[ntp:questions] Odd offset for PPS DCD w/ Garmin GPS 18x LVC

unruh unruh at wormhole.physics.ubc.ca
Tue Mar 22 19:06:18 UTC 2011


On 2011-03-22, David J Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>
> "lellis" <larry.ellis at gmail.com> wrote in message 
> news:efc6e755-c213-4a10-90f9-420384001da3 at s18g2000prg.googlegroups.com...
>> On Win XP, I am seeing an odd positive offset of about 128ms on the
>> Atom driver, relative to other sources:
>>
>>
>>     remote           refid      st t when poll reach   delay
>> offset  jitter
>> ==============================================================================
>> *GPS_NMEA(1)     .GPS.            0 l    6   16  377    0.000
>> 0.373   0.919
>> xPPS(1)          .PPS.            0 l    5   16  377    0.000
>> 125.447   0.830
>> -clock.team-cymr 172.16.65.22     2 u   39   64   77   12.989
>> -41.044  44.960
>> +ntp.okstate.edu .USNO.           1 u   42   64   77   28.906
>> -18.762  25.784
>> +navobs1.wustl.e .GPS.            1 u   38   64   77   19.133
>> 1.819  19.599
>>
>> Normally, I just use NMEA with fudge, but that was also showing the
>> odd offset so I removed fudge and added the Atom driver to help debug
>> this.
>>
>> (Normally the status shows oPPS(1) but xPPS(1) is seen above for the
>> obvious reason.
>>
>> ntpd is Dave Hart release 4.2.7p138;  I have a serialpps-ppsapi-
>> provider.dll dated 6/6/2009.
>>
>> Any ideas or recommendations.
>
> Larry,
>
> You may want to revert to the 3.20 firmware for you GPS 18x LVC - see what 
> others have written:

A .128 or .3 second offset really seems strange for this cause.  I guess
it could be if the 1 second offset from the Garmin 18x only happens
occasionaly, and this is being averaged out. 


>
>   http://support.ntp.org/bin/view/Support/ConfiguringNMEARefclocks#Section_6.1.12.2.
>
>   http://support.ntp.org/bin/view/Support/ConfiguringGarminRefclocks
>
> and there is a somewhat flawed analysis here:
>
>   http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm#gps-18x
>
> (flawed in that I was being helped by a UNIX person, and the Windows 
> driver does not work in exactly the same way as we had assumed).
>
> Cheers,
> David 
>




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