They have thrusters and "rudders" which are powered by batteries which are charged by solar. The solar will not provide enough power to keep them powering 24/7. Consequently they have to turn them off daily (I think) for a period to allow the batteries to top up. As a result they have periods of down time each day.
As for D - IMHO I think D is for Dead. I suspect they could have power issues as it shouldn't have taken this long just to upgrade software. Alternatively there might be some issues with communication. Same result.
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