[opendtv] Re: Satellite radio
- From: "johnwillkie" <johnwillkie@xxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 10:27:14 -0700
You have got to be kidding. "Terrain Blockage" That's by far the funniest
canard I've heard here.
If the satellite is low in the sky (like near the poles) and the mountains
are steep, terrain blockage is an issue if you, for example, are in the
Northern Hemisphere and are on the north side of a hill.
L.A. is at 34.25 degrees north latitude, 118 degrees w longitude. So,
assuming a satellite at 110 degrees west, the satellite point angle as
calculated here http://www.satsig.net/ssazelm.htm is a whopping 49 degrees!
How many hills are you familiar with a rise over run in excess of 100%?
That hill "might" block satellite reception on its northern side. But, few
people live (or drive) along the exact base of a mountain.
To put it simply, XM doesn't have any terrestrial repeaters in L.A. due to
terrain shielding. And, were building shielding an important issue, they
would have little need for a xmtr at 5800 AMSL 20 miles north of Downtown
LA.
As for car radios working in the Lincoln and Holland tunnel (and in major
NYC subway lines), it's because of a Florida-based company that held the
patent for, and was called, "Tunnel Radio." They installed the equipment,
and I talked with them several times at NAB in the 1980's. I believe the
patent ran out earlier this decade.
What this has to do with COFDM is that the modulation scheme employed by XM
-- as trumpeted by Bob Miller years ago -- is COFDM.
I don't know that the XM setup actually uses a single frequency, but I
suspect that it does. I don't know that their network in LA uses a SFN, but
I suspect that it does.
By the way, the North Side of Mt. Wilson is quite steep, but I don't think
it's in excess of 45 degrees.
Terrain blockage, Craig, is an issue with terrestrial broadcasting. Except
for perhaps Alaska, it is not much of an issue with reception of satellites
in the Clarke belt. And, it has zero relevance to Los Angeles or southern
California, where most steep hillsides (over 30% grade, about 15 degrees
rise over run) are blocked from development.
John Willkie
-----Mensaje original-----
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Craig Birkmaier
Enviado el: Monday, September 24, 2007 7:28 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: Satellite radio
At 10:08 AM -0700 9/23/07, johnwillkie wrote:
>XM's LA-site-number 2 is atop Mt. Wilson, where I've seen the equipment,
and
>I don't mean from the outside of the building. They're in the same
building
>that houses KCET, the southern-most one.
>
>5800 feet elevation amsl. With satellites and this boy, they still needed
>illegal transmitters. Not exactly a ringing endorsement of COFDM or SFNs.
What has this got to do with COFDM or SFNs?
Satellites cannot overcome terrain blockage.
How is it that car radios work in the Lincoln and Holland tunnels in NYC?
Every transmission medium needs help to overcome its inherent
physical limitations. That help is typically associated with
economics (hence the concentration of terrestrial satellite radio
helpers) or a perceive public safety issue (hence the retransmission
of radio broadcasts via "leaky cables" in the tunnels).
Regards
Craig
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