selector  switch  is  moved  to  the  BT  position.  The recorder  chart  drive  circuits  automatically  position the chart paper to provide correct chart registration. Recorded scale marks on the chart paper denote the temperature scale being used for each temperature recording.  The  recorder  plots  temperature  on  the vertical axis and depth on the horizontal axis of the moving chart. Recorder Range Mode The  recorder  RANGE  mode  is  used  to  obtain continuous  strip-chart  displays  of  target  echo  ranges. Range  scale  control  signals  from  the  receiver  RANGE SCALE-KYDS  switch  are  accepted  by  the  recorder sweep circuits to correlate the range sweeps. As the chart paper moves, range scale marks are recorded on the chart paper to denote the range scale being used for each range recording. Target echo video signals are  applied  to  the  styluses  when  they  appear  in  time, as related to the range sweep. The video signals are recorded  each  time  a  stylus  passes  over  the  range position  of  a  target. The  chart  advances  a  small increment  for  each  stylus  sweep. Recorder Aspect Mode The  recorder  ASPECT  mode  is  used  to  obtain continuous  strip-chart  displays  of  target  echo  signals. Timing  and  control  signals,  generated  within  the recorder, slave the receiver timing circuits to alternate sweep  ramps  between  transmit  and  receive  cycles. During each transmit sweep ramp, a train of short keying   pulses   is   generated,   and   pulsewidth   is regulated in the recorder. This pulse train is applied to the receiver. During each receive sweep ramp, the train  of  received  target  echo  video  pulses  is  applied  to the  recorder  styluses. Target  echo  signal  level  is neither  limited  nor  affected  in  the  system.  This permits  varying  intensity  recordings  (highlights)  of target structural characteristics for optimum target classification. Recorder Test Mode The  sonar  operator  uses  the  recorder  TEST  mode to check the operational status of the recorder. The TEST  mode  effectively  checks  the  operation  of  the recorder  stylus  drive,  stylus  write,  and  chart  drive operations.  In  addition,  all  front  panel  controls  on  the recorder   can   be   checked   by   the   operator   for operational  compliance  and  accuracy. MAGNETIC  ANOMALY  DETECTION Learning  Objective:  Recognize    components and   operating   principles   of   magnetic anomaly detection (MAD). By the beginning of World War II, it had become apparent that the aircraft was a deadly antisubmarine weapon.  This  was  true  even  though  the  ability  to search and detect submarines was solely dependent on visual sightings. The development of radar extended the usefulness of airborne antisubmarine measures, making detection of submarines possible at night or under conditions of poor visibility. However, visual or  radar  detection  was  possible  only  when  the submarine   was   surfaced.   Thus,   some   method   of detecting   submerged   subs   from   an   aircraft   was needed.  The  use  of  sonar  wasn’t  feasible  because there was no direct contact between the fast-moving aircraft  and  the  surface  of  the  water.  The  most feasible  way  of  detecting  a  submerged  submarine  was to detect its disturbance of the local magnetic field of the earth. PRINCIPLES OF MAGNETIC DETECTION Light, radar, and sound energy cannot pass from air into water and return to the air in any degree that is usable  for  airborne  detection.  On  the  other  hand, lines of force in a magnetic field are able to make this transition almost undisturbed because the magnetic permeability of water and air are practically the same. Specifically, the lines of force in the earth’s magnetic field  pass  through  the  surface  of  the  ocean  essentially undeviated   by   the   change   of   medium,   and undiminished  in  strength.  Consequently,  an  object under the water can be detected from a position in the air above if the object has magnetic properties that distort the earth’s magnetic field. A submarine has sufficient  ferrous  mass  and  electrical  equipment  to cause  a  detectable  distortion  (anomaly)  in  the  earth’s field.  The  function  of  the  MAD  equipment  is  to detect this anomaly. Magnetic  Anomaly The lines comprising the earth’s natural magnetic field do not always run straight north and south. If traced along atypical 100-mile path, the field twists at places  to  east  and  west,  and  assumes  different  angles with the horizontal. Angles of change in the east-west direction  are  known  as  angles  of  variation,  while angles between the lines of force and the horizontal 4-15