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AO-40 and the Sun ....clarification




More and more messages are appearing on the AMSAT-BB indicating that the 
transponders on AO-40, or even the entire spacecraft will be shut 
completely off for 3-4 months.  This has led to concerns not only about 
complete loss of use for 3+ months, but whether AO-40 would even wake back 
up!!  On reading recently published information, I can see how this could 
be confusing.  The fact is that once we leave 0/0, we won't be able to get 
back there until early April 2002.  So, we will be off-pointing at apogee 
until then and optimal conditions that we have now WON'T be back for about 
3.5 months.  That fact is where the "hiatus" comes in.  During this time 
there will be periods when it makes absolutely no sense to have the 
transponders active, so they will be turned off.  However, the middle 
beacon will remain on during these times.  In particular, early in the move 
we will drop ALAT by between -30 to -50 degrees.  In this configuration, 
squints are not good during any part of the orbit, so the transponders will 
be off.  We hope to slide under the sun in short order to approximately 
ALON=270.  We will then raise ALAT to 0.  In that configuration (~270/0), 
it should be possible to activate the transponders for about 15 to 20 MA 
units right after perigee.  Given the short range, signals could actually 
be extremely good during this approximately 1 hour window.   As the sun 
moves out of the way, we can progressively move towards 0/0, modifying 
(lengthening) the transponder schedule as we go.   So there will be periods 
of no transponder activity, hopefully just a few weeks, and there will be a 
much longer period of very limited but progressively increasing transponder 
activity before we return to 0/0.  The increase will be exponential.  As we 
approach 0/0 later in the spring the squints will dramatically improve, and 
so will the transponder times.  We will make every effort to activate the 
transponders, even if for only an hour per orbit, when conditions are 
appropriate.

Several individuals have asked if the move could be postponed until after 
the new year.  Only if you know how to change the earth's position in 
relation to the sun, or rotate AO-40's orbital plane (RAAN). :-)

Others have asked that the transponders be left on whenever the squint is 
<90 degrees.  There are a couple of concerns with this approach.  First, 
and least important, it is likely to be very disheartening to newcomers who 
may not know what good conditions sound like.  Much more importantly, with 
high off-pointing will come the overpowering temptation to really hammer 
the uplink signal to try to improve marginal downlinks.  This will overload 
the receiver AGC's potentially creating problems with commanding.   The 
stronger uplinks will also activate the S2 transmit limiter, decreasing the 
strength of the middle beacon, which will already be difficult to 
copy.  Under such adverse conditions, users having difficulty finding 
themselves are more likely to trample on the middle beacon, further 
hampering telemetry.  Thus, in order to get adequate telemetry during times 
of high off-pointing, it makes more sense to leave the transponders off.

Magnetorquing is a little like "magic" and although we will do everything 
possible to follow the above scenario, we must juggle the mystery effect, 
efforts at attitude determination, decreased torquer efficiency due to 
perigee eclipses, etc.  The simulators say this will work, but ultimately 
the above must be viewed as our current best estimate of upcoming events.
...stay tuned.




-- 
  ________________________________________________________________________
  Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
    Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
  ________________________________________________________________________

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