Us? As far as I know nothing. We used to have radars watching the oceans looking for Soviet submarine launched ICBMs. Don't know if they are still operating. I haven't heard of any deployments of defensive batteries (Patriot, Hawk, THAAD, or Aegis/BMD ships) to coastal areas where we'd get a shot at it, even if we saw it coming.
That hasn't been a priority (I guess) - more worried about countering the developing threat of ICBM launches out of NK or Iran. But with the ongoing proliferation of better and better short and medium range ballistic missiles, and unstable states developing deliverable WMD warheads... Yeah, I think closing the border and watching the oceans again are going to have to become priorities. Along with deploying systems that'd have a shot at something if it was detected.
Worse even, the image below is a mockup display of a P800 on not a military vessel, but a commercial-profile vessel.
Ironically, recent reports (last week) refer to the deployment of 'blimps'.