Re: NTP and PPS (and gpsd etc..)

Jason Rabel wrote:
David,

I have a Jupiter 8 unit that I am using the Jupiter Refclock and it works
perfect. However I looked over the specs for the Jupiter 21 and it only
supports SiRF and NMEA. AFAIK there is no SiRF refclock.

With the older Jupiter modules you have to use them in their Binary mode,
using them in NMEA can cause a 1-2 second offset because of the delay in
processing NMEA output from the module. I don't know if that is still true
for the newer modules, but I would assume it's entirely possible. Even if
the time info was wrong you can use just the PPS signal from the GPS via the
PPS/ATOM refclock, then use another NTP server for the general time info.

The catch here is: exactly what OS are you running? You said "linux embedded
machine", which by default linux does not support a PPS signal. You have to
patch the kernel to get PPS support. On the other hand FreeBSD supports PPS
and also has nanosecond precision.

Jason

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Ops, i'm sorry...i use a machine with thinstation 2.2 system and a linux
2.6.16.5 (i have patched the kernel with de Giometti's patch..).
Now i try to use NMEA with ntp 4.2.4 (Udo van den Heuvel's ntpd NMEA
patch) <http://time.qnan.org/linux2.6-pps/ntpd-nmea.patch>. I find this
procedure on http://time.qnan.org/.

I must debugging some code because i have big problem with
time_pps_create function on time_pps.h (refclock_nmea.c) and i have
created a "personal" patch (the old code try to use a inexistent filedes
even if in the comment say "/* We simply ignore "filedes" */")

Now i have a jitter that sometimes "go away" but i think it's possible
that in some case i lost a satellites cover..

Thanks to all.

--
David Anullo

Itaco S.r.l.
Via Zoe Fontana, 220 Ed. B1
00131 ROMA - ITALY
Tel. +390641294922 - Fax +39064191740
www.itacosystems.com

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