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

Mark Ngbapai lightningbolt31 at gmail.com
Tue Jan 18 20:21:06 UTC 2011

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

Thanks in advance,


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