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RE: AO-40 Computer Crash

On orbit 563, the IHU-2 was turned on and primed to take images early in 
orbit 564.  When the IHU-2 is on, it controls the middle beacon but also 
listens to the IHU-1.  Software is loaded into the IHU-2 to parrot the 
IHU-1's telemetry (with a one block delay) until the images are ready to be 
downloaded; then the IHU-2 switches to outputting D-blocks.  When the IHU-2 
is in "parrot" mode the only way to easily tell that it is actually 
(re)generating the IHU-1 telemetry is to note that the IHU-2 is ON in the 
Status window of your telemetry program.  Otherwise, the telemetry format 
is nominal.  Sometime around perigee 563/564, presumably when passing 
through the van Allen radiation belt, the IHU-2 crashed.  This caused it to 
repeatedly spit out a diagnostic block.  The sync vector is present but the 
block looks like gibberish.  The IHU-2 (when on) is currently running in 
cache mode for speed and does not have active EDAC software so it crashes 
about 1 in 5 perigees.  Unlike the IHU-1, it controls no critical functions 
and is easily reloaded from ROM in a matter of seconds.  Because the 
picture "window" was about to pass for this orbit, the IHU-2 was simply 
turned off, instantly giving command of the middle beacon back to the 
IHU-1, which was running normally in the background.

  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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