The article is seriously confused. What you are talking about is easy - chip scale atomic clocks are easy to get. I can have one shipped to me today. Hell, i have one on a time card in my basement.

Assume you want it even super accurate.

Great, 3k for an SA65 https://www.microchip.com/en-us/products/clock-and-timing/co...

Holdover would be fine for even a very long flight.

Hell, even a good rubidium oscillator doing 1PPS will stay within 200 nanoseconds over 12 hours.

If you are trying to do navigation while jammed, none of these help you.

You still need good reckoning, which is the hard part

We done solved the clock problem enough already :)