[time-nuts] Clock accuracy

[Bob kb8tq]

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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