Status, 10 January 2000
The manufacturer of the Terra High Gain Antenna (HGA), Spar Aerospace, has
completed laboratory testing of the transistors which they feared could get
damaged by the South Atlantic Anomaly (SAA). Spar sent 1 million pulses of
a few microseconds in duration through the devices--many, many more pulses
than would be seen during the normal mission life--and longer in individual
duration and with current levels about twice the value expected from proton
irradiation. The transistors survived these tests. Engineers at Goddard
performed similar tests with the same result. In light of this, normal
operations of the HGA will resume this Thursday, January 13, and the plan
is to keep the HGA powered on during passages through the SAA. In the event
of a gimbal drive reset, a telemetry monitor will automatically restart the
drive 7 minutes later. With experience, this time delay may be shortened.
If a reset interrupts a download of data, a command to repeat the download
will take place during the next contact through a TDRSS communications
satellite.
The anomaly in the propulsion system's power supply which occurred last
week was traced to the failure of a device whose purpose is to put a
resistive load on the power supply and keep it stable. There are four such
devices, and although spacecraft engineers predicted that things would work
fine with the loss of one of them, they recommended turning on a heater to
keep the load on the power supply at the nominal level. This was done, and
today's 11-second engineering test firing of Terra's propulsion system went
very smoothly! The first long-duration ascent burn remains scheduled for
tomorrow at 3:18 PM PST.
The ASTER thermal-infrared (TIR) imager was turned on yesterday, completing
the power-on of all science instruments (the other ASTER instruments, the
visible and shortwave-infrared imagers, were turned on last Friday; CERES,
MISR, MODIS, and MOPITT were all turned on earlier). Early this morning,
the cooling system provided by the spacecraft for the ASTER TIR experienced
an anomaly and shut down. The TIR itself was doing fine but was powered off
for protection since its temperatures started to increase. The cooling
system will be restarted tomorrow, and if necessary a backup system will be
called into action.
MISR's thermal engineer has completed a run of the computer model he built
to predict instrument temperatures. The model predicts the maximum
temperature of the optical bench in the current instrument configuration to
be in the range of about 27-30 degrees Centigrade. Actual telemetry is
showing the optical bench temperatures to be running about 27 degrees C at
one location and 29 degrees C at another, with variation from day to night
of about 1 degree C. What a nice result! All other engineering readings
continue to look great.
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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