You can buy a copy of the standard for $80.00 at rtcm.org.  My company is an RTCM member, but we don't have a copy.

What you are planning is pretty much the same thing any differential station does so there is no problem with your theory.  The only problem I see is that the Etrex won't take RTCM-SC104 differential corrections as far as I know.  The RTCM corrections are not standard NMEA so you can't just put them into the Etrex.  What will work is if you come up with some other simple format for your corrections and link that to a different airborn display than the Etrex.  Feed in the Etrex position and the corrections to this display or a laptop and have it do your navigation, or use a different GPS that will take in corrections.

-------Original Message-------
From: Robert Ussery <uavscience@FRII.COM>
Sent: 09/02/03 04:11 PM
To: PICLIST@MITVMA.MIT.EDU
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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