[ntp:questions] Synchronize distributed PCs with GPS 1 PPS and NTPd for OWD measurements

sandip gangakhedkar sandipfloyd at gmail.com
Fri Sep 11 15:05:29 UTC 2015


Hello,

I am trying to sync the system clocks of two desktop/laptop computers
within sub millisecond accuracy.

The goal is to have a setup for measuring One Way Delay of UDP packets
between two moving nodes, over a long-range wireless network.

My plan is to set up the two machines as Stratum-1 NTP servers using 1PPS
GPS output over RS-232 and NTP.

Specifically, I want to:
- Use an external GPS receiver with 1 PPS output and interface it via
RS-232/USB port to the desktop/laptop
- Use LinuxPPS or some other Kernel-level PPS interfacing library to
receive the pulses from the Serial port.
- Compile ntpd to use the PPS source for sync.
- Patch the Linux kernel if needed (RT_PREEMPT and PPS related stuff)

The main problem I am facing is how to choose a GPS receiver that will give
the required acuracy and fit my budget (< EUR 150 a piece). From the foll.
resources, it seems that GPS-based disciplining for NTP is possible in both
Windows and Linux:
http://www.satsignal.eu/ntp/NTP-on-Windows-serial-port.html
http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm

Ideally I would prefer to use a receiver that provides an accurate 1PPS
pulse over USB that can enable absolute accuracy of the computer system
clock under 1ms.

Any suggestions/comments on my approach are much appreciated.

Thanks,
Sandip


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