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London Underground hosts tests for 'quantum compass' that could replace GPS

theguardian.com

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Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#141

Post by tonyarkles »

contravariant wrote: post #38
porphyra wrote: post #9

I remember doing laser cooling of rubidium atoms in physics class. It's really cool how it works: * Rubidium will absorb photons of a certain wavelength and re-emit it in a random direction. Since photons have momentum, it will get some net momentum change from absorbing photons all coming from a particular direction, but the re-emissions are in random directions so they have net zero momentum. * Shine laser light at…

Maybe I'm missing something but hitting a gas just above absolute zero with lasers from every direction doesn't sound as something that could be miniaturised.

It's pretty amazing what's slowly but surely becoming possible. This company, for example, has successfully made chip-scale "fiber" ring gyros with incredible performance that would have been very large heavy units 5-10 years ago: https://www.anellophotonics.com/products/x3

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#142

Post by Reason077 »

prewett wrote: post #138
pas wrote: post #124

Earlier quoted context omitted.

There's some natural drift, and about yearly boost to keep them in their desired orbits https://space.stackexchange.com/questions/30752/do-operating... As far as I know - while their onboard clock is pretty good - they continuously get corrections from ground stations. (Once per day.) https://gpsbeam.com/ground-stations/ the "navigation message" broadcast by the sats is described here https://gssc.esa.int/navipedia/i…

> their onboard clock is pretty good They use atomic clocks, so it's accurate to something like 1 part in 10^16, or about 1 second in a billion years! A friend of mine is working on the next generation, which will be even more accurate. In contrast, the force of gravity experienced by the satellite is known to much less accuracy. In fact, changes on a monthly basis due to rainfall/rivers and tides. The GRACE satellit…

Not quite that accurate. Apparently it’s more like 3 seconds per million years for a rubidium atomic clock, and 1 second per 3 million years for a hydrogen maser. (Caesium atomic clocks are somewhere in between)

ESA use both types onboard Galileo satellites:

https://www.esa.int/Applications/Satellite_navigation/How_th...

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#143

Post by lucb1e »

echoangle wrote: post #102
lucb1e wrote: post #93

Earlier quoted context omitted.

Especially with the word "compass" in the title, I figured they made an accurate map of the earth's magnetic field and would use that to avoid needing any satellites. Then I opened the comment thread... Come to think of it, the earth's magnetic pole shifts so I guess this isn't possible? Either way, "London Underground tests new dead reckoning system" would have been more accurate it sounds like

Regarding pole shift: you maybe could have multiple static measurement stations spread around which measure the current values at known positions and synchronize the data once per day to the trains. I’m not sure how you could even determine position just from magnetic field alone, surely there are multiple positions with the same magnetic conditions?

Maybe, I don't know enough about magnetic fields to say much. In my head they're directional so you'd know how it's aligned and can measure how strong it is, and that might function similar to a gravity map, especially if you know where you're coming from: there may be two places on earth with the same amount of gravity, or in this case, the same magnetic field strength, but if those places aren't next to each other that may still be tolerable depending on the system requirements.

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#144

Post by HideousKojima »

avianlyric wrote: post #133
robertlagrant wrote: post #122

Earlier quoted context omitted.

> Neither do GNSS satellites Is it a similar problem? I hadn't thought about how GPS satellites know where they are. Do they need regular correction?

Yes, satellites are tracked by ground stations, and receive an update every 24 hours with information about their project positions in the future. GPS satellites transmit this data in the form of an almanac which includes all the high level parameters for estimating the location of every satellite, and ephemeris data which allows you to calculate the precise location of the satellite, when used in tandem with the alm…

This is also why, in a zombie apocalypse type scenario, GPS would become inaccurate past the point of usability within a few weeks, maybe a few months max.

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#146

Post by roywiggins »

Dylan16807 wrote: post #95
roywiggins wrote: post #81

Earlier quoted context omitted.

Precision is, counterintuitively, still a big deal in the logic of nuclear weapons strategy. Nuking Moscow is easy, nuking hardened targets can be a lot harder. > Before the invention of this new fuzing mechanism, even the most accurate ballistic missile warheads might not detonate close enough to targets hardened against nuclear attack to destroy them. But the new super-fuze is designed to destroy fixed targets by d…

But importantly, making sure a nuke can still hit Moscow despite geolocation countermeasures has lots of worrying consequences for normal people that are in normal cities. If nukes are hitting random countryside then a lot less people die. But when it's the difference between hitting a hardened target or being off by 200 meters, that doesn't really affect normal people.

I'm not sure there are much in the way of geolocation countermeasures against ICBMs anyway, they're inertially guided. Unless you can generate localized gravity anomalies at will, I guess.
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