First: I'm not a real pilot. I used an on-line flight planning tool, and in the brief was the following "GPS NOTAM":
GPS 11/032 ZSE NAV (CHLK GPS 15-15) GPS (INCLUDING WAAS, GBAS, AND
ADSB)MAY NOT BE AVAILABLE WITHIN A 476NM RADIUS CENTERED AT
360822N1173846W (BTY214059) FL400-UNL DECREASING IN AREA WITH A
DECREASE IN ALTITUDE DEFINED AS:
432NM RADIUS AT FL250,
375NM RADIUS AT 10000FT,
340NM RADIUS AT 4000FT AGL,
253NM RADIUS AT 50FT AGL.
I must admit I don't understand most of this, but it seems like GPS is unavailable in what seems to be a huge inverted cone of space. What's going on here?