I know I'm out of the loop on this one, but as a passing comment: be wary of covariance on manifolds (i.e., using any flavor of Kalman filter on lat/long coords or UTM). Differential geometry and information geometry are hard, and figuring out how converting coord paramaterizations affects covariance (i.e., how the distribution that used to be Gaussian in the original space is warped into a hideous abomination in the new space) is black magic. I strongly advise doing all back-end calculations in Euclidean space (e.g., ECEF), and only allowing client interfaces to use UTM/lat-long/whatever. You'll need to use 64-bit data types for ECEF, but at least your calculations will be theoretically sound. You could also center your local coord system around some point in ECEF, allowing 32-bit math to suffice, and you're still no worse off than the local coord systems used in UTM.