So a localized outage can in fact affect the entire planet. This has also implications in disaster scenarios.
Not sure how applicable this is for GPS.
Of course, if the ground station failure was a soft failure, and instead of ceasing to upload, it began uploading incorrect data, the GPS location results would be arbitrarily bad to unusable. It's not clear to me whether the Galileo outage was because of a hard ground station failure or a soft one. But given that one of the contributing factors was that the backup system was not online, it would indicate to me that this was a hard failure that GPS would have been able to correct.
I see now that the US gets to take GPS (and the fact that the Air Force pretty much runs it single-handedly) for granted. We are seeing _some_ problems (like the GPS III block upgrade and OCX, ahah), but the org chart seems to be considerably simpler than what Galileo's got to deal with.
As far as I can tell, Galileo has nothing like this. They have a small buffer of future ephemeris data, but the ground segment failure here was so long that it completely ran out. Their approach seems to be full accuracy or why even bother.
Because of that allocation, the FCC cannot guarantee that Galileo HAS receivers will not be interfered with.
In the current age, where GPS can be post-processed precisely enough to measure things like land subsidence, there are networks of base stations at fixed locations recording GPS observables and providing historical archives [1] - some operated by US government agencies, some by private companies, some by universities and suchlike.
Due to the use of GPS for things like aircraft navigation, the government also issues 'NANUs' ('Notice Advisory to Navstar Users') and provides an online archive of them [2]. So we can say with some certainty that GPS SATELLITE SVN31 (PRN31) WAS UNUSABLE ON JDAY 349 (15 DEC 97) BEGINNING 1300 ZULU UNTIL JDAY 349 (15 DEC 97) ENDING 2200 ZULU.
Someone who wanted to find the largest recent GPS outage could download the archived NANUs and parse them.
[1] https://cddis.nasa.gov/Data_and_Derived_Products/GNSS/RINEX_... [2] https://navcen.uscg.gov/?Do=gpsArchives&exten=txt
I don't have a ton of knowledge into the deep technical aspects to GPS, but I imagine we would probably have some similar clues of an outage of this scale in GPS. Maybe a little less technical details of what is happening behind the scenes, but knowing a high percentage of satellites entering a no guarantee precision mode should be possible.
