Re: Cost effective time servers

[Joe Abley <jabley () hopcount ca>]

On 21 Jun 2019, at 10:57, Quan Zhou <quan () posteo net> wrote:

> Yep, went through the same route until I figured out that GPS time is a bit ahead of UTC.

The clocks on the GPS satellites are set to GPST which I think (I'm not a time geek so this is going to make someone 
cringe) is UTC without leap seconds or other corrections relating to rotation of the earth.

However, the messages sent to GPS receivers include the offset between GPST and UTC as well as the GPST timestamp. The 
receivers can use both together to obtain a measure of UTC accurate to about 100 nanoseconds.

Seems to me (again, not time geek, stop throwing things) that the use of GPST is an internal implementation detail 
chosen because it's easier to adjust an offset that rarely changes than it is to adjust atomic clocks floating in 
space. The system (including the system-internal adjustment of GPST with the offset) still produces a reasonably 
accurate measure of UTC. Also I imagine occasional leap seconds causing GPS navigators to jump spontaneously to the 
left which is probably more amusing in my imagination than in real life.


Joe

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