Re: [art] Predictable Internet Time

[Tony Finch <[e-mail]>]

Joe Touch <[e-mail]> wrote:

> Section 3 states that in the definition of a solar day. Section 4.2
> explains that UT0 is one of many different "means", and that UT0 is the
> most common in current use.

No, UT1 is the most common form of mean solar time. (UT0 is basically
mean solar time at a particular observatory; UT1 is corrected for polar
motion so it is globally consistent.)

> GPS isn't one of the clocks averaged into TAI (AFAICT); it's a separate
> source that's sync'd to TAI to ensure the 25 ns max delta.

Well, GPS is an ensemble clock (each sat has its own clock) which is
steered with reference to UTC(USNO). UTC(USNO) feeds into the BIPM paper
clocks.

The USNO says that GPS's maximum deviation from UTC(USNO) is 1us though
in practice it is within a few hundred nanoseconds. The NAV message has
additional information that allows GPS receivers to get the UTC(USNO) time
to within 40ns.

http://www.usno.navy.mil/USNO/time/gps/usno-gps-time-transfer

> > - It is not clear to me why NTP would differ 100ms from TAI.
>
> It's in the spec.

Where in which spec?

> Uptime is just a printout of the delta of the POSIX time when a computer
> started and the current POSIX time.

CLOCK_MONOTONIC cannot be reset, so it is more reliable than that.

Tony.
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