Fringe Period -- from Eric Weisstein's World of Physics

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Fringe Period

The time-average cross-correlation of the electric field strengths received at two points from two antennas is given by

where

is the zenith (or "position") angle of the source measured from the center of the baseline and is the geometric delay, or time light would take to travel the distance . The baseline remains fixed, but changes as the Earth rotates.

Fringe, Fringe Frequency, Synthesis Imaging