Δψ versus as derived from the attitude data simulated with the DAM. In this case the B-spline representation of the astrometric attitude used a knot interval of 2.1 s. Each point represents a time-step (0.05 s) from the simulation. The central red line represents the mean value of the points within 1 μas  s^-2 bins, and the two adjacent blue lines are 1σ intervals (for the sake of clarity 90% of the green points are not displayed). These lines demonstrate that a linear fit is suitable. The large variance of the points around the line reflects the fact that Eq. (19) is just an approximation, valid only when the second derivative is constant during the integration time. The second derivative is actually only (approximately) constant during 1 s intervals (due to thruster commands).