Hi, Sandip-- On Sep 11, 2015, at 8:05 AM, sandip gangakhedkar <sandipfl...@gmail.com> wrote: > 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.
OK. > 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 RS-232 serial port and USB are very different things when it comes to timekeeping. A real serial port has hardware flow control via signals like RTS/CTS which cause an interrupt and result in fast processing of PPS. USB ports are unable to deliver the same level of timekeeping quality. > - 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) Yes. > 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 > <http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm> Note that FreeBSD isn't Linux. The BSD family tree predates Linux by quite a bit; FreeBSD also has outstanding timekeeping and kernel PPS support already available by default. > 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. Switch over to a real serial port and you should have no problems getting better than ~1ms group synchronization. Heck, if you choose your upstream NTP servers well, you can even get a stratum-2 server without a refclock to maintain ~1ms accuracy, but that depends on having a decent net link also. Regards, -- -Chuck _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions