Status 02 February 2000
This is a lengthy report so here's the short version: Plans to
achieve final orbit for Terra are undergoing revision but are
expected to firm up soon. Meanwhile, MISR continues to do well in
science mode. The cameras, still operating with the cover closed,
have detected the South Atlantic Anomaly. Large amounts of MISR data
are flowing and giving our data processing system a good workout. For
more details, read on!
Two 20-second engineering burns of the Terra thrusters were
successfully performed today. Spacecraft engineers suspected that a
contributing factor to the anomalous roll that occurred on January 11
was that the plume from one of the thrusters was impinging on the
solar array and providing an additional torque (rotational force) on
the spacecraft. Today's results confirmed predictions that take this
effect into account, and the behavior of the spacecraft controller
that directs the thruster activity is now more readily understood.
Factoring in this finding with a slip in the start of the ascent plan
announced last week means that final orbit might not be reached
before February 23. However, a more aggressive schedule which aims
for an earlier date has not yet been ruled out.
The MISR instrument continues to perform well. When we open the cover
(soon!) and view the Earth, each picture element, or "pixel", in the
charge-coupled device (CCD) detectors used in the cameras will
generate a signal (essentially a flow of electrons) that is
proportional to the amount of light falling upon it during each
exposure--putting all these pixels together is how a complete picture
is formed. CCD's are used in commercial digital cameras and
camcorders, but the ones used in MISR are many, many times more
sensitive and accurate. Other things besides light energy can cause
CCD's to generate a signal, however, such as thermal energy (this is
why we cool them) or the presence of electrically charged particles
such as protons. Many of these charged particles come from the Sun,
and the South Atlantic Anomaly (SAA) and the Van Allen Radiation
Belts are locations where variations in the Earth's magnetic field
cause these particles to become "trapped". In addition to being the
source of the occasional Terra High Gain Antenna resets, we have
found a definitive correlation between passage through the South
Atlantic Anomaly and the frequency of random "hits", or fleeting
bright points, in the MISR "dark" images. (The Terra star trackers,
and many other space instruments, have also seen such an effect.) For
MISR, the proportion of hits is small but noticeable, varying from
about 1 per million pixels outside of the SAA to about 1 per thousand
pixels inside the SAA. This means that even in the worst case, 99.9%
of the data are unaffected, and most of the time the situation is a
thousand times better. Portions of our ground data processing
software already contain procedures that will "filter out" these
random hits, and we are contemplating additional possibilities.
Enough MISR data to fill about 35 CD-ROM's arrived in the first 24
hours after the start of "global" science mode, and data continue to
flow into the Atmospheric Sciences Data Center and to JPL. Things are
proceeding well, though the volume of data has brought to light some
subtle pathologies not encountered during pre-launch testing. Our
team is working diligently to address these as they are discovered.
Most issues that have arisen are associated with handling non-typical
situations, such as data gaps, missing spacecraft navigation or
attitude information, and compensation for occasional instrument
errors. A major focus at the moment is the software that turns the
stream of bits sent out by the cameras into pictures, using
information generated by MISR's flight computer to sort the data by
camera, color, and location across the swath and within the orbit.
Checkout of the software that combines spacecraft navigation data,
camera geometric orientations, and solar position to compute
illumination and view angle information is also underway. These
supplementary data are essential for interpreting the MISR images.
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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