[ntp:questions] How to debug GPS PPS?

Thomas Laus lausts at acm.org
Thu Dec 31 15:14:46 UTC 2009


On 2009-12-30, xyz-2041 <xyz2041 at gmail.com> wrote:
> On Dec 28, 7:59?am, Thomas Laus <lau... at acm.org> wrote:
>> On 2009-12-28, xyz-2041 <xyz2... at gmail.com> wrote:
>>
>> > Plugged in the GPS unit's serial cable into a Windows
>> > computer running WinOncore12 v1.0 (Build 37):
>>
>> > ?http://www.google.com/#q=WinOncore12Installation.exe
>>
>> > Seemed to work without any problems. ?Generated
>> > all sort of graphs and charts. ?Let it run and it
>> > told me exact latitude, longitude and height above
>> > sea level.
>>
>> > Used an analog volt meter and from pin 5 (supposed
>> > to be ground), I only noticed voltage on pins 2 and 3.
>> > Pin 2 was going erratically negative once a second.
>> > I believe that this must be received data as per
>> > standards. ?Pin 3 was +5 volts, but dropping to
>> > -5 volts once per second.
>>
>> You did not mention how your Oncore receiver was being interfaced to
>> your computer or it's model number. ?
>
> GT+.  Serial port.
>
>
>>I use one of the TAPR boards and
>> Oncore UT+ combination. ?The NTP refclock driver for the Oncore includes
>> the code for receiving the PPS signal on the computer DCD pin.
>
> It doesn't need pulse stretching?
>
>
>> > I've changed my configs a bit, /var/log/ntp.log and
>> > /var/log/ntpd.log don't show any errors.
>>
>> > However, "ntpq -c pe" still doesn't show any response:
>>
>> > ?GPS_NMEA(0) ? ? .GPS. ? ? ? ? ? ?0 l ? ?- ? 16 ? ?0 ? ?0.000
>> > 0.000 ? 0.001
>> > ?PPS(0) ? ? ? ? ?.GPS. ? ? ? ? ? ?0 l ? ?- ? 16 ? ?0 ? ?0.000
>> > 0.000 ? 0.001
>> > ?GPS_ONCORE(0) ? .GPS-. ? ? ? ? ? 0 l ? ?- ? 16 ? ?0 ? ?0.000
>> > 0.000 ? 0.001
>>
>> You should not be using GPS_NMEA or PPS. ?
>
> OK.  Turned both of those off.
>
>
>>The type 30 refclock
> OK.  Used WinOncore to default.  Made sure that it was
> set to Motorola/binary.  Power cycled.  Hooked back
> up to FreeBSD server.  This is what was produced--for
> a while:
>
>      remote           refid      st t when poll reach   delay
> offset  jitter
>======================================================
>  GPS_ONCORE(0)   .GPS.            0 l   99   16   40    0.000
> 192.362   0.002
>
>  GPS_ONCORE(0)   .GPS.            0 l  200   16    0    0.000
> 192.362   0.002
>
>  GPS_ONCORE(0)   .GPS.            0 l  418   16    0    0.000
> 192.362   0.002
>
This does not look like anything is happening when NTP is trying to
comunicate with your Oncore.  You might get some insight by enabling
some of the statistics.  You add them to /etc/ntp.conf

# NTP Statistics
statsdir /var/log/ntp/
statistics clockstats loopstats peerstats
filegen clockstats file clockstats type day enable
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable

NTP will write events to these files when it runs.  The satellites that
are in view will be listed by PRN number.  It also will show any RS-232
handshaking issues.  I had to change the flag for reading the PPS pin
because it was triggering on the trailing edge instead of the leading
edge.  My TAPR board pulsewidth was OK and did not require 'stretching'.

> Re-defaulted.  Now back to this:
>
>  GPS_ONCORE(0)   .GPS.            0 l    -   16    0    0.000
> 0.000   0.002
>
>
There is still nothing in the 'reach' column, you are not talking at all
to your Oncore receiver.  Even if the PPS was incorrect, you should
still get data from the Oncore as soon as your NTP has synced with your
other hosts.  Your LED shows that your receiver has an almanac, position
and is synced to the satellites.  It is just the RS-232 link is not
working between your TAPR board and your computer.  A breakout box would
be handy in this case, but enabling the statistics may give enough
insight into the problem.

The Oncore / TAPR board is the gold standard of home user time sync
using the GPS constellation.  Myself and others have been using this
combination for many years successfully.  Does your PC have a 'real'
serial port or are you using a USB to serial adapter?  My billboard
looks like this:

ntpq -c pe

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
-198.186.191.229 64.147.116.229   2 u   38   64  377   88.338   17.680   0.609
-tantalum.srvcs. 192.43.244.18    2 u    6   64  377  112.759  -29.145   3.168
+null-routed.net 198.153.152.52   2 u   42   64  377   79.333    1.590   0.792
+packman-1.isc.o 204.152.184.72   2 u    2   64  377   77.789   -8.314   2.418
*GPS_ONCORE(0)   .GPS.            0 l   10   16  377    0.000   -0.016   0.001

Tom


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