Regarding your concern vis debris clouds damaging the constellation of GPS satellites.
The approximate altitude of GPS satellites is 20,200 km. From this chart ( https://upload.wikimedia.org/wikipedia/commons/4/49/Spacedebris_small.png
) we can observe that the current spatial density of objects greater than 1 mm in size, at that altitude, is less than 1 part in 10,000,000. That density is arguably the lowest density for any altitude (or close to it). Its fairly empty space at that altitude. Where is that debris cloud you mentioned coming from? Certainly not from existing debris.
Remember that in order to force your hand to CN, a significant percentage of the satellites must be eliminated. Under the assumption that only a few satellites are damaged, we have simply degraded our fix by (1) less accurate position due to less satellite-time intersections (2) reduced update rates, since there may be insufficient satellites above the horizon to get a fix at the time of your demand for position.
Neither of these degradations will force you into a CN regime, as the accuracy normally associated with CN is multiple nautical miles and the update rate is twice a day. Hardly a competitor to GPS.
That is, unless the debris cloud (that isn't there now) takes out nearly ALL of the GPS satellites, nearly simultaneously. Then, there is an argument for CN as the primary navigational method. Probability of occurring? Very, very close to zero.
PS as a courtesy, the chart may be found on this page
Thanks for the fact check (and splitting my opening post).
It is unclear to me why middle (or, for that matter, geosychronous) orbits would be immune to the effects of collisions resulting in a debris cloud. I would agree that it is farfetched to think that a LEO collision would result in enough pieces with sufficient energy to pose a risk to higher orbits. But a collision could occur in a middle or geosynchronous orbit, leading to these orbits becoming unusable.
In any case, there are other plausible threats to the GNSS constellations, as you point out.
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