5 ms·

It's somewhat unlikely (but not impossible) that this method is based on navigating by matching patterns of the shape of the geoid (aka variations in g). We ne

by jofer 3y ago

It's somewhat unlikely (but not impossible) that this method is based on navigating by matching patterns of the shape of the geoid (aka variations in g). 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...

krisoft 3y ago
> The biggest issue is that you also need to know absolute elevation very precisely to use the measurement of g that you get. I don't think so. What you are saying is true if for some reason you want to work with absolute values, but nobody would do that. You measure many data points as your submarine flies over the landscape for a time, and then you match the measured curve with predicted curves from bathymetric maps. This is an optimisation problem where you try to find the best match. Precise elevation is an output from this process not an input requirement. > 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 100%. This is also true for cruise missiles which fly by TERCOM[1]. And there is an interesting consequence to it. Submarines and cruise missiles don't just bumble around randomly. The navigators also know this limitation so they set trajectories which plays to their strength. In the case of the cruise missile planners they have tools to evaluate the navigational quality of a terrain contour matching algorithm over a proposed trajectory with Monte Carlo methods. Probably submariners have the same. This means in practice you can know that the submarines are more likely to take certain routes. They will prefer approaching from hilly terrain over flat, but also over extended flat areas they will prefer to overfly butes to regain navigational accuracy. This of course won't tell you precisely where the submarine is, but can help an adversary more economically allocate their ASW assets. > 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 Yeah. I mean I heard that people proposed to make measurements of stars to find your location. The fools. Haven't they heard of clouds? Sometimes you can't see as far as your own nose for days. Every navigational system ever devised have limitations and peculiarities. If you are comparing gravitational tercom with the ease and quality and simplicity of GPS then of course it will look crude and cumbersome. But of course GPS adds other complications and dangers to the life of a submariner. Used well, and in the right circumstances it can be potentially very valuable technique. 1: https://en.wikipedia.org/wiki/TERCOM https://en.wikipedia.org/wiki/TERCOM
- jofer 3y ago
  > What you are saying is true if for some reason you want to work with absolute values, but nobody would do that. You measure many data points as your submarine flies over the landscape for a time, and then you match the measured curve with predicted curves from bathymetric maps. 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.

by jofer 3y ago

krisoft 3y ago

> The biggest issue is that you also need to know absolute elevation very precisely to use the measurement of g that you get. I don't think so. What you are saying is true if for some reason you want to work with absolute values, but nobody would do that. You measure many data points as your submarine flies over the landscape for a time, and then you match the measured curve with predicted curves from bathymetric maps. This is an optimisation problem where you try to find the best match. Precise elevation is an output from this process not an input requirement. > 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 100%. This is also true for cruise missiles which fly by TERCOM[1]. And there is an interesting consequence to it. Submarines and cruise missiles don't just bumble around randomly. The navigators also know this limitation so they set trajectories which plays to their strength. In the case of the cruise missile planners they have tools to evaluate the navigational quality of a terrain contour matching algorithm over a proposed trajectory with Monte Carlo methods. Probably submariners have the same. This means in practice you can know that the submarines are more likely to take certain routes. They will prefer approaching from hilly terrain over flat, but also over extended flat areas they will prefer to overfly butes to regain navigational accuracy. This of course won't tell you precisely where the submarine is, but can help an adversary more economically allocate their ASW assets. > 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 Yeah. I mean I heard that people proposed to make measurements of stars to find your location. The fools. Haven't they heard of clouds? Sometimes you can't see as far as your own nose for days. Every navigational system ever devised have limitations and peculiarities. If you are comparing gravitational tercom with the ease and quality and simplicity of GPS then of course it will look crude and cumbersome. But of course GPS adds other complications and dangers to the life of a submariner. Used well, and in the right circumstances it can be potentially very valuable technique. 1: https://en.wikipedia.org/wiki/TERCOM https://en.wikipedia.org/wiki/TERCOM

jofer 3y ago
> What you are saying is true if for some reason you want to work with absolute values, but nobody would do that. You measure many data points as your submarine flies over the landscape for a time, and then you match the measured curve with predicted curves from bathymetric maps. 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.