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AO-40: Plans for April-June 2001



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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