But still, it's all simple really.
We just need there to be an alien civilisation about 25 light years out, who'd developed the noise/signal sorting tech by late 1978, so they could pick up Doctor Who from the start as the BBC signals reached them.
Being people... ah, aliens of taste, they would instantly realise it was so good that it should be re-broadcast on their own TV stations. Which means we can pick up their retransmissions of Marco Polo early next year, if SETI just develops the necessary noise-signal tech in time!
Mind you, it'll be really irritating that, after Marco Polo, we'll only get six more missing episodes off the alien TXes until October 2015. ;-)
First of all, I suspect that if any aliens did intercept our sci-fi TV broadcasts, they'd probably find them incredibly racist and refuse to have anything to do with us on principle.
Secondly, the notion of aliens picking up Earth TV broadcasts and re-airing them on their own planet is part of the plot of the 8th Doctor novel "Mad Dogs & Englishmen" by Paul Magrs.
Essentially, an alien poodle seduces H.P. Lovecraft into convincing J.R.R. Tolkien to rewrite The Lord of the Rings
so that its events closely mirror true events on the poodle planet, so that the deposed poodle princess can use it to foment a revolution against the current emperor. My favorite part of the book is how one of the workers on the poodle listening/transmission station has set up a secret business selling black market bootlegs of "offensive" & "kinky" films like 101 Dalmatians
& Beethoven's 2nd.
Over the years I've seen that there are a group of people who will believe whatever he says and defend him no matter how horribly abusive he gets purely because he saved some old episodes from destruction.
Yes it's great that we have them but they don't buy him a lifetime pass to be a Dick.
Not only that, but there's also his dubious contribution to 1980s TV theme music that I believe he's yet to apologize for.
But then, I guess I'm really just asking for the 1980s to apologize for being the 1980s.