[ntp:questions] pps coming in, not received by ntpd

David Taylor david-taylor at blueyonder.co.uk.invalid
Sat Jun 29 18:41:14 UTC 2013


On 29/06/2013 18:57, folkert wrote:
> Hi,
>
> Something odd is happening.
> I've connected a garmin 18 lvc to a pc.
> The system has a kernel with pps support and i verified it comes in:
>
> root at hoei:/home/folkert# cat /sys/devices/virtual/pps/pps0/{assert,clear}
> 1372528130.997131540#54
> 1372528131.097120120#54
>
> I'm running gpsd and it says the gps has a fix and it also has all
> appropriate devices open:
>
> gpsd      27762           nobody    7u      CHR               4,65 0t0       1144 /dev/ttyS1
> gpsd      27762           nobody    9r      CHR              251,0 0t0      82440 /dev/pps0
>
> gps is started with '-n'
>
> ntpd is configurated to listen to the shared memory segments:
>
> server 127.127.28.0 minpoll 4
> fudge  127.127.28.0 refid NMEA
> server 127.127.28.1 minpoll 4 prefer
> fudge  127.127.28.1 refid PPS
>
> Both nptd and gpsd look at the first 2 segments:
>
> ------ Shared Memory Segments --------
> key        shmid      owner      perms      bytes      nattch     status
> 0x4e545030 0          root       600        96         2
> 0x4e545031 32769      root       600        96         2
> 0x4e545032 65538      root       666        96         1
> 0x4e545033 98307      root       666        96         1
> 0x47505344 131076     root       666        8240       1
>
> The PPS seems to be seen by gpsd:
> root at hoei:/home/folkert# strace -fp 27872 2>&1 | grep ioctl
> ...
> [pid 27873] ioctl(9, PPS_FETCH, 0x7f1e63cc9d00) = 0
>
> and the NMEA sentences are processed correctly:
>
> x127.127.28.0    .NMEA.           0 l   10   16  377    0.000  -275.55 4.421
>
> but the pps isn't:
>
>   127.127.28.1    .PPS.            0 l    -   16    0    0.000    0.000 0.000
>
> Anyone any idea?
>
>
> Folkert van Heusden

Folkert,

Not an area I have a lot of expertise in, but two suggestions:

- shouldn't the NMEA have the "prefer", and not the PPS?  I am uncertain 
about this, but if this is the case, I have this wrong on my Web page.

- I would try to include a time1 fudge factor of 0.275 into the NMEA 
configuration, so that the offset had a mean value near to zero.  I 
presume you are comparing this to a reasonably good known reference - 
perhaps one of you other stratum-1 servers.

I hope that helps.
-- 
Cheers,
David
Web: http://www.satsignal.eu



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