Status 06 February 2000
As of Thursday, February 3, MISR has been operating under a set of
commands that turns the cameras off on the orbit's night side and
turns them back on again in daylight. These "ATC" or "absolute time
commands" are uploaded to Terra ahead of time and are issued
automatically by the spacecraft every orbit. MISR's nighttime "naps"
last about 43 minutes within each 98-minute orbit. Earlier today,
during one of MISR's dayside "wake-ups", a number of camera
temperature indicators exceeded normal limits and the operations
center commanded MISR to "safe" mode. However, this was a false alarm
resulting from an idiosyncracy of MISR's engineering telemetry
polling system. Because it takes a finite time for the readings to
update, the data can appear to be incorrect for up to 24 seconds
following a change of instrument configuration. This afternoon MISR
was commanded back to normal operations and all telemetry looks fine.
Overall, MISR continues to behave very well. We have observed two
effects in the "dark" images that we attribute to enhanced charged
particle concentrations in the South Atlantic Anomaly (SAA). The
first consists of random bright points resulting from signals being
generated in individual detector elements, most likely by proton
radiation. These make the images look like a field of faint stars.
The other occurs as a result of each camera having a circuit that
stabilizes the overall background level from line to line. If a
charged particle event occurs at a particular time, the stabilization
circuit is fooled into thinking the overall background has brightened
and compensates by temporarily making the image lines slightly
darker. This effect looks like randomly placed horizontal stripes in
the images. These two phenomena have led one of our software
engineers to refer to the SAA effects as the "stars and stripes".
Though these features are obvious now, most of them will be swamped
by the vastly larger signal from earthlight once we open the cover.
Simulations of the Terra thrusters which take into account the
impingement of two thruster plumes on the solar array have
successfully replicated the behavior that was observed during the
aborted maneuver on January 11 and during last Wednesday's 20-second
test burns. The "thruster pairing matrix", or on-board logic which
determines which thrusters will be fired and at what time will be
modified to take into account this new understanding of the
propulsion system's behavior. In light of this, the "brute force"
plan to achieve final orbit using many short burns has been
abandoned; that approach has also been deemed risky because of the
large number of communications contacts with the spacecraft that it
requires. A sequence incorporating burns of duration up to five
minutes has now been developed. It begins on February 10 and ends on
February 24. We are planning to open MISR's cover on February 25.
Since launch on December 18, Terra has completed over 700 orbits of
the Earth, and has traveled a distance of more than 31 million
kilometers. A straight path of this length would cover about
three-fourths of the distance from Earth to Venus.
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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