Ultra-precision formation flying demonstration for space-based interferometry | Hacker News Reader

Ultra-precision formation flying demonstration for space-based interferometry

arxiv.org

70 pointsPaulHoule23 comments

If you know how hard it is to align an interferometer on an optical table, where things are BOLTED to it, this looks absolutely insane.

> 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.)

> 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.

I'm not sure how different this is, but the Gravity Recovery and Climate Experiment [0] is planning is 3rd mission after 2 currently operating pairs of satellites using interferometry to map gravity anomalies, among other things.

[0]https://en.m.wikipedia.org/wiki/GRACE_and_GRACE-FO

That should be useful for stellar coronagraphs for finding extrasolar planets. The idea is to have an opaque disc that occludes the star so you can see the planets. This is usually attached to the telescope, but one could be flown out a kilometer or so with this approach. JPL was looking at this but went with a more traditional design instead.

How would JPL ensure the coronagraph could be easily repositioned? I'm thinking of attaching it to the telescope through very long cables and using lasers to nudge it into position.

As in the parent article here, it's free-flying and can be any reasonable distance from the telescope. Maybe a kilometer out from the telescope. The article says they have some very low thrust thrusters for fine positioning.

It would make sense to launch multiple shades and pack them with lots and lots of propellant so they can last as long as possible. Maybe even schedule regular fleet renewals for when the propellant runs out, and manage the coronagraphs independently, as they can position themselves between any telescope and their target.

A single one would force observations to follow the coronagraph and limit telescope observation.

Will they finally test if sum of angles in triangle is 180 deg?

Is the space even Euclidean enough for that?

That's what they'd be trying to figure out.

I wonder how precise we'd need to be to detect that the three vertices don't experience time at the same rate.

Atomic clocks can do it: https://www.latimes.com/science/sciencenow/la-sci-sn-atomic-...

Presumably the limiting factor for this is density variations in the tiny bit of atmosphere present up there...

And the fix is to fly higher...?

Much higher. Like, 1.5 million kilometers.

LEO is just for demonstration

I would expect the "serious" one would use reaction masses to save on propellant.

One is the integral of the other.

For fast and precise position control, you need both.

Obviously for staying stationary, one doesn't need much of either. It could be a piezoelectric buzzer (reaction mass) and an LED (photon pressure).

Most definitely a dual-use technology, specifically locating elusive targets via their RF emissions a la the US Navy's NOSS, France's CERES, and some of China's Yaogan military satellite clusters.

This sound like a different interferometer for gravity wave detection (see LISA project), mostly for "seeing" stars instead of ground targets.

I meant the formation flying part.

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