Status 18 January 2000
The Terra spacecraft is flying in normal operating mode under control of the spacecraft
controls computer. The High Gain Antenna remains on and testing of the link to be used for
science data transmission, using small amounts of data from the MOPITT and CERES instruments,
is proceeding well.
So far it has not been possible to identify a single, definitive cause for the excessive
spacecraft roll that occurred during last week's engine firing. Among the leads is an
indication that a pair of thrusters which maintain roll orientation may be providing
less-than-expected output; however, the Terra Project Office believes that a combination
of other factors may also be involved.
With regard to how to proceed, three options (possibly in combination) are being discussed:
- Gather more data to characterize the attitude control thrusters in a continued attempt
to isolate the problem;
- Repeat the engineering and ascent burns using the redundant set of thrusters (these were
successfully test fired last Friday night);
- Break the orbit ascent sequence into a series of short burns (each about 30 seconds in
duration) to avoid inducing an attitude error that would abort the maneuvers. In this
scenario it would take perhaps three weeks to reach final orbit.
In order to achieve an orbit that follows the same path as Landsat 7, opportunities to begin
the ascent sequence occur every 10 days. The next available date to re-start the burn sequence
is January 21, but postponement until January 31 is possible in light of present uncertainties.
An updated schedule for MISR activation will be generated once we have a clearer picture of
the ascent sequence.
MISR remains in its outgassing mode with cameras and heaters on, and MOPITT and ASTER remain
in safe modes. MODIS has now turned off its outgassing heaters, while CERES has been continuing
its functional checkout, which has been going well. This past weekend, the redundant spacecraft
cooling system to support the ASTER thermal-infrared instrument experienced the same problem
that previously caused the primary system to shut down. The prevailing theory is that a small
vapor bubble can form as a result of the thermal "shock" of starting up the system and grows
to a point where it impairs operation of the cooling loop. A more graceful though operationally
more cumbersome start-up procedure has been demonstrated during ground test, and plans are to
initiate it today.
MISR continues its good behavior. A three-percent variation in the 120-Volt DC voltage
supplied by Terra is observed when the spacecraft cycles between the solar array and the
batteries as the source of power. (During the part of the 99-minute orbit that the Sun is
not illuminating the spacecraft, the solar array's photocells do not generate electricity
and the batteries take over.) MISR's instrument power supply contains transformers to convert
the input voltage to a 28-Volt DC output, along with regulators to maintain this output at a
very uniform level. A compensating increase in the current being drawn by MISR is observed at
the same time the voltage supplied by the spacecraft drops. This is good news and means that
MISR's power supply regulators are performing as expected. (Those who have taken a course
in electricity may recall that power = voltage x current, so if the current goes up by the
same proportion the voltage goes down, the output power remains constant.)
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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