An illustration of the non-linearity of the delay (in microseconds) with offset for three baselines. Matera-Medicina (crosses), Matera-Wettzell (squares), Medicina-Wettzell (circles). The top panel shows the delay at offsets between 0.01 and 100 arcmin compared to the delay at zero offset. The middle panel shows the same delays after the subtraction of the linear interpolation between zero offset and the 0.3 arcmin value. The bottom panel shows the quotient of the first two plots i.e. the fractional error of the linear fit.