I believe in the book version of the Hunt for Red October by Tom Clancy, they mention that the Soviets did something similar but using a magnetometer.

Super cool to see the newest versions of this are going from sci-fi to fact.

https://en.wikipedia.org/wiki/Submarine_navigation

"On the surface or at periscope depth, submarines have used these methods to fix their position"...GPS being one of them.

A submarine can also navigate by the stars without surfacing.

https://www.usni.org/magazines/proceedings/2021/october/navi...

"This is precisely the information required by the traditional method of celestial navigation, and it is accomplished without the need to surface or use a sextant!"

We need accurate maps of the geoid for a lot of different reasons. (Military as well as civilian. Potential fields geophysics is super useful for all kinds of different geologic use cases, and regardless, if you want to target an ICBM, you need an accurate geoid.)

However, a super precise gravimeter doesn't help much. We have more precision than we can use already. Rather ancient spring-based instruments from 100 years ago can actually still give more precision than we can use in many cases. Modern ship-borne gravity instruments work on different principles, but the signal is very noisy for the same external reasons.

The biggest issue is that you also need to know absolute elevation very precisely to use the measurement of g that you get. A few millimeters of error in elevation significantly changes the anomaly measurement you make. Sure, submarines can get accurate hydrostatic measurements of depth, but those assume a lot of things and critically aren't absolute. The ocean has currents - that's another way of saying that the surface of the ocean isn't "sea level". Those vary through time and would require satellite information to correct for. However, once you get down in the weeds, it gets tougher still.

Remember that we're dealing with an inverse square distance relationship. Things close by matter quite a lot.

People nearby standing in different positions? That actually does affect things. Easy enough to mount the instrument away from people, though. Different distributions of mass in the submarine? Also affects the measurement. You can correct for all of these in various ways, though, so long as you have information on it. It's just more complexity and another source of noise.

In the end, the "free air anomaly" measurement you'd be correcting things to is an bathymetry map, to the first order. If there's a landslide, that affects things quite a bit, and those happen all the time.

Finally, you'd be matching a "fingerprint" time series measurement as you travel to a pre made map. That's a non unique relationship. You'd have heading/etc information to help the non uniqueness part significantly, but when things don't vary much (i.e flat topography and not a ton varying geologically), you don't have much of a unique signal to match to.

At any rate, it's a very useful tool for many other things, but I'm skeptical it could be turned into a precise navigational aid. In combination with traditional gyroscopic/etc measurements of heading and distance, it could help constrain uncertainty, but it's not an independent measurement and it's relatively noisy.

Now that I think about it, though, a fully passive "seamount proximity sensor" is rather useful, and that's something you'd get even with a noisy signal...

Not really. Subs don’t move that fast in those tight of conditions. One wrong move and the sub bumps the canyon wall making a bunch of noise. Unless they are in actively hostile waters, it just means loss of stealth and the sub needs to choose a new location to lurk.

"On 8 January 2005 at 02:43 GMT, San Francisco collided with an undersea mountain about 364 nautical miles (675 km) southeast of Guam while operating at flank (maximum) speed at a depth of 525 feet (160 m)."

https://en.wikipedia.org/wiki/USS_San_Francisco_(SSN-711)

But there are VLF antennas that Ohio-class submarines have to receive low-data-rate comms while submerged.

Any reasonable communications have to be made while at periscope depth. (Which is subtly different than "having to surface"... at PD just the thin mast is out of the water.)

I am reminded of this:

https://www.theguardian.com/science/2015/may/05/microwave-ov...

Large low-shear-velocity provinces, for anyone else wondering: https://en.wikipedia.org/wiki/Large_low-shear-velocity_provi...

It's one of these subjects where you will never get anything better that vague press releases designed to justify research budgets.

Also, look up the HN guidelines on debating your downvotes.

Welcome to HN!

If you throw a baseball in an accelerating enormous elevator in a vacuum, the baseball will follow a parabolic arc.

If you throw a baseball on the vacuum surface of a planet, it will almost (but not quite) follow a parabolic arc. The force of gravity is lower at the peak, unlike in the elevator where acceleration is constant everywhere in the elevator.

Therefore, you can distinguish gravity from acceleration

Depends on you application, but if you only send some bits of information ("stocks just crashed") having a big thumper with a very known impact profile is sufficient.

Gravity cares about distance and mass, and as large parts of the planet move their density distribution shifts affecting both of those factors

The variations due to changes in elevation are orders of magnitude larger than the variations you're relying on using for navigation. Distance from the center of the Earth is the primary signal you're measuring, for better or worse. It's not just relative vs absolute. You can probably assume the sub isn't changing depth rapidly and ocean currents are long wavelength, which would allow relative measurements to be somewhat feasible.

However, nothing about this requires a new quantum sensor. Ship-borne gravity anomaly measurements have been around for half a century. The previous methods are more than precise enough. In fact, they were done on submarines first before ships - it's easier to measure without waves.

What's triggering this now? Something doesn't add up... If it were purely based on using gravity anomaly along track measurements as a "fingerprint", it wouldn't need a new sensor. There's likely another mechanism they're using or they're using it for other reasons in addition to navigation.

Or is the drift is so minor that this quantum navigation system can operate for up to 6 months without sufficient core drift alignment calculation?

Doesn't that mean that the gravitational field is an information storage and transmission medium; a signal channel?

Is it lossy?

Is there a routable signal path to block harassing calls?

Is it slower or cheaper than nonlocal entanglement; like quantum radar?

What is the minimum energy necessary to cause a propagating thresholdable-as-binary pertubation of a gravitational field?

https://insidegnss.com/the-stars-return-draper-patents-celes...

https://www.amusingplanet.com/2016/09/this-wooden-barrel-was...

However, stars are (to the nearest approximation) point sources. If you use a telescope and zoom in to the star, then you can spread the atmosphere brightness over a large area while concentrating the starlight into a very small area, which means you can see stars during the day. Couple that with a reasonable level of high dynamic range sensor and integration over time, and it would be quite possible to observe the positions of a load of stars during the day.

Los Angeles and Virginia-class submarines are always doing something: doing exercises, transiting from one location to another, etc. And typically multiple things at once. While the boat is transiting from an exercise area to homeport, the team is doing engineering drills, or other kinds of training. Or the forward part of the boat is doing exercises with a carrier battle group while the engineering team is doing engineering drills. (There's ALWAYS engineering drills or maintenance happening.)

Fuel isn't a primary concern: a nuclear reactor is fueled for the life of the boat, so 30-ish years. That said, effective life of a reactor is something the Navy tracks closely, and depending on the life of the boat, the life left in the reactor, some boats are decommissioned as they get close to the end of their fuel life, and others get re-fueled. (And in the case of the USS San Francisco, who had recently been refueled before it hit an underwater mountain, they cut off the front half of the submarine and welded the front half of a recently-decommissioned submarine on, because the reactor and fuel was too valuable to go to waste)

They do have measures of "nuclear fuel" remaining, but it lasts about 30 years (at least in the American boats) so generally doesn't impact day-to-day considerations.

1) Attack Submarines (e.g. Los Angeles-class & Virginia-class for USN) which usually roam within a designated operations area, surveilling, tracking, and generally keeping tabs on other nations' surface & sub-surface fleet dispositions. These subs typically have multi-week sorties and may intermittently surface for surveillance & comms.

2) Ballistic Missile Submarines aka "Boomers" (e.g. Ohio-class for USN) which are given a strategic area in which to operate and their objective is to remain silent & undetected, waiting for the hopefully-never-coming order to launch their SLBMs. These subs usually have multi-month sorties and often don't surface until the end of their patrol.

I often wonder how much autocorrect will shape the evolution of language.

Suddenly you've got a viable model to use on real tactical maps, that is strikingly similar to SC2, though adapted to real life conditions. That is, if you're able to track - or in the least theorize the position of - each individual "piece" on that board (so soldiers and armour, etc).

A real life soldier does perhaps have some different qualities and abilities than a space marine, but it's still a great starting point for building a more realistic model.

So say, your model looks more like Arma 3... Only top down. Now, I've been in the army, so I can tell you first hand that many aspects of Arma 3 is scarily realistic. Other aspects, not so much. But if you add in some modding, and all that is solved.

It's the same principle. You've already have a base AI to "play the game" and now you adapt each information point to reflect real life instead of game mechanics.

Sure, this sort of AI is brand new, so there would of course be things to iron out. Perhaps you need to use other statistical models to better capture the fog of war. But at the end of it, I think it can be done. Hell, perhaps it can even be used to fight crime. What do I know, perhaps it already is, but nobody wants to talk about it, kind of like how it was such a great moment when it dropped that New Scotland Yards used graph databases to solve cases before it was cool.

https://m.youtube.com/watch?v=en-qekZX4ws

But I believe that's only good down to ~60 ft. Anything deeper requires the really long land-based ELF arrays.

Is there even a difference between a submarine and the surrounding water's gravitational fields when the sub has neutral buoyancy?

They are meant for warfare and surviving torpedo concussions as much as possible, so depending on speed, a slow scrape with terrain might not be fatal.