Top: temporal auto-correlation function of u_y for HMI RDA, LCT, and GONG++ RDA. A signal can be seen to persist for at least four solar rotations. Statistical error bars are comparable to line thickness and hence are not shown. Bottom: 2D map of the u_y auto-correlation maps, as a function of latitude and time lag. The deflection with latitude is consistent with differential rotation. The dashed vertical lines in each image indicate an integer equatorial rotation.