Refraction Error in Measurement of Angle of Arrival with the Radio Interferometer

The ``extra-path' error in the measurement of angle of arrival with the radio interferometer is analyzed as a function of target coordinates and baseline B for an exponential atmosphere. By treating the error rather than the measurement and by formulating the error in terms of well-known radar refraction parameters, the necessity for numerical integrations characteristic of earlier approaches is avoided. The table of computed errors shows that angle of arrival can be measured to an accuracy of 0.01 mrad for B?500 meters.