Time Card and PTP on a Raspberry Pi Compute Module 4
jeffgeerling.com
Time Card and PTP on a Raspberry Pi Compute Module 4
1–10 of 35 posts
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#2Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#3Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?
Yes.
For professional setups, typically you'd have a coax going from the roof somewhere to the rack inside the data center where your time server is located.
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#4Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?
I guess that's the point of the highly accurate onboard clock. Even if you don't have GPS reception for some period of the day, as long as it does connect at least once every two days you'll still get accurate timekeeping.
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#5Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?
Yes, you need to connect an antenna that usually then gets mounted somewhere outside.
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#6Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?
For non-professional setups, if you have a decent GNSS module/antenna then near a window works fine.
I built a clock with a GNSS module and it gets enough of a signal by the window to keep time fine but I wouldn't rely on it.
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#7Is anyone using PTP and its requisite accuracy in production? If so, what's your use-case?
Our central NTP servers are getting, per "ntpq -c kerninfo", sub-millisecond error:
$ ntpq -c kerninfo
associd=0 status=0615 leap_none, sync_ntp, 1 event, clock_sync,
pll offset: -0.257641
pll frequency: 7.5157
maximum error: 0.165426
estimated error: 0.000422
[…]
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#8Is anyone using PTP and its requisite accuracy in production? If so, what's your use-case? Our central NTP servers are getting, per "ntpq -c kerninfo", sub-millisecond error: $ ntpq -c kerninfo associd=0 status=0615 leap_none, sync_ntp, 1 event, clock_sync, pll offset: -0.257641 pll frequency: 7.5157 maximum error: 0.165426 estimated error: 0.000422 […] That's talking to a couple of Stratum 1, and a bunch of S2, serv…
Have you tried Chrony instead of Ntpd ?
System time : 0.000084656 seconds slow of NTP time
Last offset : -0.000029673 seconds
RMS offset : 0.000046217 seconds
Frequency : 17.760 ppm slow
Residual freq : -0.001 ppm
Skew : 0.012 ppm
Root delay : 0.001419884 seconds
Root dispersion : 0.000967303 secondsRe: Time Card and PTP on a Raspberry Pi Compute Module 4
#9Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?
Most datacenters have the GPS antennas on the roof for this reason.
Re: Time Card and PTP on a Raspberry Pi Compute Module 4
#10Maybe a stupid question; if you're using GNSS as a time source, is it difficult to get reception? Do you have to put the receiver near a window or something? Or is it able to get signal even in cases where, for instance, my GPS watch would fail to get a lock?
> is it difficult to get reception? Do you have to put the receiver near a window or something?
The answer to that is "it depends".
Windows are known to work, but not all windows are made equal. Which direction your window is facing and whether you are in an urban canyon can make significant difference. Some windows also have films or coatings.
If you are working in tricky situations, then the quality of your antenna and the quality of your receiver will make a difference (e.g. multi-constellation support, interference mitigation, multipath rejection etc. etc.).
If you do not have access to a workable window, then your alternative options are:
- Accept slightly lower accuracy and use LW radio instead (e.g. MSF, DCF etc.)
- Rent a leased line to your country's national time laboratory. They will deliver you certified, traceable time with no roof access required and microsecond accuracy (backed by an SLA, i.e. accurate or your money back).
- Run a long cable to the roof (if money is no object you can even get super fancy setups that run exclusively on fibre, no coax in sight)
- Install an internal GNSS repeater system (still involves roof install, but gives you *much* more flexibility indoors for obvious reasons).