On 27/10/07 07:35, Dennis Hilberg, Jr. wrote: > I had a similar discussion on the list recently about the GPS 18 LVC. Here > is what Rob Janssen had to say about gpsd: > > Rob wrote: > > The message-based time (SHM(0)) is very inaccurate because of > deficiencies > in most GPS receivers and their comms protocols. It is > completely unclear > what time the timestamp in the message refers to, and > usually it jitters > > wildly. > > The PPS output pulse is accurate, and the message-based time is only > > required to bring the local clock well within one second, at which time > > the PPS should take over. > > Indeed, the whole operation is much more reliable when you have a couple > > of references for NTP to choose from.
In my experience, when the GPS 18 has a fix, the GPRMC sentence received
after the PPS signal starts is correct and in sync with the PPS enough to
ensure it's received after it. When there's no fix the PPS signal is still
accurate compared to external servers also using GPS, but the GPRMC
sentence following PPS can reference the previous second...
Also, it's rather bizarre to claim the SHM refclock itself is inaccurate,
it's not and can receive time from any source :P (although I would like to be
able to automatically change refid from GPS to PPS when appropriate).
remote refid st t when poll reach delay offset jitter
==============================================================================
LOCAL(0) .LOCL. 8 l 8 16 377 0.000 0.000 0.001
*SHM(0) .GPS. 0 l 11 16 377 0.000 0.177 0.034
SHM(1) .DCF. 0 l 98m 64 0 0.000 -29.925 9.547
SHM(2) .MSF. 0 l 8 64 377 0.000 -48.509 0.294
-ntp0.nl.uu.net .GPS. 1 u 32 64 377 37.850 -0.902 4.009
-ntp1.nl.uu.net .GPS. 1 u 30 64 377 35.372 -0.768 3.456
-tik.cesnet.cz .GPS. 1 u 49 64 377 53.346 -1.301 0.535
-ntp2.sp.se .PPS. 1 u 61 64 377 58.531 0.972 0.846
+ntp-galway.hea. .GPS. 1 u 62 64 377 37.547 0.197 0.329
+ntps1-0.cs.tu-b .PPS. 1 u 53 64 377 55.847 0.236 12.696
I'm using my own code (http://svn.lp0.eu/simon/gps_nmea/, based on gpsd,
radioclkd) since I want to be able to do other things with the NMEA data too.
(I'm currently recalibrating my radio clocks directly to the PPS signal, which
takes a while to get enough samples to do so...)
--
Simon Arlott
smime.p7s
Description: S/MIME Cryptographic Signature
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