6 March 2000
We believe that the reason one of MISR's cameras (named Aa because it uses
the "A" lens design with a focal length of 59 mm and points in the aftward
direction) has been showing low signal levels is because of too short a
delay between the command to turn it on (which happens every orbit) and the
subsequent command to set its exposure times. During instrument testing
three years ago, we observed this in several cameras (including Aa) and modified
the flight software to increase the delay to 1 second. We suspect that in this
particular camera the electronic component which processes the exposure
time command is taking a bit longer to power up. Last week we successfully
demonstrated that when the exposure time command is reissued several seconds later
the camera responds normally. As of last Thursday afternoon, the command sequence
provided by the spacecraft to MISR contains an additional instruction to reset the
exposure times for this camera after each turn-on, and Aa has happily joined its
siblings in sending back images at the proper signal levels. As my mom describes
it, we've got a good litter with one runt that just needed a little coaxing.
Since opening MISR's cover, we have acquired high quality calibration data in addition
to the wonderful Earth imagery. Calibration involves deploying and stowing two panels
which reflect diffuse sunlight into the cameras, and we are carefully monitoring the
electrical currents associated with the panels' motors. Some "speed bumps" in the
current readings are seen which indicate that additional torque (rotational force)
above the average level is required for isolated short intervals. From the timing of
the events, we suspect that some insulation blankets have "puffed out" slightly in the
vacuum of space (since there is no atmospheric pressure to compress them) and some
grazing is occurring during the panels' motion. Since the additional torque requirement
is well within the capability of the motors, and analysis of the current readings implies
that no snagging or binding is occurring, we believe it is safe to continue using these
panels. However, prudence dictates that before doing so we try to understand this behavior
better, so this week we will attempt replicating the situation on our engineering model.
Since there is some possibility that over the long term the polymeric material of the
blankets could wear, we are also adjusting our calibration strategy to minimize the number
of times we need to cycle the panels.
Early last week when the cameras remained off for a few orbits a set of survival
heaters turned on. Later in the week we took some actions that caused MISR's
cameras to remain on continuously for a few orbits, and this produced
sufficient warmth to turn the survival heaters off. The optical bench is
once again at the the desired temperature of 5 degrees Centigrade.
Last Friday, a successful underflight of MISR by AirMISR occurred over the Department
of Energy Atmospheric Radiation Measurement field site in Oklahoma, and along with field
measurements acquired by other instrumentation this will provide extremely valuable data
for verifying cloud properties inferred from MISR data. Later this week we hope to observe
the same site under clear skies. Congratulations to the AirMISR and field experiment teams!
You can see earlier status reports by checking the "News" link of the MISR
web site at http://www-misr.jpl.nasa.gov.
David Diner
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