[ntp:questions] Garmin GPS18x LVC 5m - Centos 5.5 - Kernel 2.6.18 - gpsd

Q .. at ..
Mon Dec 27 17:22:34 UTC 2010


I wonder if someone would be kind enough to help me out a little here. I 
have done lots of googleing and found nothing that helps me out with the 
issues I'm seeing.

I am trying to reduce the PPS offset & delay (Which seem quite high for a 
local device) and stop the NMEA driver from swinging and becoming a false 
ticker.

Any ideas please would be great. This is only a personal project, but I was 
expecting slightly better results! Below are debug outputs, configs and some 
other 'useful' stuff if you could take a look over them and let me know what 
I could be doing wrong, or would otherwise benefit from changing.

Cheers!

GPS;
GPS18x - NMEA outputting GPRMC to serial port.
GPS18x - PPS outputting to Pin 1 of serial port.

BAUD Rate - 115200, 8N1
NMEA Sentences output - GPRMC
PPS Length: 200msec
PPS Mode 1Hz
NMEA Output time - 1 sec
NMEA Mode 2.30 - ON

Machine;
Very old AMD Duron 1Ghz CPU
384Mb RAM
Serial - '3: uart:16550A port:000002E8 irq:3 tx:20052 rx:32887544 fe:976 
pe:1318 brk:3 oe:132 RTS|DTR'

gpsd;
Sample of debug output (/usr/local/sbin/gpsd -n -b -N -D2 /dev/ttyS3)

gpsd: speed 115200, 8N1
gpsd: gpsd_activate(): opened GPS (fd 4)
gpsd: NTPD ntpd_link_activate: 1
gpsd: /dev/ttyS3 identified as type Generic NMEA (0.107275 sec @ 115200bps)
gpsd: => Probing device subtype 0
gpsd: GPS on /dev/ttyS3 is offline (0.000727 sec since data)
gpsd: PPS cycle: -753943806, duration: -753943806 @ 1293067869.993730
gpsd: PPS pulse rejected
gpsd: PPS cycle: -753743809, duration: 199997 @ 1293067870.193727
gpsd: PPS pulse rejected
gpsd: => Probing device subtype 1
gpsd: GPS on /dev/ttyS3 is offline (0.000267 sec since data)
gpsd: PPS cycle: 1000010, duration: 800013 @ 1293067870.993740
gpsd: PPS pulse rejected
<some log missing because its just too long to post here>
gpsd: GPS on /dev/ttyS3 is offline (0.000185 sec since data)
gpsd: PPS cycle: 999985, duration: 799986 @ 1293067891.993654
gpsd: PPS cycle: 999986, duration: 200000 @ 1293067892.193654
gpsd: PPS pulse rejected


ntp.conf;
restrict default kod nomodify notrap nopeer noquery
restrict 127.0.0.1

server Private1
server Private2
server Private3
server Private4
server Private5
server Private6
server Private7
server Private8
server Private9
server Private10
server Private11

broadcast 224.0.1.1 key 6
manycastserver 239.255.254.254

#Local Clock
server  127.127.1.0
fudge   127.127.1.0 stratum 10

#NMEA
server 127.127.28.0
fudge 127.127.28.0 time1 0.500 refid GPS

#PPS
server 127.127.28.1
fudge 127.127.28.1 refid PPS

driftfile /var/lib/ntp/drift
keys /etc/ntp/keys
trustedkey 6


Output of 'ntpq -p' (Several days clean running);

     remote           refid      st t when poll reach   delay   offset 
jitter
==============================================================================
+Private1   .GPS.            1 u  984 1024  377   14.012    0.205   0.101
-Private2  .GPS.              1 u 1017 1024  377   14.113    0.393   0.445
-Private3     .GPS.           1 u  974 1024  377    9.706    0.208   0.256
-Privat4     .MSF.            1 u 1013 1024  377   10.875    1.662   0.040
-Private5     .PPS.            1 u 1020 1024  377   19.916   -0.660   0.475
-Private6     .DCFa.         1 u 1004 1024  377    9.496    0.276   0.226
-Private7  195.66.241.2   2 u 1004 1024  377   14.690    0.970   0.163
 Private8 .GPS.                1 u   5h 1024    0   23.721   -0.517   0.000
-Private9  .PPS.               1 u 1022 1024  377   14.671   -3.016   0.012
+Private10  .PPS.            1 u 1006 1024  377   15.811   -0.262   0.020
-Private11 .CDMA.         1 u   56 1024  377   86.819   -4.640   0.101
 NTP.MCAST.NET   .MCST. 16 u    -   64    0    0.000    0.000   0.001
 LOCAL(0)        .LOCL. 10 l   40   64  377    0.000    0.000   0.001
xSHM(0)          .GPS.      0 l   57   64  377    0.000  -35.284  19.872
*SHM(1)          .PPS.      0 l   32   64  377    0.000   -0.488   0.046


At the moment the NMEA is being reported as a flase ticker - the offset 
swings violently from + to - sometimes its a valid ticker, others it not.

A few more readings - after the poll

xSHM(0)          .GPS.            0 l   18   64  377    0.000  -33.130 
19.161
*SHM(1)          .PPS.            0 l   59   64  377    0.000   -0.542 
0.063
xSHM(0)          .GPS.            0 l   10   64  377    0.000  -45.655 
15.924
*SHM(1)          .PPS.            0 l   48   64  377    0.000   -0.541 
0.051
xSHM(0)          .GPS.            0 l   27   64  377    0.000  -26.448 
13.995
*SHM(1)          .PPS.            0 l    1   64  377    0.000   -0.573 
0.060

If  I give it another half hour the NMEA will be 'ok' and the offset would 
have swung the other way into +ive land.

Now I know that NMEA is 'slow' but I wouldn't of expected it to wander all 
over the place like that.

The PPS also appears to have a rather large offset - If I fudge it it will 
break in a nasty way.

All of those stats are after a good few days running without being touched - 
I have mrtg graphs if they are of any help.


Fresh from an ntp restart I get the following;

SHM(0)          .GPS.            0 l   10   64    7    0.000  -12.028 
18.019
SHM(1)          .PPS.            0 l   10   64    7    0.000   -0.763 
0.034
-SHM(0)          .GPS.            0 l   20   64   17    0.000  -14.131 
13.111
*SHM(1)          .PPS.            0 l   18   64   17    0.000   -0.755 
0.033
-SHM(0)          .GPS.            0 l   58   64   17    0.000  -14.131 
13.111
-SHM(1)          .PPS.            0 l   56   64   17    0.000   -0.755 
0.033
*SHM(0)          .GPS.            0 l   14   64   37    0.000  -36.112 
17.250
+SHM(1)          .PPS.            0 l   12   64   37    0.000   -0.576 
0.199
+SHM(0)          .GPS.            0 l   21   64   77    0.000  -16.348 
12.642
*SHM(1)          .PPS.            0 l   20   64   77    0.000    2.978 
3.713
+SHM(0)          .GPS.            0 l    9   64  177    0.000  -27.085 
10.346
*SHM(1)          .PPS.            0 l    8   64  177    0.000    4.194 
4.527
xSHM(0)          .GPS.            0 l    1   64  377    0.000  -37.895 
16.898
*SHM(1)          .PPS.            0 l    2   64  377    0.000    6.115 
5.954

'ntpq -c rv'
assID=0 status=09b4 leap_none, sync_telephone, 11 events, 
event_peer/strat_chg,
version="ntpd 4.2.2p1 at 1.1570-o Sat Dec 19 00:58:16 UTC 2009 (1)",
processor="i686", system="Linux/2.6.18-194.11.3.el5", leap=00,
stratum=1, precision=-20, rootdelay=0.000, rootdispersion=14.620,
peer=23623, refid=PPS,
reftime=d0c347f4.c01e663d  Mon, Dec 27 2010 17:14:28.750, poll=6,
clock=d0c34822.437c3419  Mon, Dec 27 2010 17:15:14.263, state=4,
offset=3.465, frequency=-20.530, jitter=6.901, noise=7.660,
stability=0.189, tai=0













More information about the questions mailing list