I'm fairly certain that ICBM's use inertial guidance with predefined/computed coordinates (http://en.wikipedia.org/wiki/Inertial_guidance). There's no software involved, its all physical. Detonation is (probably) trigged by a combination of altimeter, accelerometer and other telemetry systems.
Yeah, GPS doesn't work so well through the plasma sheath around a reentry vehicle. And you don't have a whole lot of time to acquire GPS lock, or steer, when you're still traveling at hypersonic speeds and it takes maybe half a minute to hit the ground after first encountering the upper atmosphere.
It happened 175 days ago, when you made an unwise comment about killing children. All your comments since then have not been visible, or upvotable. This is why you're supposed to put contact information in your profile.
I hesitated before posting this, since from your older comments you appear to be a bit of a reddit user, and the comment itself contains several significant technical errors regarding the design of nuclear weapons, (American nuclear weapons haven't been of the "gun" type for 60 years now) which casts a poor light on the comment as a whole; but I don't think you're completely beyond hope.
Make a new account. Try not to be an asshole with it.
Cruise missiles, on the other hand... Well, let's just say there are eye-witness accounts of one flying down the street and turning the corner at an intersection before detonating in a tiny radius. All the was left of the target was a single shoe, IIRC.
Oelewapperke posted a long and detailed reply to this post, and yet it will go ignored because all his posts are automatically killed without him being aware of it. His posts may not be perfect, but I still find this kind of false positives terrible.
It's not a false positive. He's hellbanned. Hellbanned users don't know they're hellbanned - their posts appear to them "not dead." If you look at his posting history, all of his posts starting 160 days ago are dead, and it seems to have started at a post from 175 days ago.
ICBM's use inertial and (in the case of Trident) star mappers.
The reason they don't use GPS is that you would (hopefully) only start throwing ICBMs around if you are in "nucular combat toe to toe with the Rooskies" - and the way you know you are in a proper war is that all your GPS satellites have just been destroyed by the other side.