Em 3/3/2013 13:47, Mike Harrison escreveu:
> On Sun, 03 Mar 2013 11:16:40 -0300, you wrote:
>
>> Dear All,
>>
>>
>> I need to read data from a John Deere GPS. The GPS doesn't use the
>> traditional RS-232 interface with NMEA protocol, but instead it uses a
>> CAN bus at 250kbps and sends GPS data in NMEA2000 format inside CAN pack=
ets.
> GPS receivers are cheap - might it be easier to just add a second GPS for=
 your use?


The ones we are using are not cheap at all, they are dual frequency,
used for auto-piloting the tractor and cost tens of thousand dollars.
There is no excuse for that price range, but they are indeed very
different from the regular GPS. They work in RTK mode (Real Time
Kinematics) in pairs, one unit stays fixed at a known position and sends
correction information to the mobile unit. Both units are identical,
just one is configured to be stationary and sending correction
information and the other is configured to be mobile and receive the
corrections.

This way, the mobile unit can reach accuracy better than 2cm. The output
stream is also very fast and has a lot more significant digits than
regular NMEA RS-232 data.
Regular NMEA messages just output three or four decimal places for
minute of arc (1/1000 of minute =3D 1,8m or 1/10000 of minute =3D 18cm).


I need to decode the GPS information to control an implement towed by
the tractor, to start and stop operation at very precise and
programmable positions.


Best regards,

Isaac

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