[ntp:questions] Re: difference between M12 & M12T GPS receiver

Wolfgang S. Rupprecht wolfgang+gnus20040201T190459 at dailyplanet.dontspam.wsrcc.com
Mon Feb 2 03:17:26 UTC 2004

"Xuehuan Li" <libryant at hotmail.com> writes:
> I found ntpd running with the Motorola GPS receiver had a
> bug. Compared to the internet time sever, the ntpd running with a M12T
> gives the same time, while the M12 will be ahead 1 second to it. The
> hardware platform is a Pentium MMX PC104 CPU card, OS is linux
> 2.4.20-NANO, software is ntp-4.2.0 with enable_ONCORE option. I
> monitor the serial port of the GPS receiver, and the output time of
> the two cards are the same in the same time(according to the @@Ha
> message). the following text is the status of the two time server. the
> 'test' is using a M12T card, while the is using a M12 card.

Do you by any chance have the M12 evaluator board that has the PPS
inversion bug?  On some of the boards the PPS signal is inverted and
one needs to look at the negative-going edge using the "flag2"

    server prefer	# Oncore GPS driver
    fudge flag2 1

My M12 agrees with the external stratum 1's and 2's to within 4ms.
I'm on a very asymetrical line to the internet 256kbps clocking in the
upstream direction and 1.5Mbit/sec clocking in the downstream
direction.  It isn't clear if I'm just seeing the effect of that delay
or of there is some other delay in my gps->ntp path.

Just for completeness, there are also some other possible failure
modes if the GPS has just recently been powered up.  The GPS may not
yet have seen the UTC-GPS offset and be using some compiled-in
default.  With a good sky view it should take ~15 minutes for the GPS
to see a full set of messages from the satellites.

Wolfgang S. Rupprecht 		     http://www.wsrcc.com/wolfgang/
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