I wanted the dust to settle on this one prior to responding.
The US has made it quite clear, in the past, that an attack on our satellites, of any kind, is considered to be an act of war.
As to NK's ability to detonate an exoatmosheric weapon, that is undisputed, but one, or a few, such acts would only degrade the GNSS system, not destroy it. There is sufficient redundancy in the system to permit continued functionality. NK would need to be able to remove nearly all of the 30 satellites to destroy the system. Further, they would need to prevent the launch of new satellites, which restores capability. I would suggest that is far beyond NK's capability at this juncture. Kim may be crazy, but he isn't suicidal. Use of nuclear weapons by NK is going to have severe repercussions.
Selective Availability, in my view, only degraded the accuracy of the system to the extent that it would prohibit using the GPS system for targeting. It would still be somewhat useful in making landfall and port, albeit no longer useful for correct time and precision positioning. From my understanding, SA was a time smearing for each satellite in the constellation. When these time smeared broadcast values were received, they produce a position "close to" but not a position required at the level for targeting.
For a mariner, when making landfall, the AtoNs would be out of place, but not so far out as to be invisible on your radar. After 2 AtoNs showed up, in combination with a land return, a navigator would likely discount the chartplotter and never question the satellite system. Of course, SA is going to cause havoc on other systems, like GPS guided, turn by turn auto navigation or any GPS time based architecture. In my view, however, SA isn't going to affect mariners to the extent that Celestial Navigation becomes a better position finding tool. SA is a degradation to accuracy, not an on/off switch. CN will not be a backup for GPS, even when SA is applied.
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. Because we would never engage in a disinformation campaign when critical assets are in place, right ;)
I do appreciate your use of CN as stated here (and in other recent threads). I'm pleased that CN hasn't passed completely to an antiquarian or a hobbyist activity. It is on life support, but reports of its demise are exaggerated!
In posting this thread, I wanted to see if any of the illuminati would take up the cudgel for CN as a backup to GPS. To date, your response alone shows some measure of that, yet as you yourself stated, the backup to a GPS is another GPS, and that having battery powered GPS receivers was a wise idea.
I agree, the most likely cause of systemic GNSS failure would be war. I Would add North Korea to the list however. Firing off a high-altitude skyburst with a nuclear device would destroy a large number of satellites. In this case GNSS and comms would be drastically affected. However, I still think the overall likelyhood of this event is quite low.
Selective availability is something I considered (I remember those days), but Frank is correct, the use of other constellations would probably still provide enough precision for sea-navigation. As an interesting aside, I was was anchored off of Israel a few years ago and EVERY GPS receiver on the ship failed due to position loss, simultaneously. I pulled out my battery GNSS that used alternative constellations, and I had a position.
Since my navigation isn't really for fun, I take to heart the adage "never rely on a single source for fixing position." In coastal waters I take bearings and constantly watch radar ranges (via Parallel Index) to ensure I am where the GNSS/ECDIS says I am. Even now as I write, my mates use radar range and bearings, logged every hour, at anchor, to confirm the ECDIS position. I still teach and personally shoot stars periodically from the bridge underway to let me sleep easier at night.
My final thought is that GPS has not replaced one aspect of CN: compass error checks. Sure you can use various methods on the coast, but out at sea, the ONLY way to check error is by using celestial navigation. GNSS is great for telling you COG, but not so good with true heading. This point was recently driven home for me as I recently bought a doppler RDF array at home, and it has a plug in GNSS. It claims to be able to plot bearings on Google Earth, but only when you're moving...
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