feet in length and represents the flight path of a searching aircraft through the area of the submarine anomaly. If the submarine were not present, the undisturbed magnetic intensity in the area due to its assumed natural characteristics would be 60,000 gammas. (The gamma is the measure of magnetic intensity and is symbolized by the Greek letter g.) All variations in the field, when the submarine is present, would then be above or below this natural intensity. Therefore, 60,000 gammas is the zero reference drawn on the moving paper tape shown in view C of figure 4-23. Refer to view A of figure 4-23. Starting with the aircraft at point A, where the anomaly is undetectable, the earth’s field concentration decreases to an intensity of 2g (59,998) at point B. Its intensity then increases until a peak value of +45g is reached at point C. From that point it decreases to zero at point D. After point D, another zone of what amounts to magnetic rarefaction is encountered. The earth’s field is less intense than its normal value. Consequently, anomalous values in this zone are considered as minus quantities. A peak minus intensity is reached at point E, and thereafter the signal rises back to its normal, or undetectable, intensity at point F. As the varying degrees of intensity are encountered, they are amplified and used to drive a swinging stylus, as shown in figure 4-23, view B. The tip of the stylus rides against the moving paper tape, leaving an ink trace. The stylus is swung in one direction for positive g, and the other for negative g. The magnitude of its swing is determined by the intensity of the anomaly signal. Figure 4-23, view C, is a sample of paper recording tape showing the approximate trace caused by the anomaly in view A. In the illustration just given, the search aircraft’s altitude was 200 feet. At a lower altitude the anomaly would have been stronger, would have been weaker. MAGNETIC NOISE and at a higher altitude, it For the purposes of this discussion, any noise or disturbance in the aircraft or its equipment that could produce a signal on the recorder is classified as a magnetic noise. In an aircraft there are many sources of magnetic fields, such as engines, struts, control cables, equipment, and ordnance. Many of these fields are of sufficient strength to seriously impair the operation of MAD equipment.    Consequently, some means must be employed to compensate for “magnetic noise” fields. The noise sources fall into two major categories: maneuver noises and dc circuit noises. Maneuver Noises When the aircraft maneuvers, the magnetic field of the aircraft is changed, causing a change in the total magnetic field at the detecting element. The aircraft maneuver rates are such that the signals generated have their major frequency components within the bandpass of the MAD equipment. Maneuver noises may be caused by induced magnetic fields, eddy current fields, or the permanent field. The variations in the induced magnetic field detected by the magnetometer are caused by changes in the aircraft’s heading. This causes the aircraft to present a varying size to the earth’s magnetic field, and only the portion of the aircraft parallel to the field is available for magnetic induction. Eddy current fields produce maneuver noise because of currents that flow in the aircraft’s skin and structural members. When an aircraft’s maneuver causes an eddy current flow, a magnetic field is generated. The eddy current field is a function of the rate of the maneuver. If the maneuver is executed slowly, the effect of the eddy current field is negligible. The structural parts of the aircraft exhibit permanent magnetic fields, and, as the aircraft maneuvers, its composite permanent field remains aligned with it. The angular displacement between the permanent field and the detector magnetometer during a maneuver produces a changing magnetic field, which the detector magnetometer is designed to detect. DC Circuit Noise The dc circuit noise in an aircraft comes from the standard practice in aircraft design of using a single-wire dc system, with the aircraft skin and structure as the ground return. The resulting current loop from the generator to load to generator serves as a large electromagnet that generates a magnetic field similar to a permanent magnetic field. Whenever the dc electrical load of the aircraft is abruptly changed, there is an abrupt change in the magnetic field at the detector. 4-18