Status 17 February 2000
Yesterday's 320-second thruster firing went well. In consultation
with the instrument teams, the Terra Project has decided that an
additional maneuver to modify the spacecraft's equator-crossing time
will not take place until late March or April--if at all. This means
that we are back on track to open MISR's cover on February 25.
The mystery of the "jail bars" in the MISR "dark" images has now been
solved. These faint, uniformly-spaced vertical stripes show up at
camera turn-on each orbit, progressively get narrower, and finally
disappear after about 20 minutes. After camera turn-on we also see a
rise in the instrument electrical current in excess of what the
cameras draw, and this "extra" current peaks and then decreases over
a period of 20 minutes. Furthermore, for the first few orbits after
we started day/night cycling, this current spike was absent. So were
the "jail bars".
We had discovered a "smoking gun". But who pulled the trigger? And why?
The aluminum optical bench which holds the cameras is regulated to a
constant temperature by heaters that "pulse" on and off at fixed
intervals. The pulse width increases or decreases as more or less
heating is required. When we started cycling the cameras off on the
night side, it took several orbits for the bench to "feel" the loss
of this source of heat, so to make up the difference the heaters
began generating wider pulses and drawing more current. As the bench
warms up in daylight, the pulses get narrower and the current draw
decreases. Because the heaters use a lot of power, we believe that
imperfect electrical isolation permits a tiny "cross-talk" of their
pulses into the camera video signal. And if decreasing-width pulses
cause features at the same place every image line, and one line is
stacked on top of another, what will the pictures look like? Vertical
stripes that taper down, then disappear--exactly what we see.
This evidence was compelling but we wanted proof. So yesterday
morning we commanded MISR's optical bench heaters to turn off for
five minutes. Subsequent analysis of the camera data confirms that
the suspect was snared: During this interval the bars disappeared. We
are now confident that their faint but beguiling signal will be
"drowned out" by the much brighter earthlight once the cover opens,
and corrective actions are probably unnecessary. However, since the
illumination levels near the Earth's poles is low, we want to
carefully examine that data and are leaving open the possibility of
modifying how the optical bench heaters are used.
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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