10 ms·

Having an atomic clock behind your NTP server doesn't really gain you anything unless GPS is lost for an extended period of time. As long as a timeserver has a

Having an atomic clock behind your NTP server doesn't really gain you anything unless GPS is lost for an extended period of time. As long as a timeserver has a GPS lock, it's going to be disciplining the atomic clock the same way any other clock is disciplined, so you don't really gain anything. Even a cheapie timekeeping GPS chip can get down into the < 500ns accuracy range (and the Thunderbolt on my desk will get down to < 50ns). (You can gain a little accuracy if your timeserver itself is running its main clock (the one that drives the CPU and all the busses) off of a precision frequency reference, be it GPSDO or atomic clock, but that's a separate discussion) Oh, and if you're using NTP and not directly connected to a precision timesource (not over the network), you may as well give up on that level of precision anyhow. For that you need PTP.

dfc 9y ago
> As long as a timeserver has a GPS lock, it's going to be disciplining the atomic clock the same way any other clock is disciplined Not true, you need a GPS lock as well as PPS. Are you sure about 50ns?
- elfchief 9y ago
  The PPS is just something you get when the GPS chipset has a lock, though (and sometimes when it doesn't, depending on how you have it configured). Most non-timekeeping GPS chipsets give you a PPS output for free, and 100% of timekeeping GPS chipsets give you one. Pretty sure about 50ns. Datasheet for the Trimble Thunderbolt E: http://trl.trimble.com/docushare/dsweb/Get/Document-383329/022542-010B_Thunderbolt-E_DS_0807.pdf http://trl.trimble.com/docushare/dsweb/Get/Document-383329/0... ...second page, "PPS accuracy: 15ns (one sigma)" (I picked a larger number than the spec sheet simply so that hopefully nobody would go "oh, that's the ONE SIGMA value, lets argue".) My Thunderbolt, in a completely temperature-uncontrolled environment with ~20 degree daily swings, generally has a PPS accuracy of ±45ns, in what is pretty crap conditions (first rule of precision timekeeping is "temperature stability")
  - dfc 9y ago
    I should have been more specific. You need GPS, PPS and you need to correctly configure them. Its a bad idea if people think you can just plug a USB into a GPS and be good to go.

dfc 9y ago

> As long as a timeserver has a GPS lock, it's going to be disciplining the atomic clock the same way any other clock is disciplined Not true, you need a GPS lock as well as PPS. Are you sure about 50ns?

elfchief 9y ago
The PPS is just something you get when the GPS chipset has a lock, though (and sometimes when it doesn't, depending on how you have it configured). Most non-timekeeping GPS chipsets give you a PPS output for free, and 100% of timekeeping GPS chipsets give you one. Pretty sure about 50ns. Datasheet for the Trimble Thunderbolt E: http://trl.trimble.com/docushare/dsweb/Get/Document-383329/022542-010B_Thunderbolt-E_DS_0807.pdf http://trl.trimble.com/docushare/dsweb/Get/Document-383329/0... ...second page, "PPS accuracy: 15ns (one sigma)" (I picked a larger number than the spec sheet simply so that hopefully nobody would go "oh, that's the ONE SIGMA value, lets argue".) My Thunderbolt, in a completely temperature-uncontrolled environment with ~20 degree daily swings, generally has a PPS accuracy of ±45ns, in what is pretty crap conditions (first rule of precision timekeeping is "temperature stability")
- dfc 9y ago
  I should have been more specific. You need GPS, PPS and you need to correctly configure them. Its a bad idea if people think you can just plug a USB into a GPS and be good to go.