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The following from Space News indicate that a laser gyroscope failed and that software changes were developed and uploaded to the spacecraft bringing it back on line again recently:

I"m wondering for example if DSCOVR had three axes of gyroscopic measurement and if it had enough star cameras to provide the same functionality in principle, and how the cameras could be used in situations where the angular rate was significant. Gyros and star cameras complement each other, the former works well for instantaneous rate but sampling and noise issues make absolute attitude a problem, whereas star cameras are good for absolute attitude but may have issues if the spacecraft is rotating.

Question: How was DSCOVR fixed? What software changes were implemented to allow sufficiently dynamic attitude determination from star cameras? What kinds of algorithms were added to make this possible? Is this described in any detail somewhere?

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    $\begingroup$ I think, though I would need to dig for sure, the bus started with 4 gyros and they lost one. The "fix" was to upload new spacecraft attitude and control command tables that only relied on the remaining gyros. I think it primarily relies on a sun sensor, but again, I'd have to dig to figure out what precisely happened/changed. I do know it was a software "fix." $\endgroup$ – honeste_vivere Feb 22 at 23:04
  • $\begingroup$ @honeste_vivere I think even a partial answer posted will be helpful and may stimulate even more activity. Thanks! $\endgroup$ – uhoh Feb 23 at 2:55
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How was DSCOVR fixed? What software changes were implemented to allow sufficiently dynamic attitude determination from star cameras? What kinds of algorithms were added to make this possible?

The DSCOVR mission did suffer a "pointing hardware issue" in late June 2019, according to the 2020 NASA Earth Science Senior Review Report, pdf at: https://science.nasa.gov/science-red/s3fs-public/atoms/files/2020-NASA-ESSR-FullReportFINAL.pdf

Technically, the gyros had not failed but were nearing the end of their expected lifetimes so the spacecraft was intentionally placed in a safe mode. New software was uploaded and tested to rely only on star trackers alone. The spacecraft was brought back to full operations in early March 2020. Also according to the Earth Science Senior Review Report, DSCOVR appears to have enough fuel for at least 6 years from summer 2020.

Is this described in any detail somewhere?

I am guessing the DSCOVR Senior Review Proposal would have this information but I am not sure if it is yet public.

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