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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.
        DLY 2130-2230
        1511172130-1511182230

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?

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This outage is due to department of defense GPS interference testing taking place 17-18 Nov 2015. The interference generated may make the GPS signal unreliable and the outage cone described by NOTAM lets you know that you can't trust the GPS signal during this time. This interference may be purposeful jamming or live testing of devices that may interfere with GPS to ascertain what level of interference occurs.

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    $\begingroup$ How did you come to this information? Is there something in the original text that tells you to go somewhere and look up something? $\endgroup$ – Steve Nov 17 '15 at 2:37
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    $\begingroup$ @Steve I just googled for gps scheduled outages. Pilots that use GPS, particularly if they are familiar with RAIM and GPS NOTAMS are probably knowledgeable about that. $\endgroup$ – casey Nov 17 '15 at 3:41
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    $\begingroup$ As a side note, non-pilots tend to believe that GPS is 100% available. As a pilot though, I've had in flight loss of GPS a couple times and even was unable to take a planned trip due to a combination of a GPS outage and no other suitable approaches for the reported weather conditions. $\endgroup$ – Brian Knoblauch Nov 17 '15 at 14:02
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    $\begingroup$ @BrianKnoblauch Anyone who uses GPS a lot in mountainous regions or other areas where satellite reception may be limited knows that GPS isn't always available. $\endgroup$ – JAB Nov 17 '15 at 14:23
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    $\begingroup$ As does anyone who's ever tried to use Google maps for directions in Manhattan.... $\endgroup$ – Jon Story Nov 18 '15 at 1:36

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