3D engagement kinematics from Song(1994) Seong-Ho Song and In-Joong Ha, “A Lyapunov-like approach to performance analysis of 3-dimensional pure PNG laws,” IEEE Trans. Aerosp. Electron. Syst., vol. 30, no. 1, pp. 238–248, 1994. http://ieeexplore.ieee.org/document/250424/