[michaelt]

Perhaps in the cold war era.

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