Ratios c_t,fit/c_t,ref for selected locations. The top panel corresponds to the green diamond in Fig. 2, the middle panel to the black diamond 20° above, and the bottom panel to 40° above. To constrain the unknown shift, the choice where the mean ratio is 1.0 for 2.5 mHz ≤ν≤ 3.5 mHz was used. In each panel the azimuth difference between SO/PHI and HMI is given, together with the viewing angle, which is common between the instruments. As in Fig. 4, the smooth red curves show c_t,ad for a height of 190 km, corresponding to the top case. The c_t,ad values for the heights corresponding to the two other cases are indistinguishable at the scale of the plots.