I recall the 2018 US government shutdown delayed updating GPS data. How fragile are our global systems?
There are several other constellations, even if GPS is currently the biggest one. One problem, which I do not know how easy it is to overcome, is that some signals are deliberately encrypted or degraded so that it is not straightforward to get precise positioning information using a consumer-grade device.
For the US GPS system, Selective availability was turned off in Y2K (after a 1997 decision to do so). In 2007 it was announced that any new satellites would not include the SA feature at all.
Note that differential GPS can improve GPS position accuracy to as good as just a few centimeters. This requires local ground-based reference stations, meaning it’s not useful for the open ocean, but is useful for coastlines and harbors. This would mean you could get accurate motion-based headings using position-based velocity/heading measurements even at very low speeds (such as when traversing a harbor or crowded waterway, where you need it most).
Hostile entities can really mess up the system in areas they have the ability to set up their own transmitters. These entities do not have to be state actors.
E.g., there’s been some funny business in some Chinese harbors where ships notice navigational deviations that cause near collisions. Among the suspects are illegal sand dredgers trying to hide that they are are hauling up sand from unpermitted locations.
Russia has also been messing with GPS networks in regions outside it’s borders in Europe.
One should assume that in any situation where the stuff gets real, GPS from all networks will be unreliable.
One thing to keep in mind in terms of GPS accuracy is that there is absolute and relative accuracy. I.e., getting to the centimeter where you are on the globe is much harder than determining how many centimeters you’ve moved between two points.
The velocity via motion method fortunately uses the later.
Which is why differential GPS, with known, fixed base stations, is so useful.
But really, on the smallest scales (in distance and time), accelerometers are probably more useful than satellite systems, anyway.