aircraft, it is important for air traffic control to know not only   their   location   but   their   identity.   The   Selective Identification Feature (SIF) was developed to expand the IFF system. This increases its flexibility through a multiple-code    transponder    reply.    By    such    means, selective   and   individual   identification   of   aircraft   is possible, with the following results: !    Ground control of friendly aircraft !    Operational    flexibility    in    the    identification process !    A     measure     of     additional     security     in identification ELECTRONIC COUNTERMEASURES A basic rule of warfare is that for each weapon used by one side, a counter-weapon will be developed by the other side. This rule is clearly seen in the development and   use   of   electronic   countermeasures   (ECM).   The objective  of  ECM  is  to  gather  intelligence  from  the enemy's    electronic    devices    and    make    the    devices ineffective. Electronic countermeasures consist of two general types of actions—passive and active. Passive Passive  ECM  operations  are  those  that  cannot  be directly  detected  by  the  enemy.  These  include  search operations where enemy radar transmitters are detected, located, and as many of the signal characteristics as possible are determined. For example, ECM can detect a radar pulse transmission at 1  1/2  times  the  distance  the  radar  return  can  detect  a target. The signal characteristics determine if the radar is  used  for  search,  navigation,  or  fire  control.  Passive countermeasures  also  include  evasive  tactics  taken  to avoid     detection     and     methods     of     controlling     the radiations   from   friendly   equipment.   Such   measures prevent the enemy from using the signals for homing, direction finding, or any other purpose. Active Active ECM operations are actions that the enemy can  detect.  Active  operations  prevent  effective  use  of the enemy's equipment. Electronic jamming interferes with  enemy  radar  and  communications.  Active  radar nonelectronic  jamming  is  done  by  releasing  strips  of metallic   foil   (chaff   or   window)   from   aircraft.   The falling   strips   cause   many   false   targets   or   cause   the enemy scope to cover with clutter that can mask targets from search and fire control radars. Q7-24. What is the meaning of the acronym radar? Q7-25. A radar is a device used to detect objects at distances greater than the eye can see by the use of what basic principle? Q7-26. Sound    waves    travel    how    many    feet    per second? Q7-27. A  system  that  allows  a  friendly  aircraft  to identify itself automatically before approaching  near  enough  to  threaten  other naval units is known as what type of system? Q7-28. What are the objectives of electronic countermeasures? ANTISUBMARINE WARFARE EQUIPMENT (ASW) LEARNING OBJECTIVE: Identify the purpose   and   uses   of   antisubmarine   warfare equipment to include sonobuoys and magnetic anomaly detection equipment. A major problem for the Navy is the detection of enemy     submarines.     Submarine     detection     devices include (SOund NAvigation Ranging (sonar), sonobuoys, and Magnetic Anomaly Detection (MAD) equipment.    Surface    ships,    submarines,    and    harbor defense installations use sonar equipment. Aircraft use MAD equipment. SONOBUOYS The sonobuoy is an expendable electronic listening device   dropped   into   water   from   carrier-based   and land-based    patrol    aircraft.    The    sonobuoy    detects underwater    sounds    and    transmits    these    sounds    to aircraft. A surfaced or snorkeling submarine is not likely to be  detected  by  an  aircraft's  radar.  The  reason  is  the submarine's ECM detects the aircraft's radar at a greater distance than the aircraft can detect the submarine. The sonobuoy helps solve the submarine detection problem. The sonobuoy, housed in a cylindrically shaped tube, is designed   to   float   upright   in   the   water.   Upon   being dropped  from  an  aircraft,  the  sonobuoy,  stabilized  by small  blades,  enters  the  water  in  an  upright  position. Upon striking the water, the stabilizing blades eject and a  small  transmitting  antenna  erects  itself.  The  impact also  causes  the  release  of  a  hydrophone  (underwater microphone). This underwater listening device connects to the end of a cable that permits it to sink to a predetermined depth. The hydrophone receives 7-19