  Large radio astronomy multi-element interferometers are frequently used as
single dishes in a tied-array mode when signals from separate antennas are
added. Phase shifts arising during wave propagation through a turbulent
atmosphere can significantly reduce the effective area of an equivalent single
dish. I aim to give estimates of the impact of the ionosphere and troposphere
on the effectiveness of a radio interferometer working in tied-array mode.
Statistical estimates of the effective area are calculated and the power-law of
turbulent atmosphere irregularities has been used. A simple method of
tied-array calibration using optimization techniques is proposed. The impact of
phase errors on the effectiveness of tied-arrays are given for low and high
frequencies. Computer simulations demonstrate the efficacy of the proposed
calibration algorithm.

Instrumentation and Methods for Astrophysics