SXT Status During the 24-Oct-95 Eclipse Recovery  
                     
                             Serge Savy

                              13-Nov-95


The eclipse observations on 24-Oct were a great success but the special
operations associated with them led to problems with Yohkoh operational
control for several days afterward.  Normal operation was restored by
Saturday 28 October.  The problems arose from conflicting Sun sensor
signals due to incorrect orbital timer setting after the partial eclipses
ended.  This caused Yohkoh to go into safehold mode, whereby spacecraft
pointing switched automatically to its backup control system.  A mistake
was made in the attempt to recover from safehold mode the next day, causing
Yohkoh to enter "hard safe hold" mode, whereby the spacecraft pointing is
maintained by the attitude control hardware. 

Recovering from this mode required a reboot of the Attitude Control
Processor (ACP) to clear an error flag.  This reboot failed.  Working on
the assumption that the ACP software had become corrupted, the entire ACP
software package--including original code and software patches--was
restored from memory at a different on board location (the bubble file
memory--BFM).  The reloading process required careful checks, but after
hard work by senior Tohbans (especially Te.  Watanabe) helped by NEC
specialists, the ACP setup was restored in several stages over a few days. 

SXT operation was suspended during most of the recovery process because on
entry into safehold mode on 24 October Yohkoh lost fine pointing capability.  
The key times with regard to SXT data are:

Yohkoh safehold began             24-Oct-95 07:29:20 UT 
SXT off (SXT CTL MAN)             26-Oct-95 04:19 UT (KSC Pass 4)
SXT on  (SXT CTL AUTO)            28-Oct-95 04:57 UT (KSC Pass 5)
Yohkoh fine pointing regained     28-Oct-95 05:00:47 UT

Yohkoh and SXT have been operating normally since 28-Oct-95 05:00:47 UT. There
is no problem now either with the spacecraft or the instruments.