NavList:

Alex, you wrote:
"I also apologize for triggering this submarine discussion, but let me recall that it started from this important Cel Nav question: Who needed high precision Celestial Navigation, and was this really ever needed?"

While the submarine navigation question is off-topic, it's only one step removed which I think is reasonable. And as you note, it does get back to a celestial question.

You wrote:
"By "high precision" I understand less than 1 nm accuracy. Ordinary practitioners do not care about this"

Right. I have occasionally DEFINED traditional celestial navigation by including this level of acceptable error. And you're quite correct that submariners were one group who needed high accuracy celestial navigation, at least for a while. Both the submarines and the early missiles used stellar fix data to constrain errors in the inertial system. In fact, from what I understand, French ballistic missile submariners still use high accuracy celestial fix data.

You wrote:
"This is still a conjecture and there is no direct proof of this. However,
there is much indirect evidence."

There's plenty of information on these systems in the (unclassified) literature. Naturally there are other aspects which remain entirely classified, and those of use without security clearance can only speculate once we hit that wall. There's a book by Donald Mackenzie, written in 1993, "Inventing Accuracy: A Historical Sociology of Nuclear Missile Guidance" (portions available in preview on Google Books) which discusses both the technology and the implications in terms of the nuclear arms race. And those implications are not trivial. Submarine missiles were originally pure deterrence. They were "city busting" weapons reserved strictly for the deadly second strike of the doctrine of "mutually assured destruction:. In essence they were non-weapons. Submarine-launched ballistic missiles were doomsday devices, only be used in a nuclear armageddon. But from the earliest Polaris missile era in the US, there was a seemingly inevitable desire to apply technology to turn those missiles into something far more dangerous: counter-force or first strike weapons which could be used to destroy hardened military targets, like missile silos, in the Soviet Union. This logic was also driven by good old-fashioned inter-service rivalry. The US Navy was second tier in the nuclear triad with the Air Force controlling the ICBMs and the bombers. Eventually the submarine force got the accuracy they wanted. Supposedly, the Trident missiles, still deployed by the US in fourteen Ohio class submarines and by the UK in four Vanguard class submarines, have this high accuracy capability. Increased accuracy in SLBMs was highly de-stabilizing.

You wrote:
"It seems clear that INS alone, without checks is not capable of the required
accuracy. The question is what checks were available before satellites,
and also what checks are available without surfacing."

Celestial through the periscope. There are some big problems with this process which may not have been discussed yet. Once you get down to 0.1 nautical mile accuracy, you have to worry about time accurate to better than one second (and that time has to be UT1 or equivalent, not UTC unless it's corrected by DUT). But the really huge problem is the Earth's gravity field. While it's an almost perfect oblate sphere when you're working at the level of accuracy of traditional celestial navigation, the gravitational field is quite "bumpy" at the higher level of accuracy required by submariners. The field had to be mapped in order to support high accuracy missile launches. This problem applies to both high-accuracy celestial navigation and "stable platform" inertial navigation, too, since both depend on the local vertical.

-FER

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