He could have saved himself some time by looking at two different channels -- assuming the transmitters were in different locations, this would have given him two different ellipses which only intersected at a few points.
Google "Canary Wharf TV reception" and you'll see that this is not unusual. Those buildings really messed up reception.<p>Is digital TV immune to these analogue problems?<p>We actually have it worse in the part of North London where I live - we're in a dip, behind a hill, and don't have line of sight on the digital/freeview transmitters. Everyone has cable.
I didn't like the reddit-style title but really enjoyed the explanation from that guy.<p>The explanation and method are actually pretty simple but I just wouldn't have thought about it.
The physical phenomenon is called multipath and can become an incredibly difficult and important problem in many applications (e.g. radar guided missiles).<p><a href="http://en.wikipedia.org/wiki/Multipath" rel="nofollow">http://en.wikipedia.org/wiki/Multipath</a>
Actually, passive radar's getting popular for those who don't want their radar systems bombed out in the first attack run. Specifically, if you don't want stealths coming in & bombing out your radar installations, hide sensitive receivers throughout your city and let the cell towers, tv stations, etc. do the transmission for you.<p>Donno anything about the signal analysis part of this, but that's the gist of what I heard.
Isn't analog TV transmitted on an FM carrier signal? An FM receiver receiving two signals of the same frequency should synchronize on one, or possibly oscillate between the two. So it seems like his antenna is placed exactly where the two signals are in phase, making them indistinguishable to the receiver. Maybe he could put the signals out of phase by moving his antenna to the other end of the roof?
There is also a handy pub density map of mainland Britain on that site: <a href="http://www.frisnit.com/pubs/index.html" rel="nofollow">http://www.frisnit.com/pubs/index.html</a>
The punchline's very good: "My television is now an effective Canary Wharf detecting radar. Probably the most interesting thing I've seen on TV for a long time."