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AO-40 S2 Shutoff, Orbits 649-651

For the last three orbits, the battery voltage during the final 45 seconds 
of  the 25 minute eclipse period has dropped to the point  where it has 
triggered shutoff of the S2 Tx.  There are several reasons for this.  The 
batteries are warm due to the negative solar angle and to protect them the 
BCR's run them at slightly lower voltages, so they are less than fully 
charged.  As they cool during eclipse, the voltage will drop some as 
well.  We are also running the magnetorquer through eclipses which draws 
considerably more power.

The cutoff voltage for the S2 Tx is set very conservatively and the 
scheduling software has restarted the S2 Tx at MA=15 nominally each 
time.  We have allowed this to repeat for three orbits to determine its 
reproducibility and to evaluate the reliablity of the automatic 
restart.  Everything has been nominal and the shutoff has been virtually 
identical, within seconds, each orbit.  This morning, I have lowered the 
shutoff threshold by one count (0.155 volts).  This should be sufficient to 
prevent further eclipse shutoffs for this cycle, but if not additional 
adjustments will be made.

The current perigee eclipse cycle runs through June 19th.  From  August 13 
to September  18, AO-40 will experience much longer non-perigee eclipses 
that peak at 165 minutes duration.  During these eclipses the S2 Tx will be 
turned off by the scheduler.

--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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