[pps43]

I was trying to explain why it is more important for the satellites to quickly detect launches from land.

With SLBMs you have other options. You can have your attack submarine shadowing enemy's ballistic missile submarine, or surface ASW ships patrolling the area that can detect the first launch and try to sink the ballistic missile submarine before it can launch its remaining missiles.

[avar]

Sure, I'm just replying to your question of "Who do you nuke? [if you don't know who launched a missile against you]", which it seems to me is implicitly assuming that if someone nukes you you must nuke someone back right then and there.

That's nobody's nuclear posture, it's just a subset of the posture of the US, Russia, China etc.