SXT Status report
Fri, 10 January 1992
Fm: James Lemen

Greg Slater, Keith Strong, Nariaki Nitta and I all arrived at ISAS this
week.  Hugh Hudson left for the States on Wednesday for a two week
trip. Operations were resumed on 4 January following the spacecraft
holiday.  The early operations have been characterized by 3 seu's:

1. The first occurred on 4 January.  

2. The 2nd was on 8 January.  

At the time of this error, filter soft errors were detected.  The KSC
operator noted that the shutter status was "open".  He attempted to
close the shutter, but when he failed to do so, he decided to turn off
all SXT power, including the TEC.  During the last pass, Tsuneta turned
on the instrument except for the TEC.  Subsequent images have been
examined to try to determine if the shutter was indeed left open for any
period of time.  From a quick analysis, it does not appear that there has
been any dark current increase, and from this we conclude that the
shutter was in fact closed.

3. The 3rd was today, 10 January.

On this occasion there were 3 filter soft errors and a filter hard
error.  After the KSC operator failed to restore normal SXT operation,
the SXT was turned off and the Yohkoh spacecraft was off-pointed to
prevent damage to the CCD in case the shutter was open!  Tsuneta san 
executed the recovery of the SXT at a subsequent station contact and 
turned on the TEC.  Normal operations of SXT have resumed.

The occurrences of three SEU's in less than a week is somewhat of a
surprise, also given that the the orbit at the moment is "good", that is
the apogee is not falling in the radiation belt.  Sam Freeland is
currently testing code which could make the monitoring of code
corruption by SEU's more frequent and automatic.