Bruce A. Wielicki
October 28, 1998
The Good News:
- As reported earlier, there was only a small degradation of the +15V DAA voltage converter during the
6 weeks the instrument was turned off: roughly September 1 - October 13. If the degradation was due
to radiation exposure in space, then that damage should continue independent of the instrument operation,
and the small degradation over the 6 weeks would have indicated approach of an asymptotic limit: similar
to radiation damage over time of optical components. However, upon restoring operational power, it was
observed the degradation continued at a greater rate than when powered off, therefore the asymptotic
point for the degradation has not been reached. More information concerning the degradation is given below.
- An internal calibration was performed during October and showed that the calibration of the CERES instrument
remains unchanged (at the 0.2% noise level of an individual internal calibration). In analysis of the
August and October data we see no evidence of radiometric changes in the instrument at the higher voltage
levels of the DAA +15V.
- The instrument operated normally in a wide range of test modes until the orbit geometry increased temperatures
and voltage on the DAA +15V to a level of 18V where the instrument automatically went into safe mode
and operational power was removed. This automatic turn off of the instrument at a prescribed voltage
was a new operation programmed into the TRMM on-orbit computer system and worked as planned. The power
down occurred on day 5 of the test period.
- If the voltage converter becomes totally deregulated, then it will pass roughly 23 to 29V (caused by
variations in spacecraft bus voltage) to the 7 electronics components supplied by the converter. All
of these components are currently under test at 30V. The component thought to be most sensitive, the
analog to digital converter has been successfully run at 30V for roughly 4 weeks so far and that test
will continue. Two entire sets of the 7 electrical components have also begun a long-term test and have
operated successfully at 30V for 1 to 2 weeks. A third set of components may also enter the test if
sufficient spare parts at TRW are available. It is not considered meaningful to test electrical components
that are not from the same manufacturing lot.
Now the Bad News:
- There is evidence from the 5 day test that the voltage converter may continue to degrade at the rate
seen in August when the instrument is operated. Further analysis is needed to tighten up the relationship
of voltage to temperature in order to get more accurate determination of the minimum temperature of the
converter sensitivity and its time history. Work is also underway to theoretically model the converter
physics and simulate the behavior seen so far. If in fact the degradation continues at the August rate
for every day of further instrument operation, then the voltage converter may become completely deregulated
in roughly 100 days of instrument operation. Total lifetime of the instrument would then depend on how
long the 7 electrical components could last at 23-29V instead of the designed 15V. From data so far,
there do not appear to be any calibration changes at elevated voltages, so the concern is instrument
lifetime and maximizing the science return.
What are the Candidates to Explain the Problem?
- The Tiger team is focusing on two primary candidates:
- migration of conducting silver across an insulating epoxy layer, thereby causing a short in an LED, and
- a cracked resistor that is degrading with thermal cycling.
- Both of these potential problems can affect the voltage converter's output regulation feedback mechanism.
Modeling studies are underway to test which hypothesis better explains the data.
What about the other CERES instruments for EOS-AM and PM?
- The current plan is for the EOS-AM and PM instruments to have the voltage converters replaced.
Newer versions of these voltage converters eliminate the insulating epoxy layer, and would eliminate
any problems with silver migration. If the problem is a cracked resistor, it is a random part failure,
and as such is rare and cannot be predicted. Note that this is only the second of roughly 6000 of these
parts to show an anomaly.
What about overlap of the CERES data record with SCARAB and the EOS-AM CERES instruments?
- SCARAB is now starting to get some data on the Russian RESOURC platform in a morning sunsynch orbit.
Once they are routinely gathering data, we will pick a period of 1 or 2 days when the TRMM local time
of day at the equator nearly matches that of SCARAB (10 am), align the CERES scan plane with the SCARAB
scan plane (the orbital inclinations are very different) in order to maximize the intercalibration
opportunities with matched time, space, solar zenith, and viewing zenith/azimuth.
- We will make sure that we can get overlap of the TRMM CERES instrument with EOS-AM. What this means
for the future operational schedule of the instrument depends on the results of the Tiger team studies,
and on the experience with running the 7 electrical components for longer time periods at 30V.
What about the upcoming meteor shower in mid-November?
- The instrument will be placed in the stowed position.
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