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

[John Ackermann N8UR]

You'll probably get better (and more interesting!) answers on the NTP 
group/mailing list, but a first thought is that NTP takes quite a long 
time to stabilize after startup.

 From the jitter value below being larger than the offset, I'm guessing 
you took this ntpq snapshot after only a few polling intervals.  Give it 
a couple of hours and see where you end up.  (Because of the slow 
convergence, the typical laptop usage pattern is not conducive to good 
NTP performance.)

John
----

On 1/18/2011 3:21 PM, Mark Ngbapai wrote:
> 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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