Cost effective time servers

Mon Jun 24 18:14:19 UTC 2019

On 21 Jun 2019, at 10:57, Quan Zhou <quan at 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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