[ntp:questions] GPS_NEMA, but high offset and jitter?

David Lord snews at lordynet.org
Wed Feb 24 23:23:25 UTC 2010


Kelsey Cummings wrote:
> David J Taylor wrote:
>> The reach = 0 suggests that the signals from the GPS are not being seen. 
> 
> I'm not sure why it lost sync but I went ahead and upgraded to 4.2.7p5 
> from the ntp-dev port.
> 
> I'm still seeing high offset and variable jitter.
> 
> ============================================================================== 
> 
> xGPS_NMEA(1)  .GPS.      0 l   45   64  377    0.000  -311.36  75.859
> -time.sr    207.200.81.. 2 u  348  512  377    0.643    0.977   2.984
> *clock.sjc.he .CDMA.     1 u  362  512  377    6.348   -2.584   0.289
> 
> Or: http://kgc.users.sonic.net/ntp_127_127_20_1-day.png
> The initial spike was the last restart.
> 
> # Garmin GPS 18 LVC
> server  127.127.20.1    mode 0  prefer
> fudge   127.127.20.1    flag1 1
> 
> Does the GPS_NEMA driver actually make use of the PPS signal (currently 
> wired to DCD on serial port) at all or does it just use the GPS sentences?
> 
> I'm wired up following http://www.satsignal.eu/ntp/GPS-interface-2.png 
> except that I used a NPN transistor to drive the PPS led so it wouldn't 
> load the PPS signal.
> 
> -K


I'm guessing you aren't seeing pps.

My notes from last year might have been understandable
then but best I can decode from them now is:

# ntp.conf gps-18x-LVC
# gps0 -> /dev/tty00
# pps0 -> /dev/tty00
server 127.127.20.0 mode 1 prefer # NMEA $GPRMC

That was giving offset of about 70us.

No LED, just using pps direct to serial DCD. I'm still
suspicious cmos ttl out from the 18x-LVC might not be
good at driving rs232 as it requires at least 3mA/0.7V
pulldown.


ATOM driver and pps to serial DCD gave offset < 10us


Last check was with pps of my 18x-LVC direct to NACK of
parallel port without any extra components.

#ntp.conf
# mknod pps0 164 0 /dev/pps0
tos mindist 0.4 # I could reduce this now
server 127.127.20.0 mode 1 prefer
fudge  127.127.20.0 time1 0.651 refid GPSb
server 127.127.22.0 minpoll 4 maxpoll 6
fudge 127.127.22.0 refid PPSb flag3 1
server serv1
server serv2
server serv3

On a good day above cfg was giving offset = 0.000,
jitter = 0.002. NMEA offset/jitter are then very erratic
in order of 10s of ms.

Note for above I needed "fudge time1" and/or "tos mindist"
otherwise NMEA was deselected.


David




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