G. P. Roussos, D. V. Dimarogonas, and K. J. Kyriakopoulos, Distributed 3d navigation and collision avoidance 221 for nonholonomic aircraft-like vehicles, Proceedings of the 2009 European Control Conference, 2009. 222 11. D.W. Scott. Multivariate Density Estimation: Theory, Practice, and Visualization. A Wiley-interscience