[ntp:questions] NTP server with Garmin GPS 18 LVC

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Dec 19 08:03:46 UTC 2013

On 18/12/2013 19:59, Adrian P wrote:
> Thanks for the great site!
> I just noticed that mine is actually a GPS 18x LVC, as written on its
> back. It is temporary located on the inside window sill, in my
> apartment at the 3rd floor. So it can see only half of the sky,
> through the window. Later I plan to install it on the roof, but first
> I need to figure out how to route the cable outside of my apartment
> (need to drill one hole I suppose :).
>  From your graphs I could see that the best time keeper is the
> Raspberry Pi #1, running Linux/3.6.11 and the Adafruit MTK3339 indoors
> RX. Do I read them correctly?
> Everywhere is written that FreeBSD is the best OS for time keeper,
> since its kernel supports nanosecond accuracies. However, if GPS 18x
> LVC PPS signal has only microsecond accuracy, then I suppose there is
> no point on using FreeBSD, any linux would be okay as well. So what do
> you think, looking from NTP point of view only, is this nanosecond
> accuracy the only reason why someone would use FreeBSD, or there are
> some other reasons as well? I see you have experience with all kind of
> OSes.
> Many thanks,
> Adrian

Glad you like the site, Adrian!

Seeing half the sky is likely good enough for the 18x, but do beware of 
windows - some have a thin metallic coating to reduce IR incoming, but 
that can also reduce 1.57 GHz RF incoming as well!  Try with your mobile 
phone to see whether the better GPS signals are though the window or 
behind the wall!  There are GPS status programs for iPhone and Android.

The Raspberry Pi cards are all good timekeepers, largely independent of 
the GPS used (providing it offers PPS output).  The accuracy of the PPS 
is about one order of magnitude better than any of today's PCs can 
manage, so it's really a choice between soldering or not:

Soldering and has on-board antenna:

No soldering & puck antenna:

I haven't investigated why, from the graphs, Raspi-3 and Raspi-4 appear 
to be slightly worse.  Raspi-3 is also in active use as an ADS-B 
receiver and is sending data continually to two PC clients which process 
and display the data.  Raspi-4 runs a digital wall clock, but that 
doesn't take a lot of CPU or I/O, but it does mean that it's running a 
graphical display whereas all the others are SSH access.  I don't 
believe that the poorer performance is because of the particular GPS used.

In the past, FreeBSD was certainly reported to be better than Linux, but 
I don't believe that to be the case today.  Purely for time-keeping I 
see little or no difference.  Whether Free BSD has fewer variants than 
Linux, and whether that might make it easier to support may be 
considerations, but I'm not really competent to comment on those.

What accuracy do you need?  I'm after millisecond level on Windows PCs, 
which can be achieved with a PPS source, and should be even easier to 
achieve in Windows 8.1 (compared to 7 or earlier).  To sync a PC to the 
full accuracy which a time-keeping GPS can deliver would take 
considerable effort.  It would be interesting to know whether anyone has 
actually achieved that, and if so, how!
Web: http://www.satsignal.eu

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