[ntp:questions] Reasons of NTP not to use GPS source

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Sep 17 09:51:20 UTC 2013

On 17/09/2013 09:05, Igor Pavlov wrote:
> Thanks, Brian.
> I'll do all these things.
> Now will try to fix problem with PPS: it's level 3.3V, and serial port
> seems to not recognising such low level.
> Now playing with time2 parameter and my GPS now stoped getting "x".
> I can't understant one thing.
> Should system time be fluently corrected by NTP so in some term offset of
> active ("*") source would be close to 0?

You can check whether the serial port is seeing the 3.3V PPS signal with 
my program:


Note that a PPS signal of at least several microseconds duration is 
required by the PC, and perhaps 100 milliseconds or more to be visible 
on my program.  Most GPS use 100-200 ms for the pulse, which is adequate 
for seeing seen by eye.  NTP would need to be stopped while running my 
program.  On checking the manual for your device:


it seems that the maximum length of the PPS pulse is only 2 
milliseconds, and the default is just 1 millisecond, so that may not 
show on my program, but should be seen correctly by the PC using the DCD 

Yes, the offset for the selected source should be close to zero (+ or 
-).  Checking on my Windows stratum-1 PCs right now:

XP: offset 0.086
Win-7/64: offset -0.032
Win-8/64: offset -0.009

There is nothing "special" about those values, just an indication of the 
sort of offsets you might see.  My FreeBSD and Linux stratum-1 PCs show:

FreeBSD: -0.001
Raspberry Pi 1: 0.002
Raspberry Pi 2: 0.001
Raspberry Pi 3: 0.001
Raspberry Pi 4: -0.001 (user-mode PPS)

The offsets on Windows PCs being synced from the Internet will be /much/ 
larger, perhaps more than 10 milliseconds.
Web: http://www.satsignal.eu

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