The  MAD system consists of the AN/ASQ-81 MAD set, AN/ASA-64 Submarine Anomaly Detecting (SAD)   group,  AN/ASA-65 magnetic compensator group, AN/ASA-71 selector control group,  and  the RO-32/ASQ  MAD  recorder.

AN/ASQ-81 MAD Set

The ASQ-81 MAD system employs a helium magnetometer that detects submarines by measuring changes (anomalies) in the earth's magnetic field caused by the submarine. The detected anomalies are registered on a paper recorder (RO -32).  Breaking it down further, the  AN/ASQ-81  set consists  of  the  DT-323 magnetic detector, the AM-4535 amplifier-power supply,  and the C-6983 detecting set control box. 

MAGNETIC   DETECTOR

The system operates on the atomic properties of optically pumped metastable [1] helium atoms to detect variations in total magnetic field intensity. Changes in the Larmor frequency [2] of the sensing elements are converted to an analogue voltage, which is processed by bandpass filters before it is displayed to the operator. The detection  element  includes  six  separate  helium absorption  cells  and  six  IR  detectors,  arranged  in pairs, with the pairs oriented at 90° to each other. This  configuration  ensures  that  one  or  more  of  the pairs  is  at  least partially in line with the earth’s field regardless of aircraft attitude or direction of flight. The signals from all three detector pairs are combined in  a  summing  amplifier.  The  final  output  to  the amplifier-power supply  is  not  affected  by  aircraft maneuvers because of the arrangement.

AM-4535 AMPLIFIER-POWER SUPPLY.

The  amplifier-power supply  (fig.  4-26)  serves  two purposes.  One dfunction is to provide the necessary power to the MAD subsystem. The amplifier section contains the  necessary  electronics  to  detect  the  anomaly  signal from the detector output signal. There are three fail indicators on the amplifier-power supply. The FAIL light comes on when there is a fault in the assembly being tested with the BITE switch.  The  FAIL  DETECTOR  and  the  FAIL  AMP PWR SUPPLY lights indicate failure of the magnetic detector  or  the  amplifier-power  supply.  The  ALT COMP  dial  is  used  to  vary  the  amplitude  of  the altitude compensation signal. The BUILT IN TEST switch  provides  a  self-test  of  quick  replaceable assemblies  in  the  amplifier-power  supply.  The  two circuit breakers provide circuit protection for the DC power to the magnetic detector and the 115-volt AC power  to  the  amplifier-power  supply. On the right side of the amplifier-power supply, there  is  a  hinged  door  that  covers  a  maintenance panel. When this door is closed,   the equipment operates  in  the  normal  mode. On  the  maintenance panel there is a RES OSC ADJ switch that is used to manually  adjust  the  resonance  oscillator  frequency during   maintenance   procedures.   There   is   also   a MODE SELECT switch that selects various system configurations  necessary  for  proper  maintenance  and troubleshooting.

C-6983  CONTROL  BOX.

The  detecting  set control  box  (fig.  4-27)  contains  the  operating  switches and indicators for the MAD system. Across the top of the faceplate are five indicators that indicate faults in the other units. The indicator labeled 3 indicates a magnetic  detector  failure  when  lit.  The  indicator labeled  2  indicates  amplifier  failure.  The  next  two indicators  indicate  a  control  box  fault.  The  SYS READY  indicator  illuminates  when  the  system  is ready for operation. This indicator will blink during warm-up. There  are  three  toggle  switches  across  the  middle portion of the control box. The one on the right is the power   switch. This  switch  applies  power  to  the system. The middle switch is labeled CAL. It selects the calibration signal for use. The switch on the left is labeled ALT COMP. This switch is used to connect the altitude compensator to the system. The  bottom  portion  of  the  control  box  contains four  knobs.  The  two  on  the  left  side  are  labeled BANDPASS.  These  knobs  select  the  high  and  low frequencies. The  knob  labeled  REC  ZERO  is  a dual-purpose knob. Turning this knob controls the pen deflection on the recorder. Depressing the knob inhibits system output. The  bottom  right  knob  is labeled  gFS,  and  is  used  to  select  one  of  nine sensitivity  ranges  (from 0.1g to 40g full  scale)  or self-test.  In  the  TST  position,  the  self-test  function will be initiated.

(Left, Fig.  4-26) AM-4535  Amplifier-Power  Supply  . (Right, Fig.  4-27) C-6893 Detecting Set Control.  (Image courtesy Tpub.com)

The  bottom  portion  of  the  control  box  contains four  knobs.  The  two  on  the  left  side  are  labeled BANDPASS.  These  knobs  select  the  high  and  low frequencies. The  knob  labeled  REC  ZERO  is  a dual-purpose knob. Turning this knob controls the pen deflection on the recorder. Depressing the knob inhibits system output. The  bottom  right  knob  is labeled  gFS,  and  is  used  to  select  one  of  nine sensitivity  ranges  (from 0.1 gauss to 40 gauss full  scale)  or self-test.  In  the  TST  position,  the  self-test  function will be initiated.

AN/ASA-64 SAD Group

The SAD group consists of only one unit - the ID-1559  magnetic variation  indicator (MAG  VAR indicator). This indicator receives the MAD signals from  the  ECA,  along  with  roll  attitude  signals.  These signals are processed and a SAD mark is generated, which  is  correlated  with  the  roll  input.  In  cases  of excessive  aircraft  roll  rate,  the  indicator  will  generate a SAD inhibit signal. This signal illuminates the SAD INHIBIT lights on the selector control panel and the pilot and copilot’s navigation advisory panel, letting the operators know the SAD mark is unreliable.

AN/ASA-65 Magnetic Compensator Group

The magnetic compensator group consists of the AM-6459  electronic  control  amplifier  (compensator ECA),   C-8935 control-indicator,  DT-355   magnetometer  assembly,   three   compensating  coils CP-1390  magnetic  field  computer,  and  ID-2254 magnetic  field  indicator.

AM-6459 ELECTRONIC CONTROL AMPLIFIER (ECA)

The   electronic control  amplifier (ECA)  processes  standard  magnetic  anomaly  detector signals from the   MAD   subsystems, operator compensation adjustments, and  maneuver  signals from  the  magnetometer. The   ECA  provides compensation currents, which are sent to the MAD boom compensation coils.

C-8935   COMPENSATOR  CONTROL- INDICATOR

This  control-indicator (fig.  4-28) contains potentiometers for adjustment of the maneuver and correlated signals into compensating terms. The potentiometer outputs are routed back to the ECA to be amplified. From there they are sent to the  compensator coil as compensation  signals.

On the face of the control-indicator there are nine index  counters.  The  top  three  provide  the  adjustment index   for   the   potentiometers   in   the   transverse, longitudinal,   and   vertical   magnetometer   circuits. They  are  labeled  T  (transverse),  L  (longitudinal),  and V  (vertical).  The  other  six  (labeled  1  to  6)  provide compensation   adjustment   for   the   T,   L,   and   V magnetometer  circuits. The MAG TERM knob selects the magnetic term to  be  compensated.  This  knob  must  be  in  the  OFF position unless compensation is required. The RATE knob  selects  the  speed  of  the  servomotor  with  1  being the slowest and 4 the fastest. Across the bottom of the faceplate there are four toggle  switches.  The  POWER-OFF  switch  provides power to the unit. The SERVO-OFF switch provides both ac and dc power to the servomotor system. This switch   must   be   in   the   OFF   position   unless recompensation  is  required.  The  UP-DOWN  switch provides voltage directly to the servomotor selected. The  counter  indication  will  increase  or  decrease depending on which way this switch is toggled. The +/OFF/– switch provides voltage to the servo system. In the OFF position, the servomotor is operated only by the UP-DOWN switch.

DT-355  MAGNETOMETER  ASSEMBLY

The  magnetometer  assembly  contains  three  coils oriented  to  sense  magnetic  strength  in  each  of  the basic  longitudinal,  transverse,  and  vertical  axes. This results in three output signals, which are sent to the ECA. These coils are located in the MAD boom.

(Fig.  4-28) C-8935  Compensator Control Indicator. (Image courtesy Tpub.com)

COMPENSATION  COILS.

There are three compensating coils located in the boom. These coils generate the magnetic   field that opposes   the aircraft-generated   noise   fields   for   compensation. There is one coil each for the transverse, vertical, and longitudinal  fields.

CP-1390 MAGNETIC FIELD COMPUTER.

The magnetic field computer, along with the magnetic field   indicator,   computerizes   the   compensation procedure. The correlation portion of the system, the 2A5  board  in  the  ECA,  becomes  redundant  to  the computer. The magnetic field computer receives the maneuver   signals,   MAD   signals,   and   the potentiometer   outputs.   From   these   signals,   it computes the adjustment values for the nine magnetic terms simultaneously.

ID-2254  MAGNETIC  FIELD  INDICATOR.

The  magnetic  field  indicator  (fig.  4-29 below)  allows  the operator to select various weapon loads and initiate the  self-test,  auto  compensation,  and  weapon  deploy- ment programs. It  also  displays  the  most  recent computer-calculated  term  difference  value. The  PWR/OFF  switch  accesses  aircraft  power. The  DISPLAY  indicator  is  a  four-digit  numerical display  and  a  polarity  indicator.  It  shows  the  various BITE codes, term values, or calibration values. The EXEC  push  button  initiates  all  commands.  This button  must  be  pressed  after  each  selection  of  the MODE switch.

The MODE switch is a 14-position rotary switch that provides computer identification and control of fixed   compensation   functions.   The   OFF   position means  that  there  are  no  functions  processed.  The BITE  position  conducts  a  built-in  test  and  reports  the results  via  the  digital  readout.  In  the  COMP  position, pressing  the  EXEC  button  conducts  the  nine-term compensation  program.  The  WD  position  enables  the four-term   weapon   deployment   compensation program. In   the   CAL   position,   a   digital   value measurement  of  the  magnetic  coils  for  calibration accuracy is initiated. The other nine positions report the most recent computer-calculated term difference value via the DISPLAY. Remember, after selecting any of the positions on the MODE switch, the EXEC button must be pressed. The  FAULT  indicator  illuminates  whenever  a fault  condition  exists.  The  WPN  LOAD  switch  is  a nine-position   switch   labeled   0-8.   The   number   of weapons  being  carried  is  selected  on  this  switch  prior to compensation. This provides compensation for at least  80  percent  of  the  weapons  interference  field.

AN/ASA-71 Selector Control Group

The selector control group consists of two units. These units are the MAD selector control panel and the  selector  control  subassembly.

C-7693/ASA-71   SELECTOR   CONTROL PANEL.

This  selector  control  (fig.  4-30)  selects  the signal  to  be  recorded  on  the  MAD  recorder  and adjusts  the  threshold  voltage  for  the  SAD  system. The two knobs labeled BLACK PEN and RED PEN select which signal goes to which pen on the recorder. The  MAD  AUX  POWER-OFF  switch  supplies primary  AC  power  to  the  SAD  system  and  the  selector control  subassembly.  The  INHIBIT  light  indicates  an inhibit  signal  from  the  SAD  system.

Inages courtesy Tpub.com

MX-8109/ASA-71  SELECTOR  CONTROL SUBASSEMBLY.

The  MAD  signals  from  the MAD control and the SAD mark from the MAG VAR indicator  are  routed  to  this  subassembly.  The  selector control  panel  selects  which  one  goes  to  which  pen, and the subassembly routes the signal to the proper pen.  A  SAD  mark  1-kHz  tone  is  generated  by  the subassembly  to  be  supplied  to  the  ICS  system  for  the SENSOR  operator.

RO-32 MAD RECORDER

The RO-32 recorder makes a hardcopy of MAD contacts and  SAD marks.  This  recorder  has  two styluses,  one  black  and  one  red,  to  differentiate between  the  two.  The  chart  drive  is  removable  to enable the operator to remove and replace the paper tape. There are three knobs on the faceplate. The first switch  is  the  ON/OFF  switch.  The  second  controls the  intensity  of  the  internal  lights.  The  third  knob selects  the  operate  mode  along  with  the  pen calibration  modes. When B is selected on the mode knob, the black pen should trace along the zero line on the paper tape. When the mode knob is switched to the +, the black pen should go to +4. When this knob is switched to the  R  position,  the  red  pen  traces  along  the  zero  line. When it goes to the +, the red pen should swing to the +4 line. Both pens are adjustable to these settings.

FOOTNOTES:

[1] Metastable is a  state of stability that is barely stable. Metastable states may be easily stimulated to become unstable.
[2] When a magnet is placed in a magnetic field, a torque is placed on it, called a 'magnetic moment', causing it to align with the magnetic field. For an electron, however, the magnetic moment is produced by the orbital motion of the electron about the nucleus. This produces a force that causes the magnetic moment to process around the direction of the magnetic field at a frequency termed the Larmor frequency.
 

Reference:
1) http://www.tpub.com/content/aviation/14030/css/14030_100.htm
2) http://www.janes.com/articles/Janes-Underwater-Warfare-Systems/AN-ASQ-81-V-ASW-magnetometer-United-States.html

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July 24/10