[ntp:questions] garmin 18x and linux

Miroslav Lichvar mlichvar at redhat.com
Tue Sep 6 15:57:23 UTC 2011


On Mon, Sep 05, 2011 at 04:47:20PM +0000, unruh wrote:
> On 2011-09-05, Miroslav Lichvar <mlichvar at redhat.com> wrote:
> > It's from gpsd which seems to make the NMEA receive timestamp after
> > the message is processed.
> >
> Never did understand that. Timestamping the beginning of the sentences
> is cheap enough and easy enough. 
> Mind you, your fluctuations are far more than I would expect simply from 
> variations in the length of the sentences.
> Are there more sentences delivered than just the one gpsd uses?

There are other messages enabled (I like to monitor the visibility of
satellites in cgps), but RMC and GGA are transmitted first. The baud
rate is set to 115200. The measured time it takes to transmit one
batch is about 85 +/- 10 ms.

Here is another capture, this time only over couple hours, but it's
the offset to the beginning of the transfer (i.e. start of RMC).

http://mlichvar.fedorapeople.org/tmp/18x_nmea2.png

The offset still moves in a 300ms range.

-- 
Miroslav Lichvar



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