Plots of closure amplitudes at the X band for source 1807+698 (3C 371) as a function of GMST for two quadrangles, (a) NYALES20–TSUKUB32–WETTZELL–WESTFORD and (b) KOKEE–NYALES20–TSUKUB32–WETTZELL. See Figure 5 for a description of the plot design. The structure-effect pattern of the first quadrangle repeated for 1454 closure measurements in 132 sessions over 11 yr, with only minor differences. A stable pattern is also shown in plot b. These two plots demonstrate that its structure remains constant over a long time.