5 ms·

I've noticed the same thing, but I'm always surprised by it. Particularly with smartphones. A GPS is basically just a very, very precise clock and a radio rece

by jofer 9y ago

I've noticed the same thing, but I'm always surprised by it. Particularly with smartphones. A GPS is basically just a very, very precise clock and a radio receiver. Why don't smartphones use the GPS clock? I'm sure there's a good reason (and some probably do), but it sill surprises me. (Presumably it's battery related -- the GPS clock can't be used unless GPS is fully enabled or something similar.)

Lx1oG-AWb6h_ZG0 9y ago
The precise clock is located on the satellites, not receivers. The receivers figure out the correct time by listening for the signals from multiple satellites and matching them against each other and it’s own internal tables. This is the expensive part.
- Piskvorrr 9y ago
  Indeed, but the GPS fix you're getting is 4-dimensional: if you have position, you also have precise time. Therefore - listening for the GPS time is not necessary all the time, just update when a precise-enough fix is achieved.
  - deleted 9y ago
    [deleted]
  - monochromatic 9y ago
    Probably cheaper in terms of battery life to just use NTP than to periodically get a GPS fix.
    - Piskvorrr 9y ago
      Of course. I was thinking "when another app is requesting a GPS fix anyway, might as well use the temporal part, as you're getting it for free."
    - Dylan16807 9y ago
      It's worth noting that to get time with .1s accuracy you don't actually need a GPS fix, just to pick up the 50-bit-per-second navigation signal for a couple seconds, from one satellite.
      - monochromatic 9y ago
        How would that work unless you know the distance to the satellite?
        p1mrx 9y ago
        If you're on Earth, you can assume that the satellite is closer than 30000 km or so. Light takes 0.1s to cross that distance.
        Dylan16807 9y ago
        The distance to an overhead GPS satellite is somewhere in the range of .07-.09 light-seconds. The planet's width is basically irrelevant when your goal is .1 second accuracy.
        nucleardog 9y ago
        Based off of some quick and dirty calculations for a GPS satellite at approximately 20,000km above sea level, the difference in distance between a satellite directly overhead and one straight out over the horizon is around 5500km. That's actually only about 18ms difference in delay. Unless there's something I'm failing to account for here, you should be able to get more than 0.1s precision based off of a single satellite's signal. Subtract a flat ~75ms to account for the minimum delay (~66ms @ 20,000km) and half of the variable delay, and you shouldn't really be more than about 9ms out.

by jofer 9y ago

Lx1oG-AWb6h_ZG0 9y ago

The precise clock is located on the satellites, not receivers. The receivers figure out the correct time by listening for the signals from multiple satellites and matching them against each other and it’s own internal tables. This is the expensive part.

Piskvorrr 9y ago
Indeed, but the GPS fix you're getting is 4-dimensional: if you have position, you also have precise time. Therefore - listening for the GPS time is not necessary all the time, just update when a precise-enough fix is achieved.
- deleted 9y ago
  [deleted]
- monochromatic 9y ago
  Probably cheaper in terms of battery life to just use NTP than to periodically get a GPS fix.
  - Piskvorrr 9y ago
    Of course. I was thinking "when another app is requesting a GPS fix anyway, might as well use the temporal part, as you're getting it for free."
  - Dylan16807 9y ago
    It's worth noting that to get time with .1s accuracy you don't actually need a GPS fix, just to pick up the 50-bit-per-second navigation signal for a couple seconds, from one satellite.
    - monochromatic 9y ago
      How would that work unless you know the distance to the satellite?
      - p1mrx 9y ago
        If you're on Earth, you can assume that the satellite is closer than 30000 km or so. Light takes 0.1s to cross that distance.
      - Dylan16807 9y ago
        The distance to an overhead GPS satellite is somewhere in the range of .07-.09 light-seconds. The planet's width is basically irrelevant when your goal is .1 second accuracy.
      - nucleardog 9y ago
        Based off of some quick and dirty calculations for a GPS satellite at approximately 20,000km above sea level, the difference in distance between a satellite directly overhead and one straight out over the horizon is around 5500km. That's actually only about 18ms difference in delay. Unless there's something I'm failing to account for here, you should be able to get more than 0.1s precision based off of a single satellite's signal. Subtract a flat ~75ms to account for the minimum delay (~66ms @ 20,000km) and half of the variable delay, and you shouldn't really be more than about 9ms out.