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Re: Telm Diversity combining



When you mention "diversity selection scheme" are your referring to a "voted"
type of system based on signal to noise ratio? I wouldn't think the data stream
used in AO-40 would be as forgiving as voice is on switching since there is a
delay introduced on a switch.

Another factor with satellites I've wondered about is multipath - how does
multipath affect satellite reception and what can be done to counter it?
Obviously a dish with narrow beamwidth is one, but are there any other
techniques in use?

73's

Don Woodward
KD4APP
AMSAT 33535
kd4app@amsat.org
http://kd4app.webhop.org


----- Original Message -----
From: "Gunther Meisse" <gjmouse@neo.rr.com>
To: <roberts@qadas.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Sunday, January 05, 2003 19:52
Subject: [amsat-bb] Telm Diversity combining


To: Dr. Robert Suding
W0LMD
Robert,
I have been reading your mail on AO40 regarding the 40ish squint MB fades
and your thoughts regarding twin downconverters and switching to select the
preferred signal in a diversity selection scheme.
Since it is such a great mind bender, I have chosen to cc this e-mail to the
bb with the hope that it will encourage additional thinking and
conversation.

I think that what you are observing is not a phase shift of the radiated
wave, but rather the blocking of the wave front by obstruction elements of
the spacecraft as it spins.
As a basic test, put a SPDT relay on the RHCP and LHCP outputs of your new
dual polarization patch. When you listen on RHCP to the MB at a rough squint
like 45, log the cycle of the wax and waning on a real time line. It will
probably be in the order of 20 seconds, or a multiple of the spin rate of
aprx 3 rpm. Once you have the time line established, immediately switch to
the other polarization and see if the nulls of the original time line are
filled in the reverse polarization. If they are, you are onto something!!!
It is a crude test, but will answer the question: Is it is really a polarity
issue.

I don't think it is a polarization change, but rather, a wax and waning
caused by the spin of the spacecraft and the resulting physical obstruction
that exists from changing vantage angles.
Your test would answer that in a hurry.

Now, in any event, your goal is admirable and deserves more discussion
regardless of the test results.

There is great interest in a complete telemetry data stream for AO40 and, to
an even greater degree, for future GTO birds and the coming Mars bird.

I have been provided, and am running, a beta version of the AO40Rcv
telemetry decode program which has been modified to produces a third data
file which includes the complete data stream. The archivist then gathers the
files from around the world and, as I understand it, loads these new full
data records into an array, say horizontally, and then views them
vertically, eliminating the obviously bad date bits and treating the
remainder mathematically to create the best fit for a complete, usable data
stream. I have not heard how that work is coming, but it is indeed a GREAT
idea. It is sort of the ultimate DIVERSITY approach, combining and selecting
from around the world, from a multitude of look angles.

Knowing absolutely nothing about the code running on the Goddard server, I
say with some reservation: The diversity project, if successful, could even
be implemented on the Goddard system, by modest modification of the AO40Rcv
& UDPtelm programs to deliver this new raw file, in nearly real time, on the
internet to Goddard, where the same array evaluation code could function in
virtual real time, distributing the result over the internet for worldwide,
nearly real-time, use.  I would think this sort of a telemetry system would
be of massive value to the Mars venture, not to speak of our present and
near term projects.

Well Robert, let us hear the results of your tests and perhaps others will
chime in on the topic.

73
Gunther Meisse
W8GSM


Good QSO to listen to.
See ya
73
Gunther
W8GSM

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