[ntp:questions] serialPPS+gpsd+ntpd large offset & jitter

M Z mcrunch2 at yahoo.com
Fri Oct 15 05:51:40 UTC 2010


Hi. I have both usglobalsat MR350P and garmin 18xLVC connected to serial at both 
4800 and 9600.
gpsd was 2.37 but is now 2.95. ntpd is 4.2.4p7, OS: SuSE 11.2 = 2.6.31. I have 
used both
serial ports. I set to low_latency as per instructions:
http://www.lammertbies.nl/comm/info/GPS-time.html

gpsd 2.37 did not communicate with ntp for the pps (ntpq -p 'when' column always 
-). 2.95 seems to work ok. output looks good, xgps works (7 sats used), etc. 
However, jitter seems large and offset, no matter how small to start (usually +- 
20.) always creeps up to about -300 to -500.
snip from gpsd -n -N -D 2 /dev/ttyS1
gpsd: GPS on /dev/ttyS1 is offline (0.000040 sec since data)
gpsd: GPS on /dev/ttyS1 is offline (0.000017 sec since data)
gpsd: GPS on /dev/ttyS1 is offline (0.000015 sec since data)
gpsd: PPS cycle: 1000112, duration: 960102 @ 1287118259.019369
gpsd: PPS cycle: 1000109, duration: 40007 @ 1287118259.059376
gpsd: PPS pulse rejected
gpsd: GPS on /dev/ttyS1 is offline (0.000022 sec since data)
gpsd: GPS on /dev/ttyS1 is offline (0.000021 sec since data)
gpsd: GPS on /dev/ttyS1 is offline (0.000016 sec since data)
gpsd: PPS cycle: 1000138, duration: 960131 @ 1287118260.019507
gpsd: PPS cycle: 1000128, duration: 39997 @ 1287118260.059504
gpsd: PPS pulse rejected
gpsd: unknown sentence: 
"$GPGSV,3,1,11,06,27,265,21,09,36,064,34,14,53,244,29,15,06,047,25*7F\x0d\x0a"

 here is snip from gpsd -D6 > file then | grep PPS
gpsd: PPS cycle: 1000327, duration: 960315 @ 1287118037.467467
gpsd: PPS cycle: 1000330, duration: 40015 @ 1287118037.507482
gpsd: PPS pulse rejected
gpsd: PPS cycle: 1000343, duration: 960328 @ 1287118038.467810
gpsd: PPS cycle: 1000348, duration: 40020 @ 1287118038.507830
gpsd: PPS pulse rejected
gpsd: PPS cycle: 1000333, duration: 960313 @ 1287118039.468143
gpsd: PPS cycle: 1000329, duration: 40016 @ 1287118039.508159
gpsd: PPS pulse rejected

ntpq -p after a long while with GPS/PPS
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+SHM(0)          .GPS.            0 l    1   16    377    0.000  -414.55 197.047
+SHM(1)          .PPS.            0 l   16   16    377   0.000  -375.42  31.550
 LOCAL(0)        .LOCL.          10 l   24   64  377    0.000    0.000   0.001
*131.107.13.100  .ACTS.         1 u   54   64  375   55.984  -358.88 153.723
+saturn.netwrx1. .PPS.           1 u   17   64  377   64.988  -378.87 130.000
+tick.usask.ca   .GPS.           1 u   54   64  377   82.113  -357.94 136.472
+216.45.57.38    209.81.9.7    2 u   15   64  377   39.344  -374.06 127.468
+thecakeisapie.n 209.81.9.7   2 u   43   64  377   10.219  -364.89 129.425
+ds3-us.zagbot.c 204.9.54.111 2 u 17   64  377   39.226  -373.65 128.148

# cat /var/lib/ntp/ntp.drift  --> 158.230

 ntpq -p after a couple minutes without GPS/PPS

     remote           refid      st t     when poll reach   delay   
offset        jitter
 SHM(0)          .GPS.            0 l 1415   16          0    0.000  -169.04   
6.823
 SHM(1)          .PPS.            0 l 1420   16          0    0.000  -19.507   
1.432
*131.107.13.100  .ACTS.           1 u   98  128  377   55.324  -76.306  30.504
+saturn.netwrx1. .PPS.            1 u   49  128  377   61.968  -53.547  47.469
+tick.usask.ca   .GPS.            1 u  207  128  376   81.499  -79.038  26.151
-dev1-c.lax009.i 209.81.9.7       2 u   52  128  377   41.986  -100.23  36.500
+clock.trit.net  192.12.19.20     2 u   35  128  377   41.783  -78.852  34.350
==============================================================================
more time
 SHM(0)          .GPS.            0 l  38m   16    0    0.000    0.000   0.001
 SHM(1)          .PPS.            0 l  38m   16    0    0.000    0.000   0.001
 131.107.13.100  .ACTS.           1 u   35   64    1   63.182   -2.032   0.001
*saturn.netwrx1. .PPS.            1 u   23   64    1   59.459   -1.491   3.370
 tick.usask.ca   .STEP.          16 u  153   64    0    0.000    0.000   0.000
 dev1-c.lax009.i 209.81.9.7       2 u   26   64    1   43.852    1.862   0.001
 clock.trit.net  .STEP.          16 u  103   64    0    0.000    0.000   0.001
==============================================================================
more time (+47 min)
 SHM(0)          .GPS.            0 l  47m   16    0    0.000    0.000   0.001
 SHM(1)          .PPS.            0 l  47m   16    0    0.000    0.000   0.001
+131.107.13.100  .ACTS.           1 u   15   64  373   55.200    4.175   6.996
*saturn.netwrx1. .PPS.            1 u   20   64  377   61.447  -11.922   9.974
 tick.usask.ca   .GPS.            1 u   64   64   15   95.860   -9.911  15.977
-dev1-c.lax009.i 209.81.9.7       2 u    9   64  377   42.149   -9.077  12.017
+clock.trit.net  192.12.19.20     2 u   36   64  377   42.041   -0.147   6.371
==============================================================================
more time
 SHM(0)          .GPS.            0 l  57m   16    0    0.000    0.000   0.001
 SHM(1)          .PPS.            0 l  57m   16    0    0.000    0.000   0.001
+131.107.13.100  .ACTS.           1 u    3  128  367   55.288   -7.875   5.460
*saturn.netwrx1. .PPS.            1 u   78  128  377   59.112   -9.368   5.779
+tick.usask.ca   .GPS.            1 u   51  128  377   81.580   -7.873   1.362
+dev1-c.lax009.i 209.81.9.7       2 u  127  128  377   41.934   -6.816   3.922
-clock.trit.net  192.12.19.20     2 u   24  128  377   41.983  -13.802   4.239

just the servers are creeping a little but look like they are settling. 
certainly not going to -400 like it
would already be with the PPS.

I got rid of server 127.0.0.1 in /etc/ntp.conf but that didn't help. Here is 
ntp.conf:
restrict    default kod nomodify notrap nopeer
restrict -6 default kod nomodify notrap nopeer
restrict 127.0.0.1
restrict -6 ::1
server 127.127.28.0 minpoll 4 maxpoll 4
fudge 127.127.28.0 time1 0.485 refid GPS
server 127.127.28.1 minpoll 4 maxpoll 4 prefer
fudge 127.127.28.1 refid PPS
server 131.107.13.100
server ntp2.netwrx1.com  iburst
restrict 131.107.13.100 mask 255.255.255.255 nomodify notrap noquery
restrict ntp2.netwrx.com  mask 255.255.255.255 nomodify notrap noquery
server tick.usask.ca
restrict tick.usask.ca mask 255.255.255.255 nomodify notrap noquery
server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
restrict 0.us.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 1.us.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery

So, what is going wrong? What things do I still need to do? This computer will 
not serve time to any others but needs very accurate time itself. cron jobs all 
commented out. ps ax |grep ntp  looks like:

...      /usr/sbin/ntpd -p /var/run/ntp/ntpd.pid -g -u ntp:ntp -i /var/lib/ntp 
-c /etc/ntp.conf
Thanks.


      



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