Hmm, I still don't see it. The comparison to stereoscopic vision seems misplaced: we judge side-to-side motion by an entirely different (and non-stereoscopic) mechanism than depth, and neither mechanism is mathematically very similar to that of GPS. And my understanding is that GPS receivers compute their distance to various satellites, which means if all the satellites were all nearly overhead, I would expect the vertical component to be the best understood.

I Googled and found some vague and unsatisfying explanations, but, luckily, wikipedia tells us how to do the math: https://en.wikipedia.org/wiki/Dilution_of_precision_(navigat...

You judge depth not from a single image but from the difference between two images. Similarly, GPS works not by measuring signal arrival times, but by measuring the differences between arrival times.

Good point, but I'm afraid the analogy still isn't spelled out enough for me to actually make sense of it. Like why does that make a horizontal/vertical discrepancy?

If you move horizontally, you're getting closer to one satellite and further away from another -- the changes in GPS signals add, just like if something you're looking at moves from side to side. If you move vertically, you're getting closer to or further away from all the satellites at once; but since GPS relies on the differences between signal arrival times, that's much harder to pick up.

Thanks I think that's the best explanation I've read!

Thanks! It's unfortunate that the diagram on wikipedia is actively misleading since it corresponds to known distances rather than known differences of distances.