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AO-40 Update




Attitude determination on 2001-12-16 showed that ALON/ALAT = 
356/-10.5.  This was slightly more negative than originally expected, but 
such is the science of magnetorquing.  Following this attitude 
determination, we had initially planned to drop ALAT to approximately -30 
degs to assure that the solar sensors would continue to function ( without 
them we cannot magnetorque ) as we moved "under" the sun.  This would allow 
us to retain full functionality of sensors and torquing.  However, there 
are two bothersome features about this large negative ALAT.  First, the 
YACE camera will not see the earth during the orbit, and precise attitude 
determination will be very difficult and time consuming.  Second, with the 
high negative ALAT, communications will be extremely difficult, both for 
users and for telemetry gathering.  This is particularly disappointing 
given that the lighting is currently optimal for earth photography with 
SCOPE and YACE cameras, and the CEDEX system is currently supplying good 
data which we would like to continue to gather through RUDAK.

Therefore, we have decided NOT to lower ALAT any further, but to use the 
mystery effect to our advantage and simply coast through the "dead zone" 
for the next few weeks.  We can do this safely because our -10.5 deg ALAT 
means the solar angle will never be worse than -52 degs, assuring adequate 
power,  and the sun will be on the omni side of the satellite, so we do not 
have to worry about damaging the cameras.  We know the "mystery effect" at 
our current RPM will decrease ALON by 14 degs/week.  Therefore, even though 
we will lose solar lock shortly when the solar angle exceeds -45 degs, we 
will know quite well where we are.  We just won't be able to magnetorque or 
use the sun sensors again until we drift out of the dead zone.

           ALON/ALAT will look like this for the next few weeks of drifting:

       AO-40  ATTITUDE - PRECESSION BEHAVIOR
       Mystery Effect = -14.0 degs ALON/week

        DATE            ALON   ALAT    SA     ILL%
--------------------------------------------------
  2001  Dec  16 [Sun]  356.2  -10.5  -27.9    88.4
  2001  Dec  23 [Sun]  341.0  -10.6  -42.8    73.3
  2001  Dec  30 [Sun]  325.9  -10.8  -51.8    61.9
  2002  Jan  06 [Sun]  310.7  -10.9  -50.4    63.8
  2002  Jan  13 [Sun]  295.6  -11.0  -39.5    77.2
  2002  Jan  20 [Sun]  280.4  -11.0  -23.6    91.6

When we regain solar sensor lock in the latter half of January, we will 
begin station keeping to keep from drifting backward further in ALON, and 
will raise the ALAT back to 0 shortly thereafter.  We will then chase the 
sun slowly back from ALON/ALAT = 300/0  towards ALON/ALAT = 0/0, getting 
there in April as previously described.

.... the good news, of course, is that with ALAT in this range, we should 
be able to keep the transponders active for part of the orbit after 
perigee, right through this time period.  Some checking with your favorite 
tracking program will show reasonably good squints from  MA = 10-40 when 
ALON=300.   Coupled with the low range this should give acceptable to very 
good downlinks.  The negative ALAT, of course, favors the southern hemisphere.

During this time, the beacon / transponder / RUDAK schedule will be altered 
as the ALON changes.  We will keep you posted here and in the message 
blocks.  The schedule will be modified slightly in the next day or so, to 
begin this process.

----W4SM for the AO-40 Command Team



-- 
  _______________________________________________________________________
  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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