Re: [art] Predictable Internet Time

[Joe Touch <[e-mail]>]

On 3/28/2017 1:12 PM, Tony Finch wrote:
> 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.)
Yes - mistyped (the doc is correct on that - it says UT1 is the
particular mean that is in common use).

>
>> 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

The GPS spec guarantees that its max deviation is 25 ns AFAICT (see the
cited ref in the doc).

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

It's in the definition of NTP strata. Stratum 0 is the a reference clock
itself; stratum 1 is connected to the clock directly, stratum 2 is
connected to stratum 1 over the net. I'll dig up a specific citation for
that...

>
>> 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.
It is guaranteed monotonic only while the system is running. I don't
know what "reliable" means, but it isn't guaranteed to be preserved
across a boot.

http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_getres.html

Joe