[localhost]

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

[mlyle]

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.