  We study arrays of planar Nb Josephson junctions with contacts to
intermediate electrodes, which allow measurements of individual junctions and,
thus, provide an insight into intricate array dynamics. We observe a robust
phase-locking of arrays, despite a significant inter-junction separation.
Several unusual phenomena are reported, such as a bi-stable critical current
with re-entrant superconductivity upon switching of nearby junctions; and
incorrect Shapiro steps, occurring at mixing frequencies between the external
RF radiation and the internal Josephson frequency in nearby junctions. Our
results reveal a surprisingly strong and long-range inter-junction interaction.
It is attributed to nonlocality of planar junction electrodynamics, caused by
the long-range spreading of stray electromagnetic fields. The nonlocality
greatly enhances the high-frequency interjunction coupling and enables
large-scale synchronization. Therefore, we conclude that planar geometry is
advantageous for realization of coherent Josephson electronics.
