Relatedly: if you park an AEGIS BMD ship off the coast of San Francisco, how far inland can they defend with SM-3 against an ICBM? Approximately eastern Kansas. Same question, but Hampton Roads: approximately western Kansas.
The footprint defended by SM-3 is actually fairly huge.
Footprint charts here:
https://mostlymissiledefense.com/2016/06/30/strategic-capabi...
what I find suspicious though was up until 2000 or so there was a doctrine that midcourse intercept was unworkable because ICBMs could be supplemented with a large number (hundreds) of lightweight decoys that would be impossible to discriminate from a real warhead. That led to systems like Sprint that would wait until the warhead was re-entering the atmosphere and easy to discriminate because it was not slowed down so much by the atmosphere, doesn't burn up, etc. I dunno if the sensors really got better or if they just decided to go ahead anyway for political reasons.
The terminal guidance sensors could pretty trivially discriminate decoys since the 1980s, when they stopped using radar guidance and switched to broad-spectrum imagers. The decoys really only worked on radar. A decoy capable of spoofing the imagers would have to be so large, complex, and heavy that it would be tantamount to adding another warhead which defeats the purpose.
This is why they stopped testing against decoys. They had tons of test data that the terminal guidance systems could reliably discriminate them so it didn’t add anything to the tests other than cost.
This source would argue the other way
https://cissm.umd.edu/sites/default/files/2019-08/2000-UCS-C...
It seems that a warhead or decoy would fill a single pixel on the sensor until you got very close to the target, 1 km or so, at which case you have to execute a high-g maneuver in a few ms. The state of the art is a "two color" system where you could make either the warhead or the decoys any "color" you want with surface treatments and/or thermal management (worst case: put the warhead inside a shroud cooled to liquid nitrogen temperature and fire the weapon at night when it won't be illuminated by the sun.)
There was a test in the early 2000s I read about where they were able to pick out a warhead which was intermediate in properties with a set of decoys with various surface treatments. That 's great but they knew exactly what they were up against which we wouldn't know if it was a North Korean missile.
I'd have more faith in the discrimination abilities of ground-based radars in the 12 GHz range than in the "two color" focal plane imager system.
This gets deep into the classified domain, no one is going to be talking about it here. :)