In the mixed coordinates, (p_x_bar,p_y_bar), the motion of the beam follows a Lissajous curve. The spread of the markers yields direct information on the random error created by the four BPMs, with standard deviations sigma BPM, x approximately 0.66 mm and sigma BPM, y approximately 0.5 mm. The red line is the best fit of equation (1) to the experimental data, which confirms that the topology is consistent with that of a fixed line.
