3 ms·

>during missile alert without needing an external reference through precision gyrocompassing. Most ICBMs require an external alignment system to keep the INS in

>during missile alert without needing an external reference through precision gyrocompassing. Most ICBMs require an external alignment system to keep the INS in synch with the outside world prior to launch. I don't fully grasp this.

mikestew 7y ago
Until someone with real knowledge comes along, here's my take. The INS doesn't know where it is at in relation to the rest of the world/universe/whatever. So you tell it. And the INS sits there in his missile silo, constantly asking, "where am I? Have I moved since last time I asked this question? If I've moved, where does that now place me in relationship to everything else?" Of course, he's in a missile silo, he hasn't gone anywhere. But the INS drifts; small imprecisions add up to the INS thinking it moved when it didn't. So he has to sync with an external system to figure out, "whoops, haven't moved. Better rebaseline." Where this fancy-schmancy system is so good, you tell it where it's at once, and he can sit in his silo for years without drifting enough to need correction. Again, somewhat educated SWAG. Do not stake your reputation on this by repeating it.
- tolien 7y ago
  That’s my understanding too. The key is that this process of baselining takes time, which you don’t have either when there’s half a dozen 50MT Russian warheads coming your way (TFA refers to the missile sites being a nuclear “sponge”, i.e. forcing the Russians to launch on them and use warheads that could be targeted against cities instead) or when you want to destroy them in a preemptive first strike before they can be launched.
  - ghaff 7y ago
    Slightly OT but with submarine-based systems, the INS positioning is adjusted in part by doing a star sight at the top of the trajectory. https://en.wikipedia.org/wiki/UGM-133_Trident_II https://en.wikipedia.org/wiki/UGM-133_Trident_II

mikestew 7y ago

Until someone with real knowledge comes along, here's my take. The INS doesn't know where it is at in relation to the rest of the world/universe/whatever. So you tell it. And the INS sits there in his missile silo, constantly asking, "where am I? Have I moved since last time I asked this question? If I've moved, where does that now place me in relationship to everything else?" Of course, he's in a missile silo, he hasn't gone anywhere. But the INS drifts; small imprecisions add up to the INS thinking it moved when it didn't. So he has to sync with an external system to figure out, "whoops, haven't moved. Better rebaseline." Where this fancy-schmancy system is so good, you tell it where it's at once, and he can sit in his silo for years without drifting enough to need correction. Again, somewhat educated SWAG. Do not stake your reputation on this by repeating it.

tolien 7y ago
That’s my understanding too. The key is that this process of baselining takes time, which you don’t have either when there’s half a dozen 50MT Russian warheads coming your way (TFA refers to the missile sites being a nuclear “sponge”, i.e. forcing the Russians to launch on them and use warheads that could be targeted against cities instead) or when you want to destroy them in a preemptive first strike before they can be launched.
- ghaff 7y ago
  Slightly OT but with submarine-based systems, the INS positioning is adjusted in part by doing a star sight at the top of the trajectory. https://en.wikipedia.org/wiki/UGM-133_Trident_II https://en.wikipedia.org/wiki/UGM-133_Trident_II