[time] Garmin GPS 18 5Hz usability

Arthur Konovalov kasjas
Fri Nov 10 14:57:52 UTC 2006


Udo van den Heuvel wrote:
> The manual for the Garmin GPS 18 and 18-5Hz states that the PGRMC
> sentence will indicate which pulse is at the top-of-the-second. (page 13
> of 40)
> So it could work. I cannot confirm from experience, though.
>
>   

Well, due to different reasons I had to obtain GPS 18-5Hz and now ran 
into problems...
 From my point of view 5Hz has 2 differences compared to LVC:
- PPS (GPGGA and GPRMC too) 5 times per second ( .0, .2,  .4, .6,  .8 of 
each second) instead of 1
- UTC timestamp hhmmss.s instead of hhmmss

I tried on PPS-patched Linux kernel (2.6.19-rc2-g02be1c39) and 
nmea-patched ntp-4.2.2p4.

Printout of ppstest:

found PPS source #0 "pps_8250_0" on "/dev/ttyS0"
ok, sources found, now start fetching data...
source 0 - assert 1163150784.254578675, sequence: 38232 - clear  
1163150756.552520000, sequence: 38229
source 0 - assert 1163150784.254578675, sequence: 38232 - clear  
1163150784.354584000, sequence: 38230
source 0 - assert 1163150784.454590675, sequence: 38233 - clear  
1163150784.354584000, sequence: 38230
source 0 - assert 1163150784.454590675, sequence: 38233 - clear  
1163150784.554598000, sequence: 38231
source 0 - assert 1163150784.654605675, sequence: 38234 - clear  
1163150784.554598000, sequence: 38231
source 0 - assert 1163150784.654605675, sequence: 38234 - clear  
1163150784.754613000, sequence: 38232
source 0 - assert 1163150784.854623675, sequence: 38235 - clear  
1163150784.754613000, sequence: 38232
source 0 - assert 1163150784.854623675, sequence: 38235 - clear  
1163150784.954627000, sequence: 38233
source 0 - assert 1163150785.054635675, sequence: 38236 - clear  
1163150784.954627000, sequence: 38233
source 0 - assert 1163150785.054635675, sequence: 38236 - clear  
1163150785.154642000, sequence: 38234
source 0 - assert 1163150785.254650675, sequence: 38237 - clear  
1163150785.154642000, sequence: 38234
source 0 - assert 1163150785.254650675, sequence: 38237 - clear  
1163150785.354658000, sequence: 38235

But ntpd has very big jitter and vary offset:
# ntpq -p
    remote           refid      st t when poll reach   delay   offset  
jitter
============================================================================== 

*GPS_NMEA(0)     .GPS.            0 l   12   16  377    0.000  -327.93 
161.951



gpsd (svn) also refused to work complained about too short pps:

gpsd: pps-detect (DCD) on /dev/ttyS0 changed to 1
gpsd: PPS pulse rejected too short. cycle: 200015, duration: 100005
...
gpsd: pps-detect (DCD) on /dev/ttyS0 changed to 0
gpsd: PPS pulse rejected too short. cycle: 200020, duration: 100010
gpsd: polling 4
gpsd: <= GPS: 
$GPGGA,091937.4,5926.10757,N,02442.82946,E,1,03,7.7,,M,20.5,M,,*7A
gpsd: GPGGA sets status 1


Is there any fundamental reason why 5Hz not working?
However RFC2783  (Pulse-Per-Second API) said:
"In principle, the rate of events to be captured, or the frequency of 
the signals, can range from once per day (or less often) to several 
thousand per second."

Any help, please.


Regards,
Arthur




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