[aprssig] Tap into in-dash GPS Navigation for APRS?

Curt, WE7U archer at eskimo.com
Tue Aug 22 11:45:12 CDT 2006

On Tue, 22 Aug 2006, Ron McCoy wrote:

> My understanding is that you can access the GPS signal by breaking into the
> box but that you have to play with the firmware to change from a proprietary
> protocol to NEMA, and that breaks the normal OnStar functionality.

A page I saw mentioned Motorola binary format.  One should be able
to convert that to something usable with a microprocessor.

That's assuming it's not like one of the Trimble binary formats
where the unit has to query the GPS for the data:  In that case
you'd need to OR the transmit signals from the box and from your
microprocessor together so either could command the GPS to return a
position.  A bit of a pain.

If it's a continuous stream of data from the GPS then you just need
to tap into the GPS transmit line and convert it.

Found this, code that switches to NMEA mode for the Motorola:


So you could program a PIC to switch the GPS to NMEA mode, get a
position, then switch it back to Motorola...  Probably better to
leave it in Motorola mode and just convert to NMEA on the fly
though.  That way the OnStar wouldn't have a fit.

This page talks about a PIC-based project to read the Motorola
binary format:


But this is OnStar, not Alpine, so getting off-topic.

Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"

More information about the aprssig mailing list