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Abstracl--.An experiment designed to measure wavefront statistics in 3.2-mm wavelength horizontal transmission over an 18.95-km path is described. A frequency-stabilized signal is transmitted over the path and is received in two spaced antennas having variable separation. The receivers are instrumented to measure phase difference and amplitude fluctuations. A simultaneous beam-swinging experiment using a precisioncontrolled 4.57-m antenna colocated with the phase difference receivers provides data on wavefront tilt fluctuations. Preliminary results indicate that the random wavefront tilt is the predominant atmosphere-induced wavefront phase disturbance. They further suggest that spatial coherence of wavefronts is of sufficient extent that apertures in excess of 7 m in diameter are feasible at the 3.2-mm wavelength.