5 ms·

Huh, why's that?

• cperciva 10y ago
  It has to do with the geometry: GPS is measuring the differences between the distances to satellites which are mostly "overhead". In a sense it's similar to how our stereoscopic vision can resolve movement from side to side more accurately than movement towards or away from us. If you're at the top of a skyscraper you can mitigate this somewhat by looking at satellites closer to the horizon as well -- this solves the geometry problem -- but then you run into increased noise from the larger amount of atmosphere you're looking through.
  • tgb 10y ago
    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_(navigation) https://en.wikipedia.org/wiki/Dilution_of_precision_(navigat...
    • cperciva 10y ago
      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.
      • tgb 10y ago
        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?
        • cperciva 10y ago
          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.
          • tgb 10y ago
            Thanks I think that's the best explanation I've read!
            • cperciva 10y ago
              Thanks! It's unfortunate that the diagram on wikipedia is actively misleading since it corresponds to known distances rather than known differences of distances.