December 20, 1999
Turn-on of the MISR instrument and unlatching of the cover and calibration
panels has been postponed until tomorrow morning due to an anomaly in a
monitoring circuit for the spacecraft High Gain Antenna (HGA). (This has
affected turn-on of all of the instruments, not just MISR.) The HGA antenna
has a monitoring system which keeps track of slew rates, and during a slew
activity associated with a TDRSS contact when the spacecraft was entering
the South Atlantic Anomaly (a region of high radiation) a bit got flipped
to a state that caused a shutdown of the HGA drive. The manufacturer, Spar
Aerospace in Canada, has been provided the data and are analyzing the
situation. Since the currents and other indicators of the HGA hardware
performance appear normal, it looks like the problem is not with the HGA
drive itself. It is not clear why the bit flipped, but given the location
in the orbit a single event upset associated with the radiation environment
is considered a distinct possibility. Reactivation of the HGA has not yet
been attempted while the data are being analyzed, but could happen this
evening or sometime tomorrow. If necessary, a number of workaround
scenarios exist, including specialized procedures for handling of the
monitoring circuit data. The HGA also has a redundant capability, though
implementation of that option would require reconfiguration of the
spacecraft flight software. There is also capability for direct downlink to
ground stations (at reduced efficiency); however, we are quite a long way
away from having to consider that scenario. I am told that this situation
is not considered an emergency and it is apparently not unusual to
encounter this type of occurrence during mission activations.
In the meanwhile, downlink of spacecraft engineering data using the
omnidirectional antenna through TDRSS has been working at better than
expected performance. The guaranteed capability of the omni is a 1 kbps
rate, however it has been working reliably at 16 kbps, the rate required
for the telemetry associated with instrument activations. It is on this
basis that MISR turn-on and unlatch is being planned for tomorrow morning,
using the omni antenna for the data downlink. This puts off beginning of
the outgas period to sometime on Wednesday.
Terra is in a 695 km x 655 km orbit, and trim maneuvers will occur over the
coming weeks to place us in the final orbit. At present, MISR survival
heaters are continuing to keep the instrument at comfortable temperatures,
at stable levels, and with significant margins.
David Diner
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