[ntp:questions] Garmin GPS 18LVC Setup but questions on best way

Unruh unruh-spam at physics.ubc.ca
Fri Dec 26 18:50:56 UTC 2008


George R. Kasica <georgek at netwrx1.com> writes:

>I have one of these units running here under Red Hat Fedora Core 9 as
>follows based on a collection of a number of the on-line docs I've
>looked at. What I'd like to know is as follows:

>1) Is this working well or poorly?

>2) Is there any way I can use both the GPS time data AND the PPS
>signal without having to recompile the kernel as on FC9 its not that
>easy to do as it comes down as an RPM and I don't think the patches
>are built for it...

You can, but not with the standard ntp drivers. I run the gps nmea into the
serial port and the PPS into the parallel port, and use a program I wrote
to time the parallel interrupt and deliver the result to the shm driver.
But that might be a bit too complex for you.
 

>3) I'd like to share the GPS/PPS signals via gpsd with another Linux
>system if possible would that be usable or accurate enough or should I
>just synch off this one at stratum 2?

The gps/pps signals are hardware signals. What do you mean "share them?" If
you mean installing a splitter so that the same PPS signal is delivered to
the two machines, yes you can do that. If you mean something else you need
to say what.
The standard ntp network route will mean that the second clock will have
its time disciplined to about 20usec, instead of the 2usec that a direct
PPS signal would deliver. 




>3) Any other suggestions that anyone would care to make, I don't claim
>to be an expert here:

You need to say what you want. I could suggest that you give money to
Oxfam, but that might not accord with what you wanted to do. Ie, what are
you trying to do. 

>Thank you very much.

>George



>sym links from

>/dev/gps0 -> /dev/ttyS0
>/dev/pps0 -> /dev/ttyS0

>running the gpsd and the 18LVC off ttyS0 at 4800 baud settings on
>18LVC done as per quan web page docs to set NEMA and PPS auto Off etc.

>setserial /dev/ttyS0 uart 16550A port 0x03f8 irq  4 baud_base 115200
>spd_normal skip_test low_latency

>/usr/sbin/gpsd -b -n /dev/ttyS0

>./shmpps -d /dev/ttyS0 -s -l DCD -u 0 -c

>my ntpd.conf looks like:

>server 127.127.28.0 minpoll 4 prefer
>fudge 127.127.28.0 refid PPS flag3 1
> 
>server 0.us.pool.ntp.org
>server 1.us.pool.ntp.org
>server 2.us.pool.ntp.org


>And looking at ntpq stats shows me:

># ntpq -p
>     remote           refid      st t when poll reach   delay   offset
>jitter
>==============================================================================
>*SHM(0)          .PPS.            0 l   13   16  377    0.000   -0.056
>0.013
>-mighty.poclabs. 64.202.112.75    2 u  800 1024  377   12.243    2.336
>0.316
>-clock3.redhat.c 66.187.233.4     2 u  946 1024  257   59.126    2.760
>42.876
>-192.157.38.60   192.36.143.150   2 u  807 1024  377  119.111    1.110
>0.037

># ntpdc -c kern
>pll offset:           -5.7e-05 s
>pll frequency:        111.071 ppm
>maximum error:        0.009019 s
>estimated error:      1.3e-05 s
>status:               0001  pll
>pll time constant:    10
>precision:            1e-06 s
>frequency tolerance:  500 ppm
>-- 
>===[George R. Kasica]===        +1 262 677 0766
>President                       +1 206 374 6482 FAX 
>Netwrx Consulting Inc.          Jackson, WI USA 
>http://www.netwrx1.com
>georgek at netwrx1.com
>ICQ #12862186




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