Aviation Electronics Technician 1 - Historic Naval Ships Association

Figure 4-22.-Submarine's magnetic moment.magnetic strength. In views C and D, the submarine’smagnetic field distorts the natural field as shown. Thedensity of the natural field is decreased in view C andincreased in view D. The natural angle of dip is alsoaffected, but only very slightly.Submarine AnomalyThe maximum range at which a submarine maybe detected is a function of both the intensity of itsmagnetic anomaly and the sensitivity of the detector.A submarine’s magnetic moment (magneticintensity) (fig. 4-22) determines the intensity of theanomaly. It is dependent mainly on the submarine’salignment in the earth’s field, its size, the latitude atwhich it is detected, and the degree of its permanentmagnetization.MAD equipment, in proper operating condition, isvery sensitive; but the submarine’s anomaly, even at ashort distance, is normally very weak. The strength of acomplex magnetic field (such as that associated with asubmarine) varies as the inverse cube of the distancefrom the field's source. If the detectable strength of afield source has a given value at a given distance and thedistance is doubled, the detectable strength of the sourceat the increased distance will then be one-eighth of itsformer value. Therefore, at least two facts should beclear. First, MAD equipment must be operated at a verylow altitude to gain the greatest proximity possible tothe enemy submarines. Second, the searching aircraftshould fly at a predetermined speed and follow anestimated search pattern. This ensures systematic andthorough searching of the prescribed area so that noexisting anomalies are missed.Anomaly StrengthUp to this point, the inferred strength of asubmarine’s anomaly has been exaggerated forpurposes of explanation. Its actual value is usually sosmall that MAD equipment must be capable ofdetecting a distortion of approximately one part in60,000. This fact is made apparent by pointing outthat the direction of alignment of the earth’s magneticlines of force is rarely changed more than one-half of1 degree in a submarine anomaly.Figure 4-23, view A, represents a contour mapshowing the degree of anomaly caused by asubmarine. The straight line is approximately 800Figure 4-23.-A. Degree of anomaly. B. Anomaly stylus. C. Sample anomaly record.4-17