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Comment by ggm | Hacker News Reader

> The ongoing PROBA-3 (Project for On Board Autonomy-3) mission (Llorente et al. 2013; L. F. Peñin et al. 2020) (launched in 2024) aims to form a solar coronagraph using two satellites approximately 150 m apart with a relative displacement accuracy under a sub-millimeter and pointing accuracy at the arc-second level.

I expected this accuracy at km or greater. At least for radio interferometry. Maybe Optical has smaller scale?

The positional accuracy of fixed point RF interferometers in astronomy are amazing, they can probably measure continental drift (only joking) -So this is saving atmospheric attenuation but with very small angular distance between the arms of the interferometer.

Surely this has to go up to km and 100 of km to be useful? Maybe not in optical.

(the Narrabri Radio Telescope is a giant radio interferometer on a linear railway track. The SKA is fixed position, massive deployments "Square Kilometer" hence the name.)

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> they can probably measure continental drift (only joking)

They can! Continental drift measurements have been done using radio interfometry since at least the 1980s: https://www.nytimes.com/1983/07/05/science/science-using-new... (Though that 1983 article does suggest they were still working on the accuracy). Certainly radio interferometers can measure displacement of dishes after earthquakes.

I edited out only partly joking and now wish I hadn't.

There’s scintillation of the Interstellar medium, that is radio waves are affected by the interstellar medium on different paths.

For my second year degree project we did a project on space based VLBI, being just theory we were looking at a bird cage orbit, as you go out further one needs to fill out the “UV plane” (I’m just recalling bits on my 6 week study on interferometry).

ISM is a thing for continental VLBI, as pointed out by the head of Jodrell Bank when I went to a public out-reach talk in London.

Whilst not interferometry it would (might) be good to have a Gaia type galaxy mapping mission further out, although it if there were a few satellites they could account for the larger orbits.

I’m not sure if a birdcage orbit would improve things much the cost of having a high inclination orbit out of the solar plane would be huge.

> I expected this accuracy at km or greater.

On the ground? Sure. Orbiting? I'm amazed they reach sub-mm. The gravitational field of a planet or star is not uniform, subtle variations in their density are enough to impart tiny variations to an orbiting object, and they add up over time.

Can't find a reference right now but I recall someone proposed even a positioning system relying only on accurate gravimetric sensors and a (very good) map of the strength at every relevant location on Earth. Good for submarines, GPS reception is not so good underwater.

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