Aircraft measurement of electric field: Self‐calibration

Abstract

The electric field of a thunderstorm is distorted by the metallic surfaces of airplanes and rockets. The amplitude of the local, distorted electric field can be measured at selected locations on the surface of the vehicle with electric field meters. Measuring the thunderstorm field entails finding the relations between the local fields at the meters and the ambient, undistorted electric field that would exist in the absence of the vehicle. Calibration can be performed in the following steps: (1)Find linear combinations F0, F1, …, of local field amplitudes that are independent of the charge on the vehicle. (2) While the ambient electric field is constant in the Earth coordinate system, rotate the vehicle (roll and turn or roll and pitch) and record the local field amplitudes and the roll, pitch, and heading angles. Find the direction of the electric vector and the ratios between all of the calibration coefficients that best fit the linear combinations of local field amplitudes. (3) Find the magnitude of one of the coefficients by comparison with a calibrated instrument; from this magnitude and all of the ratios, find all of the magnitudes. With this method of calibration, finding out how the linear combinations F0, F1, … depend on the ambient electric field does not require a symmetric placement of the electric field meters, nor does it require any knowledge of the locations of the field meters. However, inverting the expressions to find the ambient field components as functions of the F𝒾 does place constraints on the locations of the field meters. The results of the above calibration can be used only when the signal at each field meter is a linear function of four parameters: the charge on the airplane and the three components of a uniform electric field from distant charge. When there are five or more electric field meters, the presence of nearby charge can be detected because it is possible to find linear combinations of the signals from five field meters whose value is not zero only when there is nearby charge. This paper presents one method of finding the linear combinations.