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Laser fading seems to be a known issue with ring laser gyros. With their STEREO-B probe, NASA experienced a different, "unexpected" failure mode, giving false readings. The Honeywell MIMU's were well beyond the expected lifetime but still, there may be some lessons to be learned from this. Having a second IMU in hot standby increases redundancy in critical mission phases. During extended operations however, any spare unit that ages may better be sent into cold standby.

  1. Do ring laser gyros age (in deep space environment) with laser and dithering disabled?
  2. Were laser and dither disabled on the spare unit for the STEREO mission?
  3. Can the failure mode of STEREO-B's IMU's be attributed to the dithering mechanism?

Some context to the question:

NASA's STEREO-B is in uncontrolled spin (as of Oct 2016). Efforts to regain control are ongoing following a telemetry downlink in August 2016 after two years of communication break. Control of the spacecraft was lost on Oct. 1 2014 due to an unlucky double failure during a planned reset sequence in preparation to solar conjunction (star tracker timeout + IMU fail).

The sequence of events is analyzed in this presentation, prepared by mission specialists at John Hopkins Lab.

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