about it sending radio signals and them being decoded.One of which was a star map as would have been viewed from earth 13,000 years ago.

What is it's purpose? I dunno but here's a couple of theories.
1; could it be a beacon left by an advanced off world civilization to monitor us,for what purpose? who knows.
2;Is it tied in with our moon somehow(there are theories that our moon is not natural and was brought here from elsewhere)maybe to stabilize it's orbit?