Status, 27 December 1999, 1:20 PM PST:
MISR and Terra are continuing operations in safe mode. A decision as to when
to put MISR in full outgassing mode is pending.
The problem which caused the spacecraft controls computer (SCC) to halt last
Tuesday, 21 December, one minute prior to the winter solstice, is now well
understood. The timing is not a coincidence: it was directly related to the
sun's position in the sky at this particular point in time.
As sometimes happens when doing trigonometric calculations on computers,
slight mathematical inaccuracies can cause an invalid condition to occur.
In this case, the sine of one of the angles describing the solar position
(the right ascension) was slightly less than -1.0. Since the sine properly
can only be in the range -1.0 to 1.0, when the computer tried to take the
inverse sine it detected an error and halted, resulting in the spacecraft
taking care of itself by transferring control to a different computer and
going to safe mode. This particular software bug would show up two times
a year: winter solstice and summer solstice.
This past weekend, the situation which occurred on orbit was replicated to
the exact second using a ground computer. A fix is being developed, but because
the problem would not reoccur for another 6 months (if not fixed), the Terra
Project expects to bring the spacecraft out of safe mode some time this evening
or tomorrow morning and to implement the patch afterwards.
Another High Gain Antenna tracking error occurred on Christmas Eve, and although
Terra crossed paths with a reindeer-driven sleigh carrying a red-and-white
suited man, the two events are not believed related. Rather, it was associated
with another passage through the South Atlantic Anomaly, a location where
the Earth's magnetic field causes an increase in charged particle radiation.
Proton radiation susceptibility of a particular electronic component is
suspected, and testing of spare parts will take place to test this hypothesis.
The most likely solution is to work around the problem by uploading telemetry
monitoring ("TMON") software to Terra in order to automatically reset the
HGA gimbal if this condition occurs again.
You can see earlier status reports by checking the "News" link of the MISR
web site at
http://www-misr.jpl.nasa.gov.
Further information on Terra can be accessed at
http://terra.nasa.gov.
David Diner
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