This ended up not being a problem with the GPS system at all -- it was actually a certain brand of receivers not dealing correctly with leap seconds[1] -- or, s

This ended up not being a problem with the GPS system at all -- it was actually a certain brand of receivers not dealing correctly with leap seconds[1] -- or, specifically, that it's been a while since the last leap second, which caused one of the GPS parameters (basically 'how long since/until a leap second' to roll over, which is not normally a problem, but thanks to a receiver bug, it was, on these specific receivers. This was covered in some detail on the LEAPSECS mailing list.[2] [1] https://www.flyingmag.com/collins-receivers-struck-by-failure-modes/ https://www.flyingmag.com/collins-receivers-struck-by-failur... [2] https://pairlist6.pair.net/pipermail/leapsecs/2019-June/007150.html https://pairlist6.pair.net/pipermail/leapsecs/2019-June/0071...