22 December 1999, 5:40 PM PST
What a roller coaster ride we have been on, but I have extremely good news
to report about MISR.
A retry was made this morning to unlatch the cover. The result was that the
electrical system indicated application of power to the actuator for about
32 seconds, at which point the power went off. This period of time is inadequate
to effect any significant motion of the actuator, and therefore unlatch did
not occur. While this was very disturbing at the time, the explanation for
this behavior is now well understood. When the procedure was prematurely
aborted yesterday, the software timer in the MISR computer which counts the
additional 45 second overdrive period for the actuator stopped at about the
12 second mark. Therefore, when power was reapplied this morning, the timer
simply continued the count to the timeout point and then shut off power to
the actuator. In other words, the system performed as it was designed. We
were able to replicate the observed behavior on the MISR engineering model
and examination of the software code confirmed that this would be the expected
behavior. Our tests and code review further indicated that MISR was at this
point reset to the proper state for successful initiation of a complete unlatch
cycle.
This afternoon we were given the go-ahead to try the cover unlatch procedure
again. The command which caused the problem yesterday was overridden, and
power was applied to the actuator for the required period. Everything went
perfectly and the cover unlatched successfully (an "unlatch" indication was
obtained after about 115 seconds, timeout occurred as expected 45 seconds
later, and the cover remained unlatched--its permanent state from now on).
The collective sigh of relief on both coasts has probably induced a change
in global weather patterns. In the next contact with the TDRSS, unlatching
of the North, followed by the South, calibration panels occurred. Both of
these went flawlessly.
The team's ability to reason through the cover unlatch failure scenario,
replicate it on the ground, and recover from a critical situation is a tribute
to the hardware and software resilience and the fail-safes built into the
system. Although we would all much prefer to forego the anxiety and tension
caused by this type of event, it makes the success even more gratifying.
Many thanks are due to the instrument, mechanical, and software engineering
staff and the instrument operations staff for accomplishing these major
milestones in the MISR activation sequence. I also appreciate the excellent
support we have received from Goddard and Lockheed-Martin.
On the Terra side, one of the spacecraft computers last night detected an
anomalous condition and put the spacecraft into "safe" mode, a type of standby
condition. Safing of the spacecraft causes removal of a signal called IMOK
("I'm OK") to the instruments, and when MISR detected this it automatically
went into its own "safe" mode. In this mode the optical bench heaters and
replacement heaters that mimic the heat due to the cameras (which are off)
are powered in order to keep the instrument optics and electronics from getting
too cold. At this time we are operating at comfortable temperatures and are
in a benign condition. We will get some outgassing benefit from being in
safe mode, but until the spacecraft situation is resolved we do not plan
to turn the cameras and focal plane heaters on. This afternoon's unlatching
events occurred while in the safe mode. No explanation of why the spacecraft
went to safe has been provided yet.
Another transition through the South Atlantic Anomaly occurred this afternoon
and no ill effects on the Terra High Gain Antenna were observed. The only
explanation for the previous HGA anomaly that has been offerred is that it
was a "transient" event.
During the next few days Terra and MISR will remain in safe mode to give
the crew a rest for the Christmas holidays. Another standdown period is planned
around New Year's. If, during the 3-day period between these two standdown
periods troubleshooting enables the spacecraft comes out of safe, we will
decide whether to put MISR into a full outgassing state (cameras and focal
plane heaters on) at that time or to wait until after the second standdown.
I want to wish all of you a very Happy Holiday season and a safe, happy,
and healthy New Year. May we enter the new millennium with a healthy mission
and a renewed appreciation for each other and the precious resource that
is our planet Earth.
You can see earlier status reports by checking the "News" link of the MISR
web site at
http://www-misr.jpl.nasa.gov.
With best wishes,
David Diner
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