[time-nuts] Clock accuracy

[donald collie]

Thanks to all who replied! It looks like the antioxidants will win and the
clock will fail before the 100 years are up. Assuming the "accuracy" of the
GPSDO is 1 part in 10^12 then the inaccuracy after 100 years will be up to
: 60x60x24x365.25x100x1x10^-12= 3ms [approximately] - which is probably
good enough for an old fella. I have to admit that I have an ulterior
motive for asking this question : I wanted to know what sort of long term
accuracy I could expect from the GPS constellation - looks as if 1 part in
10 to the 12th is about right.
Cheers!...............................................................................Donald
C.

On Sat, Jul 20, 2019 at 7:01 AM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> The simple answer is that your clock is locked directly to a set of time
> sources built
> into the GPS satellites. Those sources are corrected by ground stations
> via comparison
> to NRL and NIST (and indirectly other sources as well). The various ground
> reference
> time systems get measured and evaluated to form what we call the right
> time. This
> is done by BIH in Paris. That process also keeps NRL and NIST “in sync”
> with the correct time.
>
> Since everything is locked together, there really isn’t any long term
> drift. As long as
> everything is functioning (and the PPS is from GPS not some random
> divider) you
> should be “on time” to within 100 ns pretty much forever. The time
> involved could
> be GPS time or UTC depending on how you associate time stamps with your
> PPS edges.
>
> If indeed something goes wrong with GPS ( as unfortunately happened to
> Galileo
> very recently), your time could be just about anything if the error is
> undetected. If
> it is detected, your will go into holdover. The drift then depends very
> much on just
> what “Trimble” you have inside your setup. 10 us a day for the first day
> is not an
> uncommon number to see. Since it’s really frequency drift rather than time
> drift,
> the second day will be worse and it just goes downhill from there.
>
> If your PPS *is* from some random divider off of (say) 10 MHz, then every
> time power
> goes out, it will come back up at a random point in the second. If you
> punch
> a button to “sync” it, you will only be able to move it in 100 ns steps (
> the period
> of 10 MHz). If the 10 MHz edge is “right on” with GPS that’s fine. If it’s
> off by some
> random amount ….. not so fine.
>
> This gets into a vary basic gotcha: A typical GPSDO *does* get the output
> PPS from
> the 10 MHz. The PPS output direct from a GPS module probably is closer to
> “on time”
> that the GPS PPS. It will bounce around a lot more, but it likely is
> closer to being correct.
>
> Lots of twists and turns …...
>
> Bob
>
> > On Jul 19, 2019, at 1:17 AM, donald collie <donaldbcollie at gmail.com>
> wrote:
> >
> > Without wanting to show my ignorance by confusing accuracy, and
> precision,
> > etc, would some kind person please answer the following : Let me explain
> -
> > I have my prototype GPS diciplined [ Trimble inside] standard frequency
> > source connected to both a divide by 5,2,5 and 2 producing all the
> > reference frequencies necessary for the various bits of equipment in my
> > workshop, AND the 1pps
> > output connected to a 7474 "T" flipflop and thence via a 100uF capacitor
> to
> > a modified $10 analogue wall clock. Can anybody tell me this : If I live
> > another 100 years [Let`s say I take antioxidants ;-)  ] what sort of
> error
> > should I expect in this clock? [I know that it`s better than 1 second per
> > day]
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