For precise targeting, missile trajectory only matters after exiting the earth's atmosphere? Even 1st semester basic college physics suggests this to be completely untrue. That said, I'm no rocket surgeon. Just a STEM turd.
celestial navigation needs timekeeping to even work. you need to know the time to measure longitude and its celestially projected equivalent, right ascension.
Even old ICBMs and the SR-71 used star navigation systems.
And I see the SR-71 used an onboard clock, and the system can see stars during daytime.
You think today's technology would be a lot better.
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Inertial navigation should be able to handle the first few minutes until the missiles go well above any clouds ~80km.
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And given that ICBMs would be designed for a second strike contingency, that implies the national time system would have already been destroyed?