[ntp:questions] using both a PPS source and a HBG (dcf-77-like) timesource?

Dennis Hilberg, Jr. timekeeper at dennishilberg.com.invalid
Mon Jan 14 18:38:15 UTC 2008


Folkert van Heusden wrote:
>>> Well if I remember correctly someone said to me once that the
>>> time-string returned by cheap gps device (like my garmin 18 lvc)
>>> sometimes is a bit off while its PPS signal is fine.
>>>
> 
>>> Currently I'm syncing against NMEA/PPS and seeing quiet a big offset:
>>>
> 
>>>      remote           refid      st t when poll reach   delay   offset 
>  jitter
>>> =====================
> ====================================================
>>> xGPS_NMEA(0)     .GPS.            0 l    7   16  377    0.000  -185.08 
>   1.001  <---
>> <snip>
>> You can even out that delay in the GPS time by using the 'fudge' option. 
> 
>> Example:
>> server 127.127.20.0 minpoll 4
>> fudge  127.127.20.0 time1 0.185
>> I do the same thing with my GPS 18 LVC, only I use gpsd with the SHM driv
> ers.
> 
> Ok but the odd thing is: a friend of mine has the exact same garmin 18
> lvc but not this big offset?

I'm sure the offset isn't exactly the same on each GPS 18 LVC, and probably 
also depends on which and how many NMEA sentences its outputting.

I have mine fudged to 0.190, so you are right in the ballpark.  The offset 
won't be consistent, but close enough.  It doesn't have to be perfect, only 
within a few ms so that ntpd can use it as a time source.  An 'ntpq -pn' on 
my server:

saturn:$ ntpq -pn
      remote           refid    st t when poll reach   delay   offset  jitter
============================================================================
+127.127.28.2    .GPS.          0 l    5   16  377    0.000   -5.397   5.299
*127.127.28.3    .PPS.          0 l    7   16  377    0.000   -0.007   0.004
-140.142.16.34   .USNO.         1 u  461 1024  377   17.182    0.180   6.635
-132.249.20.88   .WWVB.         1 u  837 1024  377   44.128    0.621   3.173
-216.218.254.202 .CDMA.         1 u 1850 1024  376   35.322    0.252   2.332
+64.183.55.54    .GPS.          1 u  460 1024  377   59.748   -1.279   4.186

> Folkert van Heusden

Dennis

-- 
Dennis Hilberg, Jr.     \  timekeeper(at)dennishilberg(dot)com
NTP Server Information:  \  http://saturn.dennishilberg.com/ntp.php




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