NavList:

Roger, you wrote of the possibility of:
"... the intentional degradation of GPS accuracy when accessed by civilians.  I imagine they might want to do this well before an all-out war broke out, when the prospect was looming.  They could also, I’m sure, degrade GPS much more than was done routinely before 2000, or even turn it off completely (except to the U.S. military itself)."

I really think the day has passed when that logic was valid. It just doesn't make sense anymore...

Coincidentally, as I read your message, the time rang out on my quaint key-wound mechanical mantle clock, and it occured to me that this is 0h UT (8:00pm EDT). The Greenwich date has just rolled over to September 11, 2018. Seventeen years ago, during and after the unthinkable reality of the September 11 attacks, the GPS signals available to civilian users were never once degraded. So simple question: if not then, when? 

Although the US military continues to have formal control over GPS (a questionable status), they are well aware of the enormous value of the system outside military uses. There are at least three billion GPS chipsets in use around the world. Nearly all Americans depend on GPS daily in some fashion from trivial to mission-critical. If the September 11 attacks were not sufficient cause to shut it down or degrade the signals, then what would be sufficient cause?

As if that weren't enough, a bigger and better argument against Pentagon degradation of GPS during a ramp-up to war is the presence of alternative GNSS signals which are not under US control. Why turn off GPS if an adversary can instantly and automatically switch to Glonass or Galileo or Beidou?? The cheapest consumer positioning solutions already access redundant GNSS constellations. For at least six years, nearly all smartphones have included Glonass (Russian) GNSS compatibility as a standard feature, at no extra cost, requiring no special apps, and seamlessly integrated with GPS signals. Starting last year, most new smartphones have been equipped with chipsets that can process GPS, Glonass, Galileo, and Beidou signals wth the same seamless ease. If some cabal at the Pentagon decided in a fit of panic to turn off GPS, what would be the effect? No one would care, except those US government users (especially military) who are required by law to employ GPS only. It has become increasingly difficult to imagine even an extreme scenario --outside of a fit of insanity or an incompetent order from above-- where the the US GPS signals would be turned off with a reasonable expectation of achieving military advantage. The US military would be shooting itself in the foot. Worse yet, it would look foolish and incompetent for the act. 

Frank Reed