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

David Lord snews at lordynet.org
Mon Dec 27 22:10:09 UTC 2010


Q wrote:
> 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

Using NetBSD-5

I found gpsd useful for debugging but just used the normal gps
type 20 driver and type 22 for pps

######
tos mindist 0.4

server 127.127.20.0 mode 1 prefer minpoll 6 maxpoll 8
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

#######

For some time I was using parallel port for pps but didn't
find much difference parallel/serial or kernel/ppsapi.

Offset would be a few microseconds.

There are differences between versions of ntpd so that required
fudge values may not be same.


David


> 
> 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
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 




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