I recall asking if this functionality wasn't available off the shelf and the answer was that it existed, but it was difficult without also buying a lot of new equipment he didn't need. It apparently cost a lot less to have me do the data conversions (and I didn't charge much for it) on a small SBC than it would to have to buy new equipment and deal with installation, fitment issues, etc.

https://community.emlid.com/t/using-a-pair-of-reach-units-as...

He was post processing the data, but part of RTKLIB is RTKNAVI which is real time and I haven't used it but in theory I'd think RTKNAVI would support moving baseline as well. Post processing should be slightly more accurate as you can use measurements in the future as well but it should still be very accurate real time.

As others have mentioned, There's much better you can do if you're going for optimal and you have the raw satellite timing information from both receivers.

You can go to the extreme and build a full blown IMU that's tolerant to GPS outages. If it's sensitive enough, you can even measure the rotation of the planet. Big expensive ships presumably have that. The boats on a tracker that glitch their orientation when stopped seemingly don't have that, or at least don't have it integrated with that tracking system.

That can get you xyzt position, if you want your three dof direction too, a second receiver/fe/antenna can do a second fix and the differential between the two can be used to find the orientation.

You can do this with a smaller antenna separation better by going down the stack into the guts of the receiver math but it’s difficult because these things are implemented in hardware and encumbered by arms control regulatory hurdles.