[ntp:questions] Ignore one server except in extreme conditions?
snews at lordynet.org
Wed Nov 16 20:25:07 UTC 2011
A C wrote:
> On 11/16/2011 03:20, David Lord wrote:
>> A C wrote:
>>> Is there a way to configure ntpd to ignore a particular clock unless
>>> there is no other choice? What I'm thinking is to ignore the GPS
>>> receiver NMEA data and use only the PPS plus Internet servers for most
>>> of the synchronization. But if the Internet servers go down, accept
>>> the GPS NMEA data for seconds numbering and let the tick be controlled
>>> by PPS.
>> Doesn't ntp do that already?
>> If you have nmea + pps the pps will be used for sync.
>> If your gps + pps is down your internet servers will be used for sync.
> Not if PPS and NMEA are independent clock sources. Two lines in the
> config file, one is 127.127.22.1 and the other is 127.127.20.1 (not
> including the Internet servers). The PPS is synced almost all the time
> but the choice of clock sources moves around from Internet to NMEA. In
> my case the NMEA sentences wander but the NMEA clock is always listed as
> a member of the accepted clocks (the "+" indicator) even if its own data
> is so bad that another clock with similar offset and jitter has been
> labeled an outlier/false ticker.
*GPS_NMEA(2) .GPSb. 0 l 29 64 377 0.000 -52.888 11.213
offset: -0.000001 s
So are you saying that the -52.888ms is making a significant
contribution to the offset of -0.000001s ?
> Note I'm not talking at all about the GPS going away. I'm talking about
> the NMEA sentence wandering around dragging the clock with it because
> it's part of the overall computed solution. By way of example, one of
> the Internet servers in my peer list has an offset of 1.887 and jitter
> of 0.586 and is labeled an outlier. The NMEA data at that same moment
> has an offset of 21.863 and a jitter of 10.290 and it's labeled as
> What I'm looking to do is to ignore the NMEA in favor of the Internet
> clocks as long as they are available then fall back to NMEA in case the
> network drops out.
> Before anyone asks, the GPS itself is fine and functioning normally. It
> was designed as a PPS reference for a fixed location telemetry network
> over GSM. The PPS portion is very accurate but it sacrifices some
> stability in the NMEA data to achieve the PPS stability (PPS pulse
> generation is higher priority for the CPU than NMEA generation). Since
> the telemetry device was intended to be in a fixed location it wasn't
> considered important to have very precise NMEA data, only PPS pulses to
> control timing and slotting. Jitter on the PPS signal is reported by
> ntpd to be 0.061 ms (ntpd never shows jitter numbers lower than this for
> any source, must be something in the floating point calculations) and
> doesn't wander away from that (I can see the same thing on an
> oscilloscope, the pulse is quite stable).
More information about the questions