[gonzo41]

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

[mlyle]

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.