[ntp:questions] Garmin 18 LVC: whether to fudge

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Wed Feb 11 20:27:09 UTC 2009

Kevin Oberman wrote:
> Also, the jitter on your time is extremely high for PPS which should be
> < 1 us. I suspect that this is a system problem, but I can't be sure. I
> never see this much jitter on any of our PPS synced reference clocks (of
> which we have about 25 scattered across the US). If the jitter on any of
> my reference clocks using PPS exceeds .005, I consider this a big
> problem. I use FreeBSD which has excellent PPS support.

I have setup a bunch of FreeBSD based refclocks over the years, mostly 
Oncore UT+ timing receivers (15-50 ns) but also Gar 18LVC as well as an 
Endrun CDMA-based clock in the US.

I just got my homebuilt 18LVC back in operation today, after moving to a 
new office building: I use this the monitor all my official refclocks by 
configuring them as servers to my local box, so I can compare with the 
GPS results.

Since the Garmin isn't a timing-optimized receiver, I cannot put it in 
Zero-D like the Oncores, and it has a less than optimal window sill 
location (pointing NE in Oslo, at 60N), but I'm still getting offset 
mostly around +/- 2-3 us, and sub-5 us jitter.

PS. I had lost my original documentation on how to setup a server with 
this GPS, but a little Googling found lots of howto, including some that 
quoted my old description. :-)

- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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