[ntp:questions] Strange jumps in PPM

A C agcarver+ntp at acarver.net
Wed Aug 21 06:53:05 UTC 2013

On 8/20/2013 23:33, mike cook wrote:
> Le 21 août 2013 à 08:03, A C a écrit :
>> Ok, so I saw the below event in the loops on my system which is
>> controled by ATOM, SHM, and several internet servers.
>> I suspect it has to do with the GPS PPS output glitching during
>> extreme satellite events (bad constellation arrangement, fringe
>> signal, etc.)
>> Is there a way to have ntpd ignore the PPS quickly if it spins out
>> of control?  The system recovers fairly quickly and the clock
>> doesn't go too far out but it would be nice to avoid these jumps.
> This symptom is common where a receiver's antenna is not well placed.
> I suspect that you do not have a 360° sky view and probably
> obstructions or reflecting objects ( buildings etc) in the signal
> paths. If that is the case and you can get a better antenna
> placement, that might fix it. I think modern receivers are better at
> excluding multi-path signals as I have worse results with a UT+ or VP
> than with a newer one.
> Another possibility would be to profile your horizon and configure
> the satellites used dynamically, selecting only those  with direct
> LOS.
> If you have a good uninterrupted sky view, maybe its something else.

Good thoughts.  I don't have the best sky view.  There's a mountain 
behind me and trees.  I'll have to try and redo the placement of the 
antenna and see what happens.  I don't have many options to configure 
the receiver for anything except an artificial global horizon (I can 
specify the minimum elevation but no more).

I do keep an eye on the signal from time to time.  I happened to be 
sitting at the machine when this particular event happened and there was 
no obvious loss of signal.  But then again multipath doesn't always show 
up as weak signal.  Need a longer cable first I think.  I may give it a 
shot this weekend.

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