[ntp:questions] Ignore one server except in extreme conditions?

David Lord 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 and the other is (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 
> accepted.
> 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).

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