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Comment by stinky613 | Hacker News Reader

I think that would depend on how precisely they need to be calibrated, right? IIRC even military GPS is still only accurate to within a few feet.

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RTK GPS is 1cm RMS. And I still can't get over how awesome it is. It's magic.

It’s 1cm in altitude! In lat long, it can be better. It really is awesome.

I had no idea what RTK was before your comment. Thanks! Here's a link[1] for others who might be curious. Looks like the key is being within ~6mi of a RTCM (Radio Technical Commission for Maritime) station that transmits GPS correction data. If not, the cool thing is that you can create your own station as well by daisy-chaining off of an existing station that's within 6mi.

[1] https://learn.sparkfun.com/tutorials/what-is-gps-rtk/all

For these purposes, you could just have a single reference receiver at the site.

That is, with two GPSes, you can measure their vector distance very precisely by correlating their measurements.

But it doesn't help you precisely measure angles, which is a big part of this problem.

Survey quality GPS is able to have centimeter accuracy. https://en.wikipedia.org/wiki/Real-time_kinematic

But centimeter accuracy is of limited value. It doesn't tell you how the mirror base inclination has changed due to subsidence and thermal effects.

If every mirror has drifted a small angle, and you're trying to point them all at a very small target-- it's hard.

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