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