https://phys.org/news/2018-11-grace-fo-resumes.html > Both GRACE-FO satellites are equipped with redundant systems in their MWIs, which are their primary measurement instruments. The MWIs very precisely measure the distance change between the two satellites. > The switchover to the backup system was required after an anomaly occurred in a component of the primary system. The primary unit was powered down on July 19, when an instrument fault monitor detected that it was using less current than expected. After a full investigation by an anomaly response team, the mission team began a series of procedures required to switch over to the new unit. The backup system in the MWI was powered up on Oct. 19.