Stages, GPS, INS, Cruise, Descent, Ballistics...
, it seems to me that GPS *might* be used, but if it is, it would be during launch. That makes sense since during launch, if a launching nation's GPS system is still intact, a missile can know its position AT launch and maybe for a few minutes during the ascent phase. Otherwise, I presume that GPS inputs are fed to the missile for as long as GPS is not knocked out.
Then, during ascent, the missile is constantly calculating/computing all along its path. At some point, during cruise (if cruise is well above the atmosphere, far enough to count as having "exited" the Earth), it may or may not launch one or more MIRVs, and then, during descent, launch off one or more, or all remaining warheads, which may or may not be finned for maneuvering. If finned, or if using thrusters, they then would be MaRVs, or Maneuvering Re-Entry Vehicles. I thought, in my past thinking that is, that MarVS maneuvered not so much to improve targetting accurace, but to ACTIVELY and not passively evade countermeasures or anti-missile missile systems. I knew in the past of decoys in MIRVs, but thought that I in the past read the MaRV systems were good enough and fast enough to not need decoys, meaning more RVs could be carried, or they could be lighter RVs to move a bit faster before launching the warheads.
I though that the MaRVs could do things like cork-screw maneuvers, too, not just fire off decoys and counter measures. Been a long, lonnnng time since I last read this stuff, since maybe the mid 80's. I even forgot about the MX missile. The article was a nice refresher about the diffs tween SLBMs and ICBMs and how the USAF distinguishes ICBMs by ranges (Short, Medium, Long) whereas the USN does not.
Also, it was interesting to read that the V(#) rockets, some 580 of which were launched into the UK (I suppose as terrror devices, not anti-military infrastructure weapons) were, to this date, the deadlies ballistic missiles.