This is an open source Kalman filter C++ library based on Eigen3 library for matrix operations. The library has generic template based classes for most of Kalman filter variants including: (1) Kalman Filter, (2) Extended Kalman Filter, (3) Unscented Kalman Filter, and (4) Square-root UKF..
Why do you think that https://github.com/cantaro86/Financial-Models-Numerical-Methods is a good alternative to OpenKF