There are atomic clocks that weigh only about 35 grams. They cost $15K though, so probably too expensive for munitions, except nukes, but fine for aircraft and

There are atomic clocks that weigh only about 35 grams. They cost $15K though, so probably too expensive for munitions, except nukes, but fine for aircraft and ships, and probably for some land vehicles. Having ±5.0E-11 accuracy and <1E-11 @1000s short term stability. So enough for any aircraft sortie or ship action in a region of probable GPS spoofing (precise enough for days). Even OCXO will be enough given short duration of stay in a spoofed environment (typical OCXO will give you 50 seconds, but it is more than enough time to be within 300 meters from a spoofer, unless you are on foot). Or 500 seconds within 3000 meters, which is same speed - 6 m/s - which you have to maintain to be able to filter out spoofing. Several times more in practice because these are all RMS, not max errors. But 30 m/s is 5 sigmas which is military (nuclear, to be precise) definition of 'safe', so all aircraft/munitions are safe, and ships are big and expensive enough to justify $15,000 atomic clock.