Background:

The guide telescope for TRACE is used for attitude control while in Fine Sunpoint Mode. Contained within the guide telescope are 2 sets of 4 photodiodes (1 primary, 1 redundant set) that provide the means for fine sun acquisition to the accuracy required 20 arcseconds. The diodes are situated on the positive and negative X and Z axes.

If one of the diodes were to fail to a high positive or zero value, the ACS would likely slew to zero out the resulting error signal created by the good diode. The result of a failed high value would likely cause the spacecraft to move off the sun, attempting to zero out the high signal, until the ACS Failure Detection and Handling caused a transition to Inertial Sunpoint (3° sun angle violation). If the diode failed to zero, the ACS would slew to null the error signal, probably moving the good diode off the sun and leave the spacecraft still in Fine Sunpoint, but skewed to the edge of the sun.

Key Telemetry:

The diode signals can be found on the "guide" or "inst_ov" pages. The mnemonics are IUISDIO1-4 and the values will be in counts. When the sun is centered on the photodiodes, the signals will not differ much between the 4 diodes with opposite pairs balanced. Values between 130-100 are nominal depending on the time of year.

Recovery Procedure:

Recovery will be performed after consultation with an ACS representative and EOF personnel. The redundant set of diodes will have to be used and can be commanded with the command ICISRDIO. There should be no danger in switching to the redundant set while the ACS is in FSP. A command to FSP will be needed if the spacecraft has transitioned to ISP.

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