4 ms·

> 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 sa

> 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.
- krisoft 3y ago
  > However, nothing about this requires a new quantum sensor. Sure. The news can be many thing. Maybe they got a marginally better gyroscope. Maybe it is not better in terms of better error characteristic, but cheaper, or smaller, or lower maintenance. Or maybe they got a new software package which brings them benefits, but the way they could sell it to the leaders is by telling them that the sensor they are using is "quantum". Or maybe none of the above and they are just trying to fake out an adversary to trigger a costly mistake.

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.

krisoft 3y ago
> However, nothing about this requires a new quantum sensor. Sure. The news can be many thing. Maybe they got a marginally better gyroscope. Maybe it is not better in terms of better error characteristic, but cheaper, or smaller, or lower maintenance. Or maybe they got a new software package which brings them benefits, but the way they could sell it to the leaders is by telling them that the sensor they are using is "quantum". Or maybe none of the above and they are just trying to fake out an adversary to trigger a costly mistake.