[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
something?


Thanks in advance,

Mark




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