On Thursday, May 10, 2018 at 6:08:15 PM UTC-7, wrote:
GPS testing is done from several military bases and that testing can and does involve skewing of signals, denial of service or anything else that may be deemed necessary in the National interest or to evaluate the impact on a battlefield, where enemy combatants may be using US assets to inflict damage on the US or our allies.

Remember that this is a service provided at no charge (except to the US taxpayer) for a benefit that is utilized worldwide. There are other systems (Google Glonass) but the most prevalent and capable system is still the US Air Force administered constellation of satellites that provides reasonably dependable and accurate position data to civilian air traffic, both commercial and private, to the point that airliners and other Instrument rated pilots and aircraft routinely use these data and allow for "GPS Approaches" to the centerline of the runway.

In times of war or other national emergencies, the civilian and/or commercial service may be disrupted or canceled completely either worldwide or in specific areas, while leaving the US military in a situation whereby they can still utilize the signals. GPS jamming is possible by anything from a concerted effort by opposing forces to truckers (or anyone else) on the highway who purchase a cheap signal blocking device to mask their behavior on the road from the home company or other signals analysis entities. (You can buy GPS blockers on eBay.)

I remember a notable presentation at Oshkosh several years ago entitled, "GPS Stole My Brain." Total reliance on this system with no actual physical and mental map reading skill is foolhardy.

As far as people (users) being "inconvenienced" or (administrators) dealing with a "career limiting event," I will side with the US soldier every time.

We deal with GPS testing on a fairly regular basis from Holloman Air Force Base and the White Sands Missile Range. The schedule for testing, the approximated affected area and altitudes are published well in advance and are available from a number of websites. I check the schedule, just I check for NOTAMs and TFRs. I suggest you do the same.

During the second Clinton administration, the Selective Availability was ended by executive action. If the military doesn't want you to have GPS they jam an area instead of turn the entire GPS network off or to selective availability. Also, as the boys flying the Sentinel over Iran found out GPS signals can be spoofed. Many military weapons systems still use inertial nav only checked occasionally against GPS.