NTPsec flags my GPS as a falseticker

Hal Murray hmurray at megapathdsl.net
Thu Mar 19 20:08:05 UTC 2020

nick.burkitt at nanotok.com said:
>>Something fishy there.  I'd fix that just to make sure you are using the
>> correct lines from the config file.
> Yeah - I have no idea where that's coming
> from. The PPS and GPS devices  are each mentioned only once in my ntpd.conf
> file. 

It could be a bug in our code, but that seems like the sort of thing that 
would get noticed.

ntpd does scan a directory, something like /etc/ntp.d/, but I thought that 
added a log message if it found anything.  gpsd may have put something there.

>> The combination of 0 and 2 is a bit strange.  The usual pairing is 0 and 1
>> for the first GPS device and 2 and 3 for the second GPS device. 
> That one cost me a lot of time. :-) gpsd assigns two shared memory  segments
> to each device listed on its command line. Since my PPS device  is separate
> from my GPS device, I have to pass both devices on the gpsd  command line,
> GPS (/dev/ttyACMx) followed by PPS (/dev/pps0). gpsd  assigns shm unit 0 to
> the GPS and shm unit 1 to the (non-existent)  associated PPS, shm unit 2 to
> the PPS, and it's not clear what it does  with the shm unit 3. 

There is probably a way to tell gpsd how to find the PPS device.  My guess 
would be that it looks for /dev/pps0 when it opens the first serial port and 
uses if if it finds one, else does the ldattach dance to make it.

If nothing else, there is a HOWTO on setting up a Raspberry Pi and that has a 
separate PPS via GPIO.

> I'd like to understand why that is. So far, I've  managed to improve things a
> bit by adjust the GPS offset 

That's the mindist stuff in my second message.  I forgot to include this line 
from my ntp.conf

tos mindist 0.250                      # default is 0.001 (1 ms)

>>server prefer path /dev/ttyAMA0  mode 0x010011
>>fudge flag1 0            # disable PPS
>>fudge time2 0.600        # Fixup offset
>>server                    # PPS signal, needs prefer
>>fudge flag2 0            # Rising edge
> So you define your GPS and PPS as peer servers, rather than refclocks.  How
> does that affect the way ntpd handles them? 

No, vanilla refclocks as per the server/fudge lines.

These are my opinions.  I hate spam.

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