These ships use new and improved radar, sonar, electronic countermeasures, and communications systems  to  enhance  their  detection  capabilities. Another  major  surface  unit  is  the  aircraft carrier,  with  ASW  aircraft  embarked.  A  carrier can monitor midocean areas beyond the effective range  of  land-based  patrol  aircraft. ASW   AIRCRAFT Aircraft have the ability to investigate distant contacts  rapidly  and  are  relatively  invulnerable to  submerged  submarines.  They  also  have  the advantages  of  speed,  relatively  long  range,  and weapons-carrying capability. Therefore, they may fulfill  the  antisubmarine  mission  independently or  in  coordination  with  other  types  of  anti- submarine units. The three basic antisubmarine warfare aircraft are   long-range   patrol   aircraft,   medium-range carrier-based  aircraft,  and  helicopters.  We described  some  of  these  aircraft,  primarily  the P-3C  Orion,  the  S-3A  Viking,  and  the  LAMPS III   helicopters,   earlier   in   this   chapter.   These aircraft  use  a  wide  variety  of  electronic  devices to  detect  submarines. The  magnetic  anomaly  detection  (MAD) device is used mainly for submarine classification purposes. Depending on the height of the aircraft and other variables, it can detect a submarine by variations in the earth’s magnetic lines of force. Because of its limited range, MAD is unsuitable as  a  device  for  open  area  searches.  However,  it is   effective   when   used   in   geographically   or tactically   defined   or   restricted   small   areas. Aircraft  normally  use  the  MAD  device  to  detect the  specific  location  of  a  submarine  before  they attack it. Expendable  sonobuoys,  used  with  measured success  against  submarines  of  the  last  war,  are very  useful  against  submarines  in  a  variety  of tactical  situations.  Sonobuoys  are  tubes  containing a  hydrophore  and  radio  transmitter.  As  aircraft drop them into the water, the hydrophores pick up  sounds  and  broadcast  them  to  surface  craft or   aircraft.   Each   sonobuoy   is   on   a   slightly different  frequency.  An  active  buoy  is  also  used that emits a sound signal and listens for the return echo. Since  helicopters  are  capable  of  hovering,  they use a different piece of equipment. The aircraft, by  means  of  a  long  cable,  lowers  a  cylindrical sonar  transducer  into  the  water  while  hovering over  the  suspected  contact  area.  With  this  gear the  helicopter  can  listen  or  echo-range  (determine the  location  of  a  submarine). Other  methods  of  detection  include  infrared detection   and   explosive   echo   ranging   using sonobuoys. In   all   types   of   airborne   electronic   ASW devices,  proper  training  of  both  operating  and maintenance  personnel  is  paramount  to  successful application  of  the  equipment.  Certain  applications require  special  techniques  for  effective  use  of sonobuoys  and  other  sonic  devices.  Proper  and accurate   sound   identification   and   spotting   of snorkel  targets  on  radarscopes  are  examples. ASW   SUBMARINES The  submarine  itself  is  perhaps  the  most effective  antisubmarine  vehicle.  It  operates  in  the same medium as the target and shares the target’s advantages  of  concealment  and  passive  detection. (Passive  sonar  depends  entirely  on  the  target’s noise as the sound source rather than the returned echoes  of  a  transmitted  signal.)  The  submarine can detect enemy submarines while working with other  ASW  forces  or  while  working  independently. Submarines can precede carrier strike forces into enemy waters, function as ASW screens, and operate as minelayers. Fleet ballistic missile submarines are used to destroy enemy targets when ordered by the President  of  the  United  States. SOUND NAVIGATION AND RANGING  (SONAR) The use of sonar (sound navigation and ranging) is the principal method of submarine detection. We have two types of sonar—passive and active. Sonar is an electronic device that either detects underwater sounds or transmits them. Passive sonar detects sounds originating under water. Active sonar is an electronic device that can transmit (through the depths)  a  sound  wave  which,  upon  striking  an object, will reflect. Submarines use passive sonar to  enable  them  to  detect  noise-making  objects without  transmitting  a  telltale  ping  themselves. To understand how sonar works, you must first understand sound. Sound is the physical energy that causes the sensation of hearing. It travels in the form of waves away from the point of origin, as ripples travel out in all directions from a pebble tossed into a pond. Echoes are created when sound waves strike objects through which they cannot travel and therefore bounce back to the source. 12-14