Dec. 27, 1999
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