def reverse_velocity_kalman_model():
    """Return a KalmanState set up to model going backwards in time"""
    om = np.array([[1, 0, 0, 0], [0, 1, 0, 0]])
    tm = np.array([[1, 0, -1, 0], [0, 1, 0, -1], [0, 0, 1, 0], [0, 0, 0, 1]])
    return KalmanState(om, tm)