Figure 4-24.—A. Stationary detector boom. B. Extendable detector boom. C. Cable-deployed towed detector

COMPENSATION Regardless of its source, strength, or direction, any magnetic field may be defined in terms of three axial coordinates. That is, it must act through any or all of three possible directions—longitudinal, lateral, or vertical—in relation to the magnetometer detector. Compensation for magnetic noises is necessary to provide a magnetically clean environment so that the detecting system will not be limited to the magnetic signal associated with the aircraft itself. Experience has shown that the induced fields and eddy current fields for a given type of aircraft are constant. That is, from one aircraft to another of the same type, the difference in fields is negligible. These fields may be expected to remain constant throughout the life of the aircraft, provided significant structural changes are not made. In view of these factors, it is present practice for the aircraft manufacturer to provide compensation for induced fields and eddy current fields. Eddy current field compensation is usually achieved by placing the magnetometer (detecting head) in a relatively quiet magnetic area. In some aircraft the detecting head is placed at least 8 feet from the fuselage. This is done by enclosing the detecting head in a fixed boom (fig. 4-24, view A), or in an extendable boom (fig. 4-24, view B). Helicopters tow the detecting head by use of a cable (fig. 4-24, view C). Induced magnetic field compensation is accomplished by using Permalloy strips. The aircraft is rotated to different compass headings, and the magnetic moment is measured. The polarity and the variation of the magnetic moment are noted for each heading, and Permalloy strips are oriented near the detector magnetometer to compensate for field changes due to aircraft rotation. Additional 232.179 Figure 4-24.—A. Stationary detector boom. B. Extendable detector boom. C. Cable-deployed towed detector. 4-19