Simulation of the astrometric model response to a step change U(y) in the position of chip 2 for the example sky field of dw14 offset by 73 px from the detector center. Dots show the response in the epoch residuals of reference stars, and the function W_RF is the smoothed response in proper motions to the step change of proper motions of chip 2 stars. The function W_CAL is the same, but for the reference stars distributed evenly over FoV.