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Comment by Animats | Hacker News Reader

"There are very few applications requiring such precise guidance and independence from external references. In fact, beyond ICBM guidance, none have been identified."

Ballistic missile submarines need very low drift inertial navigation, because they stay submerged for a long time without outside references. The submarines provides a starting position for missiles, so missile error is added to submarine error. Size and weight aren't major issues, so they can use much larger gyros.

There's continuing interest in inertial navigation. Projects show up on DARPA.[1] The military assumes that GPS will be jammed much of the time in wartime, so they want inertial backup.

[1] https://www.darpa.mil/program/micro-technology-for-positioni...

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Aren't missiles launched from the surface, enabling a quick calibration just before launch? When there aren't clear skies to see stars, what other sort of non-GPS calibration could be done at sea?

TIL submarines can fire ballistic missiles while submerged. It helps them maintain stealth. Here's a video from the 60's showing Polaris firings:

https://en.m.wikipedia.org/wiki/File:1960-07-21_First_Polari...

Among others (Tomahawk being notable), https://en.wikipedia.org/wiki/UGM-27_Polaris was designed to be launched underwater, in service from 1961.

Not always, but we I understand it even a wire periscope can provide excellent location data.

AFAIK submarine launched ballistic missiles aren't launched that far below the water surface and even a traditional periscope can reach 15+m

> When there aren't clear skies to see stars, what other sort of non-GPS calibration could be done at sea?

It is speculated that you could keep navigational errors bounded with gravimetric measurements. Essentially if you have a detailed pre-surveyed map of the underwater landscape, and high precision gravity measurements you can match your measurements to the undulations of the expected gravity field. Somewhat similar, at least on the math level, to how TERCOM works with cruise missiles.

Wouldn't the movement of the boat cause too much "noise" for these kinds of measurements? How would they cancel out small movements due to e.g water currents?

The short and true answer is that if I would know for sure I could not write about here. Hence all of this is speculation.

Very likely they would use the best motion isolation money can buy. Also the submarine is quite slow (especially at navigation speeds) so the terrain changes under it slowly. Water currents and the motion of people and machinary happens at much higher rate, so you can probably try to eliminate the quick changes and keep the slower changes with filtering and statistics.

It isn't as hard as you might think.

Most gravity-gradient measurements are differential. Common-mode accelerations can be substantially-rejected.

Also, if you're in the precision-measurement game, navigationally-relevant signals can be much larger than the precision of modern instruments. People in the prospecting business sometimes fly gravity-gradiometers in airplanes.

During the day, you can use the sun position as a reference (like it's done using a sextan). You only need an accurate clock, and can determine the sun's location through the clouds.

I suppose you can also do it with the moon. On a dark, cloudy night though? You could risk using the sonar since the launch is going to make a ruckus anyway. Compare measures against seafloor maps.

Boomers aren't supposed to surface for their missile launches for half a century now, it's just too dangerous.

Nope, they're launched from just a bit below the surface to make it harder to spot and chase the sub via aircraft/helicopter. Here's footage: https://www.youtube.com/watch?v=ppz_jj2uw7k&t=25s

Pre GPS navigation methods include sextants, celestial navigation, and LORAN (navigation via bearings to multiple radio towers).

Sub launch ballistic missiles actually have an automated version of celestial navigation: there's a star spotting camera that compares what it sees to a reference database of the couple hundred brightest stars. So even if the missile starts out with a lot of ambiguity about where it's launched from, by the time it gets above the clouds it can correct this.

The missile could still use GPS to correct its trajectory ...

Yes, as long as the GPS (Navstar) satellites haven't been vaporized...

Ballistic missile submarines are primarily second-strike weapons. By the time they launch, our GPS satellites might already be destroyed by anti-satellite weapons. So far no one has demonstrated a working weapon that can knock out satellites in medium orbit (like the Navstar constellation) but I have to assume that China will have such a capability soon.

For routine operations, modern subs do carry a towed buoy which can be used to exchange radio messages and update GPS position while the sub remains deep underwater. This carries some risk of detection, but less than coming up to periscope depth.

I'm failing to see the difference on the medium orbital range? It's still "just" an intercept. I assumed all tests were done at low orbits to help ensure the junk deorbits as quickly as possible while still fulfilling 100% proof of effect, but that is just an assumption and I'd certainly be curious to know if/why/how I was wrong!

The basic physics are the same regardless of altitude, but the launchers used in current known ASAT weapons systems are only capable of reaching LEO. Need a bigger booster. I assume the US / China / Russia have secret programs working on this right now.

USA Boomers use LRG for INS and SLBMs use star shots for mid-course correction. It’s good enough that the D5 was counter force, which is to say it can hit enemy siloes.

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