And Now, a Brief Diversion
Just so we have something to talk about while we're waiting for the Truthers to get their act together, let me clean up and retire the satellite dish questions from a page or two back.
"Slewing" and stuff.
By the way, why would a satellite dish be non-functional if at 0.5 to 2 degrees? Wouldn't that depend on how just-over-the-horizon the satellite is?
Slew rate refers to how fast you have to swing your antenna to keep a target in view. The beams formed by high-gain comm antennae (that is, the area that is "in focus") are narrow, typically on the order of a degree for commercial satellite television receivers, and down to about 0.04 degrees, or two minutes of angle, for space communication in X-band. This is about the size of a postage stamp fifty meters away --
very small. Thus, as the Earth rotates and the satellite moves in its orbit, you have to move the antenna to keep up with it. This movement is moderately to very precise depending on the performance you need.
At 0.5 to 2 degrees of elevation, you are unlikely to be able to communicate. Several reasons for this. First, you're forcing the beam to go through a much thicker slice of atmosphere than you would at higher elevation (say > 20 degrees). Weather affects peformance too, and will be much worse. Second, you're now going to pick up interference from the ground which may swamp your weakened signal. Third, at that low angle and pointing west, you will have a Doppler problem that your receiver may not be able to compensate for, due to Earth's rotation, talking to anything but a geosynchronous bird.
Now, these are all things that
maybe you can compensate for. At NASA we use a lot of tricks to recover passes, like slowing down the transmission data rate when our signal-to-noise ratio drops too far, but we get to do this because we're the only thing the satellite is talking to. Using a commercial satellite, they don't care about you, they're designed to much different conditions. Still, even we don't try to get an X-band signal at zero elevation. Go ahead and try it, you won't receive a thing.
This is moot, however, since Abbottabad is surrounded by hills, and all of these reach more than 2 degrees apparent elevation from the spot in question. This particular antenna, pointed how it is, cannot see anything.
It is, according to the source, pointed at 260-265 deg west. The elevation is stated as 0.5-2 degrees, but looking at the picture, I would estimate it to be nearer 5-10. At any rate, it is plausible that this direction will intercept a geostationary orbit, somewhere. Where that is requires a bit of math, and much more precise data. It is not a very large disk, so it is doubtful that they can get a reasonable signal; not only is the low beam noise-prone, but it is way out of a satelite's normal beam.
Correct. I was referring to the satellite beam width in my initial assessment.
The other issue, as you note, is lobing. If you try to talk to a geosynchronous satellite, but you do so from the very edge of the planet (from its perspective), then you're insisting that the satellite is illuminating
the entire Earth with its signal. This is almost never the case. To do this a satellite needs a beam width of about 16 degrees, which is simply inefficient (about +23 dB). Most geosynch comsats run around +30 dB antennas or so, though there are a few (mostly satellite telephone applications) that do in fact cover a good chunk of the Earth's surface.
If we ignore this and look for where it
could be pointed, it's fairly straightforward to compute that, given Abbottabad's location (73 degrees East) and antenna pointing almost true west (assume 0 degrees for now), this intersects a geosynchronous position hovering over about 13 degrees West. There are a
number of satellites in that location, some of which are military comm sats (well, duh, it is over the Atlantic), and some are quite mundane.
I think it's pretty clear, however, that Usama simply had satellite TV. It's hardly rare among the wealthy in that part of the world. The dish itself is quite unremarkable with respect to size, placement, feedhorn, or anything else. This is just another made-up mystery. One may as well speculate the walls outside his compound were actually the
trillions of dollars of gold stolen from the WTC, cuz', you know, nobody would actually build walls like that.
