3 ms·

There's some natural drift, and about yearly boost to keep them in their desired orbits https://space.stackexchange.com/questions/30752/do-operating-gps-satell

by pas 2y ago

There's some natural drift, and about yearly boost to keep them in their desired orbits https://space.stackexchange.com/questions/30752/do-operating-gps-satellites-ever-make-orbital-maneuvers-for-station-keeping https://space.stackexchange.com/questions/30752/do-operating... As far as I know - while their onboard clock is pretty good - they continuously get corrections from ground stations. (Once per day.) https://gpsbeam.com/ground-stations/ https://gpsbeam.com/ground-stations/ the "navigation message" broadcast by the sats is described here https://gssc.esa.int/navipedia/index.php/GPS_Navigation_Message https://gssc.esa.int/navipedia/index.php/GPS_Navigation_Mess...

prewett 2y ago
> their onboard clock is pretty good They use atomic clocks, so it's accurate to something like 1 part in 10^16, or about 1 second in a billion years! A friend of mine is working on the next generation, which will be even more accurate. In contrast, the force of gravity experienced by the satellite is known to much less accuracy. In fact, changes on a monthly basis due to rainfall/rivers and tides. The GRACE satellites measure the change, although I couldn't find any information on how accurate their measurements are, except that they can measure the distance between the two satellites to within a micrometer (10^-6), so substantially less precision than the clock!
- Reason077 2y ago
  Not quite that accurate. Apparently it’s more like 3 seconds per million years for a rubidium atomic clock, and 1 second per 3 million years for a hydrogen maser. (Caesium atomic clocks are somewhere in between) ESA use both types onboard Galileo satellites: https://www.esa.int/Applications/Satellite_navigation/How_the_Galileo_atomic_clocks_work https://www.esa.int/Applications/Satellite_navigation/How_th...
Reason077 2y ago
Amongst other things, the clocks need to be corrected for relativistic effects. Due to the net effect of both kinetic and gravitational time dilation, clocks onboard a satellite advance more quickly than they would on Earth (when observed from Earth). https://en.m.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Relativity https://en.m.wikipedia.org/wiki/Error_analysis_for_the_Globa...

by pas 2y ago

prewett 2y ago

> their onboard clock is pretty good They use atomic clocks, so it's accurate to something like 1 part in 10^16, or about 1 second in a billion years! A friend of mine is working on the next generation, which will be even more accurate. In contrast, the force of gravity experienced by the satellite is known to much less accuracy. In fact, changes on a monthly basis due to rainfall/rivers and tides. The GRACE satellites measure the change, although I couldn't find any information on how accurate their measurements are, except that they can measure the distance between the two satellites to within a micrometer (10^-6), so substantially less precision than the clock!

Reason077 2y ago
Not quite that accurate. Apparently it’s more like 3 seconds per million years for a rubidium atomic clock, and 1 second per 3 million years for a hydrogen maser. (Caesium atomic clocks are somewhere in between) ESA use both types onboard Galileo satellites: https://www.esa.int/Applications/Satellite_navigation/How_the_Galileo_atomic_clocks_work https://www.esa.int/Applications/Satellite_navigation/How_th...

Reason077 2y ago

Amongst other things, the clocks need to be corrected for relativistic effects. Due to the net effect of both kinetic and gravitational time dilation, clocks onboard a satellite advance more quickly than they would on Earth (when observed from Earth). https://en.m.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Relativity https://en.m.wikipedia.org/wiki/Error_analysis_for_the_Globa...