[time-nuts] Not getting microsecond accurate time in Linux with GPS setup

cook michael michael.cook at sfr.fr
Tue Jan 18 22:58:50 UTC 2011

   A couple of things not mentioned in the previous responses:

  " precision" here is not how accurate your laptops clock is. 
"precision" is a measure of how much the time changes during successive 
reading of the system clock as viewed by ntpd. Its minimum value is 1us 
due to the  timeval structure variable types.

   Your ntpq data is showing a high offset 24,5 ms but that it also has 
high jitter , adding the two gives you around 67 secs reported.

   Laptops often have variable speed oscillators to save power when not 
much is going on. This will cause unreliable timing and may be the 
source of your jitter. If your machine has that facility I suggest you 
turn it off (probably in the bios) and see if the stability improves.

  As mentioned , the time source is not optimal. I am pretty sure you 
only have the NMEA sentences to get time info and they are sent after 
the 1sec GPS mark. Check to see if the SHM driver has a fudge variable 
to take that into configuration. If you are seeing a constant offset of 
25ms and the shm driver has a fudge to allow for it, then configure it.
  However I doubt that you will be able to get better than 10s of ms 
accuracy with the hardware you have.

  You should be configuring more time servers. Take them off the ntp 
pool for instance over the network.

Le 18/01/2011 21:21, Mark Ngbapai a écrit :
> Hi all. I've grown interested in precise timekeeping so I decided to
> buy an inexpensive Transystem iBlue 737 GPSr clone with MTK 3301 +
> 3179 chipset (32-channel, -158dBm tracking sensitivity, Silicon Wave
> Bluetooth 1.2 chipset) for use with my Fedora 12 Linux Netbook (An
> Acer Aspire One D150). Having lock indoors of 5/9 satellites I've
> succeeded connecting the device via rfcomm to my netbook and using
> gpsd for parsing the data. I restart the nptd server in the machine
> and after a few minutes I get:
> [root at PHOENIX Streamer]# ntpq -p
>       remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> *SHM(0)          .GPS.            0 l    -   16  377    0.000   24.511  42.977
> If I execute ntpstat, it shows:
> [root at PHOENIX Streamer]# ntpstat
> synchronised to modem at stratum 1
>     time correct to within 67 ms
>     polling server every 16 s
> In /var/log/mesages I see the lines:
> Jan 18 20:38:39 PHOENIX ntpd[6898]: ntpd 4.2.4p8 at 1.1612-o Wed Dec  9
> 11:49:22 UTC 2009 (1)
> Jan 18 20:38:39 PHOENIX ntpd[6899]: precision = 5.448 usec
> Jan 18 20:39:28 PHOENIX ntpd[6899]: synchronized to SHM(0), stratum 0
> So why my system is telling me the time is correct within 67 ms and
> not 5.44 usec? My GPSr is located at 1-1.5 meters from my netbook
> (GPSr battery lasts around 40 hours, low power is not an issue). Does
> my Linux installation need special Kernel patching or I'm missing
> something?
> Thanks in advance,
> Mark
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