Coherence plots for the Space Radio Telescope at 22 GHz. The data shown are for the time interval 17:23:00−17:33:00 UT. Top panel: S/N in the FFT fringe search step of a global fringe fitting algorithm as a function of integration time. Grey dots show the S/N of individual solutions, while large black dots indicate the averages. The dashed line shows the expected -dependence of the S/N. The data starts to deviate from the dashed line at ∼8 min integration time, which marks the maximum averaging time that still improves the S/N. Longer averaging times do not improve the S/N because of increased coherence losses due to a drifting visibility phase. Middle panel: Average visibility amplitude (after applying the corresponding fringe fit solutions). Bottom panel: S/N after the least-squares minimisation step of a global fringe fitting algorithm as a function of integration time. Symbols are the same as in the top panel.