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

David Lord snews at lordynet.org
Wed Feb 24 22:52:13 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 guessing you aren't seeing pps.

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

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

That was giving offset of about 70us.

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

> 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 not sure that's a good idea since rs232 is current
driven/voltage triggered with a relative low input
resistance depending on spec of interface. Having an
LED+series resistor load might give a better pullup.
Having said that, I can't remember what interface circuit
if any I was using last year.


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.


David




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