"A GPS signal could be spoofed or blocked for instance. When you're thinking about nuclear submarines, it usually best to consider the worst."
Actually, when you are thinking about submarines, you probably don't want to be using a GPS in your navigation system. A blocked signal is "what happens", not "the worst", because sea-water is basically impenetrable to EM waves.
And if you are thinking about ballistic missile subs, I hope you've factored into your thoughts the near-certain fact that in just a few years the enemy will be able to track those with one of their mass-produced robotic drones and so their locations while out on patrol will cease to be "totally undetectable" (as they have been for the last half-century or more) and instead be "posted on the internet, by the enemy, just because they can, for shits and giggles".
Biting the hand that feeds IT
