Hi Robert:

The Motorola GPS receivers output a Motorola DGPS correction that can be
used by another Motorola GPS receiver by direct interconnection.  This
binary format is documented in the Motorola manuals.  Both the older 8
channel units and the new 12 channel units generate this correction
message.  There are a number of different RTCM SC104 packets, but the
idea is to send a range and range rate correction for each satellite.

If you have the option of using two Motorola GPS receivers then all you
need to do is provide the data link.

If you want to have standard RTCM SC104 packets they use the DGPS
Reference Station board from TAPR.  It uses a Motorola GPS receiver
board and a uC (maybe a PIC) and generates the RTCM104 packets.  As far
as I remember it's an 8 channel version.

http://www.tapr.org/tapr/html/gpsf.html

Yet another option is to do a self referenced DGPS correction.  This
idea was patented by CSI, See:
http://www.pacificsites.com/~brooke/GPSpat.shtml#Pat

You could send just one correction packet and use it for quite some time.

Have Fun,

Brooke Clarke, N6GCE



>Date:    Tue, 2 Sep 2003 14:11:17 -0600
>From:    Robert Ussery <uavscience@FRII.COM>
>Subject: [OT:] RTCM SC104 interface
>
>Hello, everyone!
>After Googling around for a while, I've not had any success finding any info
>on the RTCM DGPS interface.
>Do any of you have some general DGPS tutorials and some specific RTCM
>protocol references?
>What I'm planning to do here is use my handheld GPS (<3m accuracy w/WAAS) as
>a DGPS groundstation to feed correction data to an airborne GPS unit. The
>handheld GPS unit is a Garmin Etrex Legend which supports most of the
>standard NMEA sentences. I assume there's some way to use the DOP data along
>with realtime position error to generate RTCM data with a microcontroller. I
>plan to have the Etrex mounted in a portable base station (which will remain
>fixed when providing DGPS corrections). I will get an approximation of
>actual position by averaging the GPS position output over a period of time
>(15 mins or so) through a Kalman filter using the DOP data to weight the
>data. I will then compare this realtime estimate of position with the
>reported position from the GPS to get the approximate position error, and
>generate RTCM data from this error signal.
>Does this sound feasible? I know it won't get me much more accuracy than
>just using a WAAS-enabled airborne GPS, but I'd like to get as much
>precision as possible without too much expense.
>Thanks for your help!
>
>- Robert
>
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