[ntp:questions] remote NTP server

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Jun 27 20:41:36 UTC 2008

kvdh wrote:
> RE: ntp serving.  How exactly does the server sync to information
> provided by a GPS receiver?   Does it simply read the RMC string or
> equivalent on a serial port

You generally need either the NMEA string or some other source of time 
to differentiate between seconds.  You can use just NMEA, but, with most 
receivers, that has a lot of jitter, and you will have to configure a 
large correction for systematic error.

> or does it also use the PPS as in interrupt and if the later is
> necessary, is that received via an SBC's parallel port bit.    Does

Generally PPS is read using a modem control line on the serial port.

Note that ntpd then phase locks the software clock to this information, 
so individual measurements will, typically, have a larger error than the 
time on the software clock (except for systematic errors).

> the server then make time information available on request?


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