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2003-09-25 - Terra SSR Anomaly Two Printed Wire Assemblies (PWA) in the Terra Solid State Recorder (SSR) have failed. The first failed September 24, 2003 (2003/267), at approximately 23:10 GMT. This PWA contains supersets #50 and #51 which are assigned as MODIS buffer locations. The second PWA failed in the morning of September 25, 2003 (2003/268), at 07:30 GMT. This second PWA contains supersets #114 and #115, which are assigned to the ASTER instrument.
The data being written to these supersets are not recoverable. The Terra FOT is in the process of analyzing diagnostic memory dumps to confirm the extent of the anomaly. Late on the 25th, the four failed supersets were moved to the "bad" list and will no longer have data written to them. This will reduce MODIS' total SSR buffer space from 36 supersets to 34 supersets and will result in a short data loss while the transition is made.
Note that MODIS started the mission with 35 supersets of SSR space. A similar PWA failure occurred in May of 2001 with the net result of MODIS gaining 1 superset (up to 36) when the SSR was recycled in June of 2001. It is believed that recycling the SSR will return all of the failed SSR supersets (currently 6) to working operation. However, the process to recycle the SSR requires approximately 4 hours to complete during which time the SSR will not be recording housekeeping or science data.
Other recovery options include "borrowing" supersets from other Terra instruments to "share" the burden of a smaller usable buffer size. A follow-up anomaly meeting will be held in the afternoon of September 25th at 4:00 EDT to discuss these options.
For more information, please contact Chad Salo. His contact information is listed in the Science Team Directory.
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