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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