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Re: S-band receiving requirements



Good points. I think the explanation for the silence of the experts (or
lambs) is that  "hindsight is always 20-20" ..... 8-)

----- Original Message -----
From: Estes Wayne-W10191 <W10191@motorola.com>
To: <amsat-bb@AMSAT.Org>
Sent: Thursday, May 17, 2001 3:04 PM
Subject: [amsat-bb] S-band receiving requirements


> I think I now understand why so many of us had unrealistic expectations
about our S-band receiving systems.  It boils down to a series a cascaded
assumptions that were incorrect.
>
> After the V-transmitter failed, a lot of us cobbled together S-band
receiving setups based on the available information which was mostly
pre-launch examples described in the book P3G to P3D and many other
articles.  For instance, P3G to P3D describes S-band downlink "Example 1" as
follows: 60 cm dish + preamp with 1.5 dB NF to get 23 dB S/N. (this is the
BETTER-performing of the two examples given)
>
> The first mistake was not realizing that these examples assume a best-case
situation of the S1 downlink and minimal squint.  The actual AO40 orbit is
likely to require frequent use of the S2 downlink which has 8 dB less gain
(because the low inclination orbit will require seasonal off-pointing of the
antennas).
>
> The second mistake occurred more recently.  The seductively strong S2
middle beacon fooled many of us into believing that our "pre-launch Example"
receiving setups were satisfactory.  We didn't know that we actually need to
copy signals 10 dB weaker than the Middle Beacon.  This was not widely known
until AFTER the experimental UL/S2 transponder operation commenced.
>
> Many of us discovered through trial and error that "pre-launch example"
S-band receiving setups are woefully inadequate for reception of the
existing spin-stabilized S2 transponder.  More uplink power will result in a
readable downlink signal, but a higher authority has (correctly) decided for
us that the solution must be to use receiving setups that are significantly
better than the pre-launch examples.
>
> It remains to be seen if "pre-launch example" receiving setups will be
adequate part of the time with the final satellite configuration.  Squint
near perigee will improve dramatically if 3-axis stabilization is
successful.  There may be seasonal periods when the S1 antenna can be used
for a portion of each orbit.  It is certain, however, that RELIABLE S-band
reception requires a SIGNIFICANTLY BETTER receiving system than the
"pre-launch examples".
>
> In retrospect, the only thing that mystifies me is why today's "alligator
critics" were SILENT when months of amsat-bb traffic was dominated by
discussions about building helixes, modifying surplus converters, etc.  One
or more of these silent experts should have spoken up to explain why those
setups won't be good enough.  Then for the last few weeks the experts
remained silent when several people reported modest success hearing the
Middle Beacon.  Again, the experts should have pointed out that we need to
hear much weaker signals than the Middle Beacon.  The "alligator critics"
are vocal now, but they were strangely silent when their advice would have
been most helpful.
>
> 73,
> Wayne Estes W9AE
>
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