NavList:

Brad, you wrote:
"I know the public statement is that the US will not use SA, but those satellites are under direct control of the US Air Force.  It would be very easy to command an equivalent to SA."

But again, how would this play out given that there is one fully-operational alternative GNSS available, and there are two others on the verge of being fully-operational. Suppose a DOD source calls up the GPS command at the Air Force and insists in a panic that they turn off the GPS satellites. What would the effect be? I would estimate that roughly 95% of users would see only a slight drop in accuracy in their position fixes since they would automatically, seamlessly, and without interruption continue to receive and process position information from the other GNSS constellations. The actual number may well be higher than 95% but that depends on how many systems are using chipsets manufactured before 2012 or so, and it depends on how many systems would not encounter software bugs when "Old Reliable" is missing, and it depends on how many US govt users are required to use devices that only access GPS signals. So even if GPS were turned off or placed in a mode equivalent to SA, the outcome for navigation would be minor --except for the US military itself!

What would happen if the system were hacked to produce highly inaccurate data? From my limited understanding of the system, standard software (hard-coded into the chips) will ignore outlier signals, just as any prudent celestial navigator would ignore a wildly out-of-line altitude observation, and once again most receivers would fall back on the remaining GNSS signals. Yet another hacking mode could produce an effect similar to GPS Spoofing yielding a fixed but otherwise consisent (and seemingly accurate) position offset at least in some part of the globe. That signal offset, combined with signals from other GNSS satellites could yield a real degradation of position fixes unless software is designed to detect such a thing (it could, but I highly doubt that this is currently available in off-the-shelf tools), but then we're into the territory of GPS Spoofing generally. And as I have said --and have emphasized in my apps-- this is a case where celestial can still have an impact. I don't mean that we should then fall back on 1970s celestial navigation with paper tables and plotting but simple comparison between expected altitudes and observed altitudes. And note, too, that well-designed GPS Spoofing can spoof the signals from all GNSS satellite constellations simultaneously, which is far more dangerous than anything that the USAF could do by manipulating the GPS signals only. 

Pirates with spoofing tech are a big risk. Degradation of GPS signals by the USAF simply wouldn't accomplish much. It's possible in panic-mode, but probably irrelevant. Does that make sense?

Frank Reed