[mythtv-users] Satellite footprints
Mike Perkins
mikep at randomtraveller.org.uk
Fri Jun 22 18:34:45 UTC 2007
Rod Smith wrote:
> On Friday 22 June 2007 12:15, Mike Perkins wrote:
>> Dan Ritter wrote:
>>> And why the .na. in the middle? Because the overlap in
>>> programming between Washington DC and La Paz is probably
>>> small. And satellites visible across the US, northern Mexico and
>>> southern Canada are probably not visible from Rio de Janeiro.
>> You do know that all those satellites are positioned above the equator,
>> no? Otherwise they're in an inclined orbit, which means they would
>> wander up and down in latitute, and it would be a devil of a job to keep
>> the dish aligned to them...
>
> Yes, but communications satellites (at least those used for digital TV, which
> is the technology with which I'm most familiar) typically serve a subset of
> the hemisphere that's visible from the satellite. Think of it like a
> flashlight beam; the satellite might point its "flashlight" at the US to
> cover that market, with a good deal of splashover into Mexico and Canada.
> That beam won't reach as far south as Brazil, though. US satellite providers
> use even smaller "spot beams" to carry local stations. This enables the same
> frequencies to be re-used to carry stations for different markets -- viewers
> in Boston and Seattle might tune to the same transponder to watch their
> respective local stations and be physically unable to receive each others'
> locals. I don't know the extent to which frequencies are re-used between
> nations in this way, but I'd be surprised if there weren't any re-use,
> particularly between extreme northern and southern nations.
>
Point taken.
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