TM 1-1510-225-10 3-8 Section III.  NAVIGATION Not Applicable Section IV.  TRANSPONDER AND RADAR 3-5.  TRANSPONDER – APX-100. a.   Description.      The   transponder   (APX-100) system  is  a  position  tracking,  identification,  altitude reporting,  and  emergency-tracking  device.    This  set receives,  decodes,  and  responds  to  interrogations  by search  radar.    Range  of  the  set  is  normally  limited  to line-of-sight. b.  Operating   Controls   and   Functions.    The operating   controls   of   the   transponder   system   are described as follows and illustrated in Figure 3-3. (1)   TEST GO Indicator – Illumination indicates successful completion of built-in-test (BIT). (2)   TEST   /    MON   Indicator   –    Illumination indicates unit has malfunctioned, or interrogation by a ground station (MON). (3)   ANT Switch. Place the TOP / BOT / DIV switch in the DIV position  during  flight  operations.    Do  not use  the  TOP  or  BOT  position.    This  does not preclude  using  the  TOP  or  BOT  switch position for ground based system checks.  (a)  TOP  – Selects use of top antenna. (b)  DIV    –    Selects    diversity    operation using both antennas. (c)  BOT – Selects use of bottom antenna. (4)   RAD   TEST   OUT   Switch.      Enables   an appropriately equipped transponder to reply test mode interrogations from an AN/UPM-92 or similar test set. (5)   MASTER Control. (a)  OFF – Turns set OFF. (b)  STBY     –     Places     set     in     warmup (standby)   condition.      Red   NO-GO   light   is    ON   in standby position. (c)  NORM .        Operates    set    at    normal sensitivity. (d)  EMER.    Transmits  emergency  reply code. (6)   STATUS   ANT   Indicator    –    Illumination indicates the BIT or MON failure is due to high VSWR in antenna. (7)   STATUS    KIT    Indicator    –     Illumination indicates  the  BIT  or   MON  failure  is  due  to  external computer. (8)   STATUS   ALT   Indicator   –    Illumination indicates  the  BIT  or   MON  failure  is  due  to  Altitude Digitizer. (9)   IDENT MIC Switch. (a)  IDENT   –   Activates   transmission   of identification (IP) pulse. (b)  MIC    –    Enables    the    POS    IDENT button   located   only   on   the   pilot's   side    CONTROL wheel  to  activate  transmission  of   IDENT  signal  from transponder. (c)  OUT – Disables IDENT MIC. (10)   MODE  4  REPLY  Indicator  –  Illumination indicates  that  a   REPLY  has  been  made  to  a  valid MODE  4  interrogation  or  will  flash  when  in  standby position and interrogated for MODE 4. (11)   MODE 4 AUDIO / LIGHT / OUT Switch. If the MODE 4 computer is installed in the aircraft,  place  the  AUDIO  /  LIGHT  /  OUT switch   in   the   AUDIO   or   LIGHT   position during  flight  operations.    Do  not  use  the OUT position. (a)  AUDIO  –  Permits  aural  and  REPLY light  monitoring  of  valid  MODE  4  interrogations  and replies. (b)  LIGHT   –   Permits   REPLY   indicator only monitoring. WARNING WARNING