They are. They do not use GPS. They use old school navigation systems based on internal dead reckoning principles with no dependencies on exterior signals. "Fly for 18 minutes at such and such a heading (established by internal gyroscopes), at such and such an angle" Type of thing.
Portable tactical nukes are the same way. Their exact position on the ground is established with theodolites, sighted off things like local mountain peaks with known positions. This data is then fed into computers which calculate a firing solution and upload to the missile before launch. Again, this firing solution is very simple and comprised of just headings and time codes.
Anyway,
I read an interesting (fiction) book recently, wherein China is suffering a food shortage and has essentially no choice but to take over Taiwan and some other territories in the Philippines. Knowing the US would not normally allow this, they launch a large scale cyber attack on the US, hoping to trigger an economic depression large enough that the us would be forced to circle its wagons and see to its own needs first rather than foreign intervention. They assume American culture would respond to such an situation the way Asian culture would, with increased unity and socialism.
The complete underestimate the US domestic situation and instead of an economy depression, they trigger complete collapse and civil war, destroying the US in all but name and prompting total retaliation which kills hundreds of millions of Chinese and WWIII.
I found this a very interesting scenario as I don't see any path where China would deliberately start WWIII, but I could see them starting it on accident.