[rescue] In Realtime: Saving 25, 000 Manuals ??? August 15, 2015

[Bob Darlington]

Well, gps is pretty good but you still need to keep it temperature stable
as well as your computer talking to it for it to get that kind of
stability.  Long term, sure, it'll asymptotically approach a cesium beam
because that's what it uses as a reference.  Check it today, then come back
in 10 years and it'll be within a small fraction of a second away from what
the cesium beam says.  That's because it's constantly checking and
comparing itself.  It gets interesting in the sub 1 second accuracy
measurements, second to second.  Nanosecond level measurements over a multi
year span might be useful for something I guess.   But nanosecond level
accuracy in a second or small number of seconds is much more useful for
more interesting things.  TDOA direction finding, radio telescopes,
positioning (which is why gps uses atomic clocks), and the like.

You CAN absolutely make very high quality clocks on the cheap at home but
you do need to deal with temperature swings.  My oscillators are very good
thermometers.

Check the time-nuts archives at https://www.febo.com/pipermail/time-nuts/

Join if it's up your alley.  I recommend lurking for 6 months to a year to
get a feel for it.  The whole point was to get a high SNR and when that
doesn't happen, people start to very vocally complain.  NTP discussions are
distractions much of the time.  Not always!  But much of the time.  It just
depends on which weird aspect is being discussed.  The list is very
hardware centric but software discussions happen regularly too.

-Bob

On Fri, Aug 21, 2015 at 11:21 AM, Phil Stracchino <phils at caerllewys.net>
wrote:

> On 08/21/15 13:11, Bob Darlington wrote:
> > But what's useful?  Do our filesystems use anything better than one
> second
> > precision?  Surely a few hundred microseconds won't matter.  And it
> doesn't
> > unless you're a time-nut.    These guys are used to measuring distance in
> > nanoseconds (in particular, wire and cable lengths).  How temperature
> > effects measurements.  It's very common for me to notice significant
> > differences in measurements solely because of a temperature change in my
> > lab.   It's a pain.
>
> I hear you.  Practically speaking, it makes little difference whether
> all of the (sync-able) clocks in the house are synced within a second or
> within .01 second.
>
> But from a geekish "Because I can" standpoint, and honestly from
> perfectionism, I'd be much happier if they were all synced within at
> least single-digit microseconds, and to a reference source that I know
> remains accurate to at least that level even if my uplink goes down.
> GPS starts to look really attractive from this standpoint because it can
> provide near-atomic-clock levels of timekeeping accuracy for a
> thousandth of the cost, and without requiring an environmentally
> controlled isolation room.
>
>
> --
>   Phil Stracchino
>   Babylon Communications
>   phils at caerllewys.net
>   phil at co.ordinate.org
>   Landline: 603.293.8485
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