4 ms·

So many comments on here asserting that leap seconds are bad and should be abandoned and that it would be simpler if they didn’t exist, but… You realize this i

So many comments on here asserting that leap seconds are bad and should be abandoned and that it would be simpler if they didn’t exist, but… You realize this is about the GPS system, right? The thing about GPS satellites is that they are in space, orbiting around the earth. If the earth’s rotation speed changes their orbital speed doesn’t (not immediately, anyway). GPS absolutely needs to adjust for the variable rotational speed of the earth - otherwise the GPS coordinate grid would gradually move relative to the surface of the earth. So GPS doesn’t exactly need leap seconds but it really does care about how long a rotation of the earth takes which… amounts to the same thing.

daddylonglegs 5y ago
> So GPS doesn’t exactly need leap seconds but it really does care about how long a rotation of the earth takes which… amounts to the same thing. I don't think this is right. GPS explicitly uses a timebase that does not include leap seconds [1]. On the subject of the article: my modest and very reasonable proposal is that we apply a leap second every six months without fail, dithering between positive and negative leap seconds so as to remain close to sidereal time. That way we would flush out bugs every six months and wouldn't have them accumulate and hit us all at once. Or we could be boring and use TAI or GPS time as the system clock every where and apply leap second corrections when we go from the system clock (currently UTC) to local time. [1] https://en.wikipedia.org/wiki/Global_Positioning_System#Timekeeping https://en.wikipedia.org/wiki/Global_Positioning_System#Time...
- jameshart 5y ago
  I mean, the article is literally about GPS satellites transmitting leap second data as part of their messages. So yes, the basic clock is just counting seconds but leap seconds are a pretty important component of the GOPS model.
a1369209993 5y ago
> GPS absolutely needs to adjust for the variable rotational speed of the earth Sure, but that's a spacial thing, not something that's relevant to timekeeping. (There's technically a change in relativistic time dilation, but it's less than a rounding error - about five millimeters per second at the equator makes 1.000000000002388402 versus 1.000000000002388457 if I've got my math right. (The relativistic time dilation from the earth's rotation is itself a rounding error.))
- jameshart 5y ago
  And when you’re using GPS to find your location on the surface of the earth… spatial things matter, right? GPS is for figuring out your location, not what time it is. You can use it to figure out what time it is, but that’s a side effect not the goal.
fanf2 5y ago
So, GPS involves a few things: precise clocks that can be used to triangulate position; a model of where the satellites containing the clocks are in space; and a model of where the earth is in space. UTC has basically the same inputs (the clocks are on the surface instead of in orbit, but one of the main time labs [NIST] is in Colorado and its altitude needs to be taken into account), but the calculations and logistics, the timebase of the model, are all very different. So you can’t derive UTC from the GPS ephemeris, because there’s a human in the loop who decides when leap seconds happen. So GPS needs to include UTC as a separate signal, alongside the ephemeris and everything else. (In fact, if I remember correctly, GPS includes nanosecond-level corrections between GPS time and UTC, because atomic clocks are not completely perfect.)
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

daddylonglegs 5y ago

> So GPS doesn’t exactly need leap seconds but it really does care about how long a rotation of the earth takes which… amounts to the same thing. I don't think this is right. GPS explicitly uses a timebase that does not include leap seconds [1]. On the subject of the article: my modest and very reasonable proposal is that we apply a leap second every six months without fail, dithering between positive and negative leap seconds so as to remain close to sidereal time. That way we would flush out bugs every six months and wouldn't have them accumulate and hit us all at once. Or we could be boring and use TAI or GPS time as the system clock every where and apply leap second corrections when we go from the system clock (currently UTC) to local time. [1] https://en.wikipedia.org/wiki/Global_Positioning_System#Timekeeping https://en.wikipedia.org/wiki/Global_Positioning_System#Time...

jameshart 5y ago
I mean, the article is literally about GPS satellites transmitting leap second data as part of their messages. So yes, the basic clock is just counting seconds but leap seconds are a pretty important component of the GOPS model.

a1369209993 5y ago

> GPS absolutely needs to adjust for the variable rotational speed of the earth Sure, but that's a spacial thing, not something that's relevant to timekeeping. (There's technically a change in relativistic time dilation, but it's less than a rounding error - about five millimeters per second at the equator makes 1.000000000002388402 versus 1.000000000002388457 if I've got my math right. (The relativistic time dilation from the earth's rotation is itself a rounding error.))

jameshart 5y ago
And when you’re using GPS to find your location on the surface of the earth… spatial things matter, right? GPS is for figuring out your location, not what time it is. You can use it to figure out what time it is, but that’s a side effect not the goal.

fanf2 5y ago

So, GPS involves a few things: precise clocks that can be used to triangulate position; a model of where the satellites containing the clocks are in space; and a model of where the earth is in space. UTC has basically the same inputs (the clocks are on the surface instead of in orbit, but one of the main time labs [NIST] is in Colorado and its altitude needs to be taken into account), but the calculations and logistics, the timebase of the model, are all very different. So you can’t derive UTC from the GPS ephemeris, because there’s a human in the loop who decides when leap seconds happen. So GPS needs to include UTC as a separate signal, alongside the ephemeris and everything else. (In fact, if I remember correctly, GPS includes nanosecond-level corrections between GPS time and UTC, because atomic clocks are not completely perfect.)

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