[Namaste-dev] Re: Howie's question

Roger Thompson rogert at traceroad.net
Fri Jun 13 11:33:11 PDT 2008


I find this interesting.

"We need the processing gain we get by using the 1/(R^2) loss advantage 
by demodulate decode at the satellite over 1/(2R)^2 path loss we would 
get from a bent pipe."

When I first heard about this project, I wondered right away why so much
complexity was being put into orbit by choice.  Added gain needed from
processing at the satellite explains why, but only if there are constraints
at the ground end of the link that force this design choice.  What are these
constraints?  My opinion so far is they don’t really exist.

A selection of commonly available “18 inch” DSS offset feed dish antennas
seems to be at the center of this issue, so let’s test why this choice may
have been made.  One reason certainly is that they are freely available,
being discarded by the thousands every year as subscribers change video
delivery services.  Another reason I’ve heard discussed is that these are
acceptable, somehow, wherever the private agreements variously called
CC&R’s, deed restrictions, or covenants would otherwise apply to amateur
radio antennas.  This isn’t so.

The reason the small dishes are so common is that Congress directed the FCC
in the Telecommunications Act of 1996 to write a rule to “prohibit
restrictions that impair a viewer's ability to receive video programming
services through devices designed for over-the-air reception of television
broadcast signals, multichannel multipoint distribution service, or direct
broadcast satellite services.”  This rule has been in effect since October,
1996.  In 2000, the FCC extended the rule to cover fixed wireless end user
devices.

The FCC rules clearly say amateur radio is not included in these rules.  See
http://www.fcc.gov/mb/facts/otard.html#links for more information.  Yes, it
is possible no one would notice if one of the small dishes was being used
for ham radio, however the legal process, should the deed restrictions that
prohibit antennas be applied by a neighborhood association or property
committee, could not call out these FCC rules on broadcast antennas as a
defense.  A more serious issue may be that the installation owner might be
held to be in knowing violation of the FCC rules.  I don’t know what the
consequences of this for the manufacturers of the equipment might be.  The
availability of these antennas and legal issues in other countries is also
unknown to me.

Deed restrictions are of vital importance to those who have chosen to
purchase real property so encumbered, but system design decisions that might
appear to have been made to accommodate, maybe unintentionally, a violation
of these restrictions don’t seem valid.  Probably more importantly, I
suspect there is a much larger potential user base than those who live under
deed restrictions, and remote operation is a growing alternative for this
hampered user base anyway.  What does this mean for processing at the
satellite?

If we discount the apparent self-imposed requirement to use the smaller
“DSS” dishes, the ground station designs would be free to use larger
antennas, and the satellite system design could be repositioned to be more
along the “bent pipe” model.  This should decrease the system costs, provide
a more robust system, accommodate future changes better, allow for simpler
redundancy at the satellite, and provide for more types of user terminals.
There are likely many other reasons we would prefer most of the complexity
of any communications system to be accessible. 


Roger Thompson
AD5T


-----Original Message-----
From: namaste-dev-bounces at amsat.org [mailto:namaste-dev-bounces at amsat.org]
On Behalf Of Bob McGwier
Sent: Friday, June 13, 2008 12:20 AM
To: namaste-dev at amsat.org
Subject: [Namaste-dev] Howie's question


I cannot see where anyone answered Howie's question. No, you have it 
almost completely wrong. It is definitely a demod/decode/recode/modulate 
satellite.

We need the processing gain we get by using the 1/(R^2) loss advantage 
by demodulate decode at the satellite over 1/(2R)^2 path loss we would 
get from a bent pipe.

Bob

-- 
ARRL SDR Working Group Chair, AMSAT VP Engineering.
Member: ARRL, AMSAT, AMSAT-DL, TAPR, Packrats,
NJQRP, QRP ARCI, QCWA, FRC.
“Trample the slow ....  Hurdle the dead"

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