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Re: AO-40: Plans for April-June 2001

Peter and all,

First thanks very much for the update.  We need those frequently.  The
team is doing a first rate job.  Congratulations to all team members.

I will start the discussion with some spectulation about the alon changes.  
Is it possible that something has come lose on the s/c and is acting like
a rudder in the very thin atmosphere?

Any one else?


Dr. Jim Akers
Dept. of Electrical 
and Computer Engineering
Miss. State Univ. 

On Tue, 3 Apr 2001, Peter Guelzow wrote:

> Dear All,
> enclosed some important information from the AO-40 command team
> (special thanks to James Miller G3RUH, Stacey Mills W4SM and
> Karl Meinzer DJ4ZC) about the upcoming activities.
> 73s Peter DB2OS
> President AMSAT-DL
> .......................................................................
> AO-40  Plans 2001 April-Jun
> ---------------------------
> Plans have had to be changed in the last few days.  We have found that there
> is a significant effect at perigee caused (we believe) by the atmosphere.  It
> causes the alon to decrease by some 3/perigee when the spin rate is 2 rpm.
> The alon already reduces 0.7/perigee due to precession of the orbit plane,
> so we we seeing nearly 4/perigee, or some 5/day.
> Despite eclipses, the magnetorquer is a little bit stronger than this effect,
> and we found we were just able to counteract this, and increase the alon by
> about 1/perigee.  But shifting alon from 170 to 270 at that rate was clearly
> going to take a very long time, even given that our assumptions could be
> extrapolated to different geometry.  Meanwhile, perigee height is decreasing
> steadily due to luni-solar perturbations, and we'd probably lose the
> advantage during the manoeuvre.
> So the drive from alon 170 to alon 270 by increasing alon has been put on
> hold.  Instead we are going to change alon the other direction, taking it
> down to 90 and then through 0 and hopefully off to 270 later in the year.
> There are a number of benefits of this plan.
>  * Firstly we can use the atmospheric effect to augment the magnetorquer and
>    achieve a more rapid change in alon.
>  * Second, communications will improve rapidly due to the improved alon.
>  * Third, as we approach alon = 0 we are in a position to try out some
>    transponder operation sooner rather than later, which will surely be
>    appreciated ;-)
> During this procedure, the spacecraft will go into "hibernation" again.  This
> is the name we've given to the state where the Sun sensor system cannot see
> the Sun, so the s/c cannot be magnetorqued by the normal means.  However, if
> the atmospheric effect continues to work as it evidently did during the
> previous hibernation, this period of poor ( > 45 deg) Sun angle will be short
> lived.  Also at this time, there is a possibility that the Sun will be nearly
> coaxial with the spin axis.  However the spacecraft bottom will be
> illuminated, not the top, so the cameras will not get fried.
> Expected Timetable
> ------------------
> The following is our best estimate of the way things will evolve.  The Sun
> angle will reach a point where the Sensor will stop seeing the Sun around
> April 5th (-0, +3 days).  Then we wait perhaps 4-6 weeks for the Sun angle to
> reach its nadir, and then recover again.  By this time the alon should favour
> some decent beacon communication.  Although the Sun sensor will not give
> data, the temperature profile gives Sun angle clues, as can be seen from study
> of the historic telemetry record.
>   DATE          ALON/ALAT    SUN AZ/EL    SA     ILLUM
>   -----------------------------------------------------
>   2001 Apr 05     146/0        280/5      -44      72%  ( lock loss )
>   2001 Apr 18     110/0        289/11     -79      19%
>   2001 Apr 25      90/0        294/14     -63      45%
>   2001 May 03      70/0        299/17     -39      77%
> The above table is an estimate.  Everything is an estimate.  AO-40 may be
> nearly half a year old, but nevertheless we are still learning, and she is
> trying to teach us.
> Footnote
> --------
> The command team welcomes informed discussion as to why the alon is rapidly
> decreasing i.e. the attitude vector direction changes clockwise as viewed
> from above [+Z] the orbit plane.  This change only occurs close to perigee.
> The phenomenon is clearly observed, but is not explained.
> ----
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