Magnetometers based on diamonds will make navigation easier

> Our calculations suggest that at least a power of several petawatts is needed

Good to know

Drifting like crazy might not be such an issue if one is looking for an AC signal, such as could be produced by 130m x 13m metal objects moving against the stationary background.

Huge and impractical sounds like SQuIDs might still be preferable. LN2 is a byproduct, so it's cheap.

Much of that is answered in the article.

Field strength pretty quickly drops of over distance, and even the most powerful magnets don't go much beyond 100T (and even that's a stretch). Assuming you could somehow achieve 100T 1m away from the magnet (which is charitable, usually the high teslas are only achieved at the exact centre) then just a kilometre away it will have dropped to 1 microtesla, well below the 25~65 microtesla of the Earths gravitational field.

So you'd need those kinds of magnets on a kilometre spaced grid, which doesn't seem realistic. And that's with some rather charitable assumptions about building a strong magnet, to be honest I expect you'd have difficulty turning a few compasses in the same building, let alone miles away.

It depends on how far away they are. Magnetic (dipole) fields fall as the cube of distance. Since the earth's pole has a dipole moment (8.0 * 1022 Am^2) and is detectable ~30uT at the earth's radius, thousands of km from the core. Even a 1T field at a radius of 1m would be billion times smaller ~1nT a km away or 1/30k of the earth's field.

A giant 6T MRI would be comparable to earth's field at 27m.

They could using a small EMP weapon. These happen to have short range and are explosively pumped, so at that point it’s easier to just disrupt the magnetometer by initiating rapid explosive disassembly.

Based purely in my layman understanding of magnetism, you’d need quite a disturbance and that same energy would likely not make the rest of any electronics happy.

1. No, anyway, your phone would have to be engineered to be non-magnetic to the same level that you wish to make measurements. 2. Yes, faster is better. Flying lower is also more specific if you have a sufficiently good magnetic map.

Compasses used in vehicles need to be calibrated to null the magnetic mass of the vehicle. This is true for airplanes and ships but also cars, although cheap car compasses often settle for poor accuracy to avoid this hassle. This whole issue is why the compass in airplanes is usually mounted in an odd place like above the glare shield or below the roof, so that it's farther from the other instruments to make the magnetic field situation simpler.

One of the earliest methods to do this has the amusing nickname of "Nelson's Balls," the binnacle (compass holder) on ships often has two large metal spheres mounted on its sides that serve to compensate for the fore-aft magnetic mass of the all the hardware in the ship.

The magnetic compass in an airplane is subject to a certain amount of variance due to installation (essentially calibration error), but that error should be quite small. More problematically, airplanes move fast enough that the magnetic compass shows errors due to the math of magnetic flux when you're moving relative to a magnetic field. The theory is somewhat complicated but pilots often remember it with acronyms like "ANDS" for accelerate north/decelerate south - basically, if there's any appreciable east-west component of your flight path, when you accelerate your compass will turn towards the north, and when you decelerate it will turn towards the south.

This is why airplanes are equipped with a heading indicator, also called a directional gyro, which uses a gyroscope to simulate a compass. The heading indicator isn't subject to the weird moving errors that the magnetic compass shows so it's more useful for maneuvering. On the other hand, the gyro does slowly drift over time, so every once in a while it needs to be reset against the magnetic compass - during straight and level flight to avoid moving errors.

In aircrafts it's common to use navigation instruments based on gyroscopes. See:

* https://en.wikipedia.org/wiki/Gyroscope * https://www.experimentalaircraft.info/articles/aircraft-gyro...

Lidar's range is measured in hundreds of meters. That might be a bit limiting for flight. Radar could be used, but you might not want to have a "hey all, I'm here!"-beacon for military purposes.

You could of course use cameras, pointing either up (stellar navigation) or down, but that's not going to work all the time.