Examples of 288-day averaged cross-correlation function obtained from MDI (left) and HMI (right) as a function of distance, Δ, and time lag, τ. They are computed in the south-north direction and averaged over ± 15° longitude and ± 15° latitude. A 20-min window that isolates the first-skip branch of the cross-correlation function is used in the fitting (20 data points for MDI and 27 data points for HMI). In both panels, the blue curve represents the fitted phase time and the red curves indicate the border of the fitting window for each Δ. The center positions of the fitting window and the fitted phase times for every 3° of travel distance are listed in Table A.1.