[amsat-bb:74694] AO-40 Update, 2004-01-27

["Stacey E. Mills" <w4sm@xxxxxxxxxxxxx>]

Initial attempts today at recovering the S2 beacon on AO-40 have so far not 
been successful.  However, these will be repeated many more times from 
multiple locations in the days ahead.  We have a good "fix" on ALON/ALAT 
and we are spinning at 3.5 RPM, so we have quite a few days/weeks before 
the squints become problematic due to the mystery effect.  Currently, the 
mystery effect is working for us to improve solar angle. Attempts will also 
be made in the next few days to bring the K-Tx on line and search for its 
signal.  The command team has a series of simple machine code commands 
that, if heard, should activate transmitters, switch receivers, switch 
batteries, etc. without having to load the flight software into the IHU-1.

By way of review, the command team had been watching the main battery for a 
number of months and noting that its behavior, while not alarming, was not 
fully as expected.  In particular, there were voltage fluctuations with 
spin under relatively light power loads.  When panels 1-6 received sunlight 
their 1/2 complement of solar cells would often lead to a voltage dip, 
which would pop back up when the fully arrayed panels were in the 
sunlight.  This suggested that perhaps one cell in the 20 cell main battery 
might be weakening.  On Sunday, we noted that the battery was not 
maintaining its set voltage and this led to the safety software shutting 
off the passbands.  Some minor adjustments were made to the solar array 
voltage offsets and this helped some, but we stayed in passband off mode to 
study the matter further.  It appears that at this time we may have 
acquired a second bad or shorted cell, though we are analyzing this 
further.  During eclipse on orbit 1487,  at approximately 1930 utc on 
2004-01-26 the battery voltage hit the extreme low voltage trigger.  This 
caused safety resets of the battery and solar array set points and the 
auxiliary battery was tied in to the main battery.  This all appears to 
have happened benignly, and when I got AOS at MA=8, about an hour and a 
half later on the following orbit, I noted that the extreme low voltage 
flag had been set ,as well as the other changes noted.  Voltages on both 
batteries at this time were similar to other post-Sunday values, as were 
the charge currents.   At this voltage level, the auxiliary battery was 
considerably undercharged.  We appeared to be in a stable, though not fully 
nominal mode, and I was in the process of noting the status of things to 
the command team to decide on the next course of action when, at 00:39:59 
utc the battery voltage suddenly dropped from its post-Sunday level of 25.5 
to 26.5 volts, down to 18 volts.  This was accompanied by an expected 
increase in charge current.  The safety software shut off the S2 Tx at the 
next MA change, and I brought it back up manually twice, noting that the 
voltage was relatively stable at about 14 volts with heavy charge 
current.   After letting things sit for a few minutes, a third attempt to 
start the S2 Tx was not successful.

Our current best understanding is that we suffered a catastrophic failure 
of the main battery which is clamping the buss voltage at a low 
level.  Accordingly, we have been concentrating our efforts on trying to 
connect the auxiliary battery to the main buss and disconnect the main 
battery, placing it on trickle charge for further analysis.  If we can 
manage just 10 volts on the main buss to activate the IHU-1 we should be 
able to accomplish this with the simple machine codes noted above, and the 
voltages should come up to levels where  the S2 will function.

If you examine the telemetry during this time, 10 minutes after the 
low-voltage "event",  the temperature of heat pipe 4 + X + Y takes a 34.2 
deg.C apparent increase in temperature over a span of 14 seconds (1 block 
interval), and continues to rapidly increase almost to the top of the 
scale.  Given the timing of this change, 10 minutes after the low-voltage 
"event",  its rapidity, the thermal inertia of the large, circumferential 
heat pipes, etc... it is difficult to believe that this is real.  Most 
importantly, this heat pipe sensor sits in a "benign" area at the apex of 
panels 1 and 6, near the high-gain end of the spacecraft.  There are no 
batteries or BCR's in this region, only the controller for the liquid 
rocket motor (which was powered off!), and the L1 receiver, which shows no 
accompanying change in temperature.  All of the BCR's and batteries are 
much closer to heat pipes 1 and 2 at the omni end of the 
spacecraft.  Therefore, this may represent a failure of the 
software/hardware due to low voltage, rather than a real temperature 
increase.  However, this will be studied further.  No other telemetry 
anomalies have been noted but we continue to look.

--W4SM for the AO-40 Command Team



-- 
  _______________________________________________________________________
  Stacey E. Mills, W4SM    WWW:    http://www.keplerian.com
   Charlottesville, VA     PGP key: http://www.keplerian.com/key
  _______________________________________________________________________

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