Measured average travel time around the disc centre as a function of the implanted velocity. Top: Plot for mode td4 at a distance of 9.9 px. The linear fit and its properties are also indicated. The slope of the linear fit corresponds to the total integral of the appropriate sensitivity kernel. Bottom: Similar plot for the p₃ mode and Δ = 15 px when using several dates. Different dates are grouped around virtual lines that are shifted vertically, as indicated by the time stamps.