Hi Depending on what you do or don’t consider GPS is somewhere in the 3 to 30 ns range without corrections and better than that with corrections. Your GPSDO has some internal “stuff” that also gets into the act.
Indeed if you went to a dual band receiver and used any of the many free correction services, you could get well below 3 ns. Over your 100 year span, 30 ns comes out to about 1x10^-17. Bob > On Jul 19, 2019, at 6:20 PM, donald collie <[email protected]> wrote: > > 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 <[email protected]> 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 <[email protected]> >> 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] >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
