[ntp:questions] Too much offset from GPS?

Unruh unruh-spam at physics.ubc.ca
Tue Mar 11 15:07:08 UTC 2008


chowalit.lab at gmail.com (chowalit.lab Chowalit Lab Linux) writes:

>Thanks all,
>      So It's wrong on my GPS (GPSmap60). Oh... So If I still use this
>GPS I should add offset with fudge option. Can I get other source from
>this GPS?

No idea what you are asking. GPS without a PPS (Pulse per Second) option
CANNOT ever give very good timing accuracy. Ever. It sends the signal down
a serial line ( which takes many many milliseconds) and does so somewhat
randomly. Ie, it is good for maybe 1/10 second accuracy, but you should not
trust it for more. IF you want better, get a gps with a PPS output (the
standard is the Garmin 18LVC which needs a bit of soldering to set up for
your system) and you can get microsecond accuracy.
You gps is good for navigating with. Use it for what it  is good for.



>Thank Again
>Chong

>On Sat, Mar 8, 2008 at 10:46 PM, Unruh <unruh-spam at physics.ubc.ca> wrote:
>> isw <isw at witzend.com> writes:
>>
>> >In article <Z_jAj.87237$C61.22584 at edtnps89>,
>> > Unruh <unruh-spam at physics.ubc.ca> wrote:
>>
>> >> Steve Kostecke <kostecke at ntp.org> writes:
>> >>
>> >> >On 2008-03-07, Hal Murray wrote:
>> >>
>> >> >>ATTRIBUTION MISSING wrote:
>> >> >>
>> >> >>>I have problem with my stratum1 box. My box include: freebsd
>> >> >>>5.2-RELEASE without PPS , ntp-4.2.0_1, Intel Pentium III, 40 G IDE,
>> >> >>>Germin GPSmap60 , connect with serial com. In GPSmap60 set as NMEA.
>> >> >>>Freebsd box can get NMEA input string. My configuration of ntp.conf
>> >> >>>like this:
>> >> >>>
>> >> >>>server 127.127.20.0 mode 1 prefer minpoll 4
>> >> >>>server  127.127.1.0 minpoll 5
>> >> >>
>> >> >> Most NMEA devices (or at least all the ones I've worked with)
>> >> >> send their text with a significant offset.
>> >>
>> >> >It gets even better ... NMEA sentences are output by the GPS device
>> >> >when it is not busy. This means that there is no fixed relationship
>> >> >between the top of the second and the NMEA sentence. And there will be
>> >> >significant jitter.
>> >>
>> >> Ie, NMEA is good for getting the second, not for getting any better
>> >> accuracy (well maybe 1/10 sec) If you want accurate time get a gps with PPS
>> >> output.
>> >> Otherwise there is no advantage of NMEA over a network ntp connection.
>>
>> >Well, except for the "no wires" part...
>>
>> Then buy a gps with ppm. The GPS18LVC is only about $60. And you get
>> microsecond accuracy rather than 1/10 sec.
>>
>>
>>
>> >Isaac
>>
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