I wonder if solvePNP would actually give you better translation estimates than the differential GPS does. I suppose it depends on the scale of the world points you are imaging and the quality of your intrinsic calibration. I also wonder if the GPS coordinates you get back are relative to the nodal point of the camera, or some other point on the camera, and if not the nodal point, how far off is it? And how much would an incorrect value for the tvec influence the orientation computation.
Have you tried using solvePNP directly and comparing the tvec it generates with the one your DGPS gives you? Are the angles you are getting back not as accurate as you need?
I obviously don’t have any real understanding of what you are trying to achieve, but I do have to wonder if you might be solving a problem that is “in the noise” so to speak.
-Steve