4 ms·
They do (that's literally how GPS/GNSS works), but they drift, for various reasons. One of the parameters that GNSS networks broadcast is a clock correction (ho
by elfchief 7y ago
They do (that's literally how GPS/GNSS works), but they drift, for various reasons. One of the parameters that GNSS networks broadcast is a clock correction (how far off each individual satellite's clock is, how fast it's changing, and how fast how fast it's changing is changing), and those parameters come from the ground stations.
It seems like a little drift shouldn't make that much difference, but keep in mind that the speed of light in a vacuum (or air) is about 1 foot per nanosecond, so for every nanosecond one of those clocks has drifted, that's a foot of positional accuracy you no longer have...
- Salgat 7y agoBut GPS can run up to 60 days with no ground station contact at all.
- elfchief 7y agoNominally, yes, but the positioning error (according to the spec) is expected to be up to 425m after 14 days, growing worse over time until it hits a positioning error of ~10km at 180 days. (later generation satellites have an 'autonav' mode that presumably makes this less severe (by using SV-to-SV ranging and such so they're not completely blind to changes), but Galileo may not have that capability or there might not be enough SVs to use that capability yet)