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A comment on 5G Is Likely to Put Weather Forecasting at Risk. Parent comment

Re:5G Is Likely to Put Weather Forecasting at Risk by mdorazio on Tuesday April 16, 2019 @02:23PM (#19676003)
You might want to look into what the NRO is already putting in space (supposedly) [1]. If the reports are to be believed, 100m dishes are already there, just pointing at the earth instead of into deep space. To your second point, it seems that relative positioning is mostly a solved problem at least for satellites in GSO (that's a core part of how GPS works). My personal opinion is that space-based radio astronomy is mostly a problem of cost, not of available tech.

[1] https://en.wikipedia.org/wiki/Orion_(satellite)

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Re:5G Is Likely to Put Weather Forecasting at Risk by krapht on Tuesday April 16, 2019 @06:56PM (#19678050)
Not a solved problem, GPS is too inaccurate for interferometry. The problem is you want to do correlations between radio signals; consider a 1 GHz signal, which has a 30 centimeter wavelength. Error compounds as you integrate longer and longer signals. To do anything decent, you need to know absolute position within millimeters, and it can't be drifting too much.

Also, the positioning error isn't the whole story; there's also timing drift. Rubidium GPS-disciplined oscillators actually drift too much instantaneously to be useful...

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Re:5G Is Likely to Put Weather Forecasting at Risk by scentoni on Wednesday April 24, 2019 @03:54PM (#19742042)
Laser interferometry will pinpoint the distance to far less than a millimeter.
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Re:5G Is Likely to Put Weather Forecasting at Risk by orbital-decay on Wednesday April 17, 2019 @10:34AM (#19682907)
Military spacecraft are almost always ahead of scientific ones, and a 100m foldable dish might be an engineering marvel, but it's a bit different from a 100m telescope. An actual telescope needs to guarantee that your data are correct within certain tolerance, and not just good enough to get a good SNR from another spacecraft.

Besides, as another poster noted, VLBAs need extremely precise positioning which is just another rabbit hole. Spektr-R orbit determination was really tricky for its baseline of 300000+ km.

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