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

The science is cool, but I wonder if a specialized popular science magazine of yester-year would do a better job at presenting it? "We made a high-precision accelerometer that uses lasers and quantum effects; let me tell you everything about it." Right now, it reads more like "Dr. C has this very peculiar quirk when he goes to the London underground. Admittedly, it is not as bizarre as his habit of drinking coffee instead of tea, but going to the underground carrying an aluminum tube instead of a newspaper is just a tad flummoxing..."

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

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.

US ICBMs been inertially guided for decades. They predate GPS. Once launched, they have no radio inputs.

Some weapons do use GPS. The Joint Direct Attack Munition, the US's most common "smart bomb", is GPS-dependent. There's an inertial backup, but it's not that good. With GPS, it can hit a bunker or an artillery emplacement or a parked vehicle. Without GPS, small targets are out.

DARPA has been working on small, low-cost inertial systems with enough accuracy to remove GPS dependence. Exactly how good those systems are and which weapons systems now use them is not talked about much. Here's the project poster.[1] This is an improved form of MEMS accelerometers and gyros, not new physics. DARPA has a project for that, too, but the near term goal is small, cheap, and disposable for munitions.

There's much current interest in weapons which combine gun, bomb, and missile technology to boost range. Bombs with a small jet engine and wings for extra range exist. Both the US and Russia have them. Artillery rounds with a rocket upper stage are becoming available. The general idea is to have stuff you can shoot into heavily defended territory.[2]

[1] https://eri-summit.darpa.mil/docs/ERIsummit2020/posters/25_P...

[2] https://www.youtube.com/watch?v=0vIPNElDkns

> US ICBMs been inertially guided for decades. They predate GPS. Once launched, they have no radio inputs.

That's how the original Third Reich V-rockets worked as well, and they were shockingly inaccurate. It didn't matter for the Nazis, as their intention was to spread terror among the British population instead of actually hitting specific military targets, but in a modern war waged under the rules of war, "collateral damage" can actually be a war crime, and even if not, it will provoke significant opposition.

This can at the moment be seen in Ukraine, where Ukraine regularly (and rightfully, IMHO) claims that Russia's strikes violate the rules of war by hitting civilian infrastructure en masse, and where Ukraine's strike capability against Russia is severely impeded by Russian GPS jamming (that also causes serious safety issues for civilian airlines [1]), but also in the current Israel/Palestine war, where Hamas regularly claims that Israeli strikes are too imprecise [2] or unguided [3], while Israel claims it hits hidden weapons caches that subsequently explode and cause a lot of secondary damage [4].

[1] https://www.newsweek.com/finland-estonia-gps-jamming-russia-...

[2] https://www.reuters.com/world/middle-east/israeli-attack-raf...

[3] https://www.lemonde.fr/en/international/article/2024/05/31/i...

[4] https://www.pbs.org/newshour/show/military-experts-discuss-i...