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