  In the Ehrenfest wind tree model, a point particle moves on the plane and
collides with randomly placed fixed square obstacles under the usual law of
geometric optics. The particle represents the wind and the squares are the
trees. We examine the periodic version of the model. Previous authors analyze
the dynamical properties of the model using techniques from algebraic topology
or ergodic theory. In contrast to these works, we adopt a signal processing
viewpoint. We describe the phenomenon of the long-term trajectories by using a
3-state hidden Markov model.
