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Hi,
I'm trying to provide a Garmin GPS 15L with dgps data extracted from a Novatel oemv-1.

The technical specification for the Garmin 15L only says:
"Position accuracy of less than 5 meters can be achieved with the GPS 15H & 15L by using Differential
GPS (DGPS) real-time pseudo-range correction data in RTCM SC-104 format, with message types
1, 2, 3, 7, and 9. These corrections can be received by the GPS 15H & 15L on COM 2. The RTCM data
must be received at the same baud rate as the COM 1 port. For details on the SC-104 format, refer to
RTCM Paper 134-89/SC 104-68 by the Radio Technical Commission for Maritime Services."

I've got the RTCM paper, but there are hundreds of possible ways to present the data! I can extract
dgps data from the oemv-1 in binary format, but what exactly is the data supposed to look like?

I can't find any examples of this, any ideas?

dgps data Garmin is talking about are obtained from the GROUND stations using external receivers. It's rather old technology that was in use before WAAS. There are dgps receivers that receive correction data from the ground stations and send it to target GPS via com port as text messages. Not sure that oemv-1 can do it. Or if it can receive dgps data at all.

The oemv can generate a lot of different kinds of data and I've tried them all (almost ) without any success.
http://www.novatel.com/Documents/Manuals/om-20000094.pdf (4Mb)

I'm looking for some kind of example of what the Garmin GPS wants. I guess I then can check the Garmin GPGGA data to make sure it uses the dgps data I provide...

Have you looked at "Differential GPS" by Dale DePriest? (link)

I have found through many experiments and tests that even though there is inherent latency built into the Type 9-3 correction message format, DGPS is consistenly more accurate than WAAS SBAS especially when the Reference Stations are nearby.

DGPS is certainly not "old technology" and frankly DGPS is much more reliable than WAAS for ground station differential.

Differential is based on RTCM and has very little to do with NMEA sentences (such as GPGGA). I would be surpised if the garmin did not accept RTCM but without checking the spec would not be sure. I fI get time I will have a look.