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Re: [gpsd-dev] gpspipe improvement proposal for hi-res timestamps -2-


From: Andreas Merz
Subject: Re: [gpsd-dev] gpspipe improvement proposal for hi-res timestamps -2-
Date: Sat, 4 May 2013 01:17:05 +0200
User-agent: SquirrelMail/1.4.18


> Nice work.  All I had to do was fix your German accent in the commit
> comment. :-)  Merged.
> --
>               <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>
>
>

Hi Eric,

thank you very much for your kind feedback.
This strongly encourages me to contribute
more frequently to open source projects in the future :-)
Likely, still with some German accent... ;-)

It may be interesting to see what funny things can be observed
when investigating nav data with high resolution time stamps.
The two referenced figures below show the timing behaviour I
observed for two different GPS receivers.
For this purposse, I investigated the
timestamps attached to RMC messages when using
gpspipe -ur or gpspipe -uw, respectively.

One GPS is a Navilock NL303 with a SiRF chip. It is interesting
to see that the RMC messages (derived from MID2) are not
equidistant in time but follow a pattern with a 3 sec period. [1]

The other GPS is a good old Garmin GPS12 attached to the RS232 port.
Its behaviour is even more strange.
The RMC messages step around within a one second interval.
This will make it tricky to synchronize a local clock. [2]

Have fun

Andreas

p.s.: I did not dare to attach 500 kBytes to this posting, so here is a link:

[1] http://www.dada.franken.de/openseamap/logging/NL303P_RMC-timing.ps
[2] http://www.dada.franken.de/openseamap/logging/GPS12_RMC-timing.ps

or as PDF:
http://www.dada.franken.de/openseamap/logging/NL303P_RMC-timing.pdf
http://www.dada.franken.de/openseamap/logging/GPS12_RMC-timing.pdf





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