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CERES +15V Power Converter Anomaly



Bruce Wielicki and Phil Hess

October 14, 1998

  • The MAM Assembly temperature rose to a peak of about 17.1C. The start of the DAA +15 volt converter's sensitivity to temperature occurred at about 16.4C. The voltage followed the MAM Assembly temperature above this minimum level with the same sensitivity seen in August data. The current Tmin value is about 0.8C lower than seen at the end of August, and indicates some further degradation of the DAA +15V converter, but only about 14% of the degradation rate seen through most of August. This again indicates that the degradation of the converter has slowed greatly from that experienced in August (roughly a drop of 0.13C per day in Tmin from Aug 3 to Aug 29).

  • Tommorrow the instrument will be operated in a higher temperature orientation (greater sunlight incident on the DAA electronics box) and we expect roughly 2C higher temperatures to be reached. Voltages are expected to still be well below our new limit of 18V. Given today's data and an expected 2C rise in MAM assembly temperatures, we might see maximum voltages of around 16.6V. The instrument is now set to go to safe mode if the voltage exceeds 18V, even during periods when the spacecraft is not in direct communication with the operations center.

  • Over the next 6 days the instrument will be operated in a variety of modes designed to isolate the impacts of individual instrument functions (elevation scan motor, azimuth scan motor, SWICS calibration lamp, etc), culminating with the full RAPS scanning mode on day 6 of the test. The tests will include 2 calibration sequences to verify the calibration stability over the last 6 weeks.

  • This next 6 days should also help to determine if further degradation in Tmin during instrument operation occurs in Tmin at the rate of 0.13C/day (August) or closer to the 0.02C/day since then.

  • The latest quick-look data for the last three orbits have revealed that the DAA+15-volt converter is now starting to show expected variations. Some observations: (provided by Phil Hess)
    1. The voltage is essentually following the MAM TOT Assy temperature cycle as has occurred prior to the September 1 shutdown.
    2. The start of the voltage variations (i.e., "lose" regulation) for each of the three orbital temperature peaks occurred as the MAM TOT Assy temperature increased through ~16.5 degrees C. The end of the voltage variation cycle (i.e., "regain" regulation) occur as the temperature drops below ~16.4 degrees C.
    3. Some approximate peak data values for these three orbits (time is GMT):

      Time DAA+15-volt MAM Temperature
      13:0615.0916.7
      14:3715.3717.1
      16:0815.3917.1

    4. There is a noticable positive "jump" of ~0.1 volts at sunrise (coincident with a "sharp" ECA Radiator Temperature peak). This seems to be in the opposite direction that we've seen prior to September shutdown. (There the "jump" was in the negative direction, not the positive direction.) We'll be looking into this in further detailed, so I wouldn't want to draw any conclusions about this just yet.
    5. Based on the MAM data, it would suggest that the diurnal peak has passed. We will know when the next quick-look data arrive.


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