Errors in ambient electric field measurements related to position of four shutter-type sensors on an instrumented airplane are analyzed. It is hsown that sensors' positions affect significantly the propagation of errors (1) in estimates of coefficients of the matrix equatins that relate the ambient field to the local fields, and (2) in the signal processing into ambient electric field measurements. Positioning field mills on or nea crossing points of the lines of the airplane's electrical symmetry minimizes the amplification of experimental errors. A computer-simulated block model of the NASA F-106B research airplane, placed in a uniform electric field, is used to calculate the matrix coefficients. The accuracy of this method is evaluated and compared with that of small-scale model measurement and in-flight calibration. A new calibration procedure to determine the net charge on the airplane is suggested. ¿ American Geophysical Union 1987