Average calibrator density, considering sources graded P or S and with a smoothing kernel of 3°, for three different stations: (top) DE605/Jülich, Germany, about 220 km from the core station at Exloo, Netherlands; (middle) DE602/Unterweilenbach, near Munich, Germany, about 580 km; (bottom) SE607/Onsala, Sweden, about 600 km. Contours are at 0.05, 0.1, 0.2, 0.5, and 1.0 sources per square degree, with the yellow contour at the highest density. The red lines indicate Galactic latitudes of ± 10°, and the two large gaps in coverage are close to the bright radio sources Cyg A and Cas A. Some small apparent gaps in coverage at higher Galactic latitudes are due to non-participation of the relevant station in one epoch of observations.