3 ms·

Sorry, but your comment is just nonsense. :( GPS time doesn't include leapseconds: It's effectively TAI with a different offset. GPS indeed has to correct for

by nullc 5y ago

Sorry, but your comment is just nonsense. :( GPS time doesn't include leapseconds: It's effectively TAI with a different offset. GPS indeed has to correct for all sorts of orbital stuff, but that is done with equations on top of a stable atomic timebase. (And it has to be that way-- each satellite's orbital parameters are different).

jameshart 5y ago
Knowing your location relative to the satellites is only so useful if you’re actually standing on the surface of the earth (or trying to fly to a particular point on its surface). You also need to know which way the earth is facing right now. And to do that you need to know how many times it’s gone round since some reference time - which varies, but amounts to ‘number of days taking into account smeared leap seconds’
- nullc 5y ago
  That isn't at all how the ephemeris data is represented in GPS. The satellite locations/headings are specified in earth-fixed geocentric coordinates. So, it's as if the earth was sitting still and the satellites were swarming around it. The effect of earths very slow changes in rotation speed would effect the accuracy, but the ephemeris data is periodically updated based on observations of satellite locations by ground stations, there are many larger sources of positioning error than changes in the earth's rotation. This is also why first fix from a cold (memory wiped or long offline) receiver takes a long time even when it knows your location, unless you have AGPS (e.g. via the cell network): it doesn't know what satellites are overhead and it has to do a brute force search before it can find the signals and download the ephemeris data. (There is a side channel that carries info on leap seconds, so that civil timing stuff can derrive UTC from the GPS time. ... but that isn't used by GPS itself at all). Here is a nice page that shows the current GPS time and notes that it does not include leap seconds: http://www.leapsecond.com/java/gpsclock.htm http://www.leapsecond.com/java/gpsclock.htm

by nullc 5y ago

jameshart 5y ago

Knowing your location relative to the satellites is only so useful if you’re actually standing on the surface of the earth (or trying to fly to a particular point on its surface). You also need to know which way the earth is facing right now. And to do that you need to know how many times it’s gone round since some reference time - which varies, but amounts to ‘number of days taking into account smeared leap seconds’

nullc 5y ago
That isn't at all how the ephemeris data is represented in GPS. The satellite locations/headings are specified in earth-fixed geocentric coordinates. So, it's as if the earth was sitting still and the satellites were swarming around it. The effect of earths very slow changes in rotation speed would effect the accuracy, but the ephemeris data is periodically updated based on observations of satellite locations by ground stations, there are many larger sources of positioning error than changes in the earth's rotation. This is also why first fix from a cold (memory wiped or long offline) receiver takes a long time even when it knows your location, unless you have AGPS (e.g. via the cell network): it doesn't know what satellites are overhead and it has to do a brute force search before it can find the signals and download the ephemeris data. (There is a side channel that carries info on leap seconds, so that civil timing stuff can derrive UTC from the GPS time. ... but that isn't used by GPS itself at all). Here is a nice page that shows the current GPS time and notes that it does not include leap seconds: http://www.leapsecond.com/java/gpsclock.htm http://www.leapsecond.com/java/gpsclock.htm