Visibility amplitude of 3C 84 as a function of time (first corrections and amplitude calibration applied). Top: amplitude measured by the baselines between the GBT (GB) and a VLBA station (HN, LA and NL). Bottom: amplitude measured only with VLBA baselines. It can be seen how the amplitude measured by the baselines between the GBT and a VLBA station changes with time because of the GBT primary beam response (see text for details). The amplitude measured only with VLBA baselines remains roughly constant.