TM 1-1510-223-10 2. Air-to-air/air-to-ground switch - A/G. 3. Channel selector knob - Select desired channel. (3)   TACAN  air-to-air  ranging.     Air-to-air  ranging may be accomplished between two airborne aircraft when each   is   equipped   with   an   appropriate   TACAN   system. Air-to-air  ranging  is  activated  by  placing  the  two-position mode selector switch on each aircraft's respective TACAN control unit in the A/A position and tuning to channels that are  63  channels  apart  from  each  other  (such  as  channel 20 and channel 83).  The X/Y mode switches will have to both be in the X or Y mode position. 3-27. INERTIAL NAVIGATION SYSTEM. a. Description.   The inertial navigation system (INS) is    a    self-contained    navigation    and    attitude    reference system.    It  is  aided  by  (but  not  dependent  upon)  data obtained    from    the    global    positioning    system    (GPS), TACAN, encoding altimeter data, and heading information from the gyromagnetic compass system The position and attitude  information  computed  by  the  INS  is  supplied  to the  automatic  flight  control  system,  the  electronic  flight instrument system (EFIS), and to mission equipment The INS  provides  a  visual  display  of  present  position  data  in universal    transverse    mercator    (UTM)    coordinates    or conventional   geographic   (latitude-longitude)   coordinates during  all  phases  of  flight  When  approaching  the  point selected for a leg switch, a WPT will be displayed on both EHSI's   and   "way-point   alert"   will   be   displayed   on   the multifunction   display   (MFD)   informing   the   pilot   of   an imminent  automatic  leg  switch  or  the  need  to  manually insert   course   change   data   The   INS   may   be   manually updated  for  precise  aircraft  present  position  accuracy  by flying  over  a  reference  point  of  known  coordinates.    The INS may be updated automatically by the TACAN system or the GPS.  Altitude information is automatically inserted into the INS computer by an encoding altimeter whenever the INS is operational. The  INS  consists  of  a  navigation  unit  (NU),  mode selector unit (MSU), and a battery unit (BU). The  aircraft  survivability  equipment/avionics  control system (ASE/ACS) keyboard (fig.  4-3) provides a means for  entering  data  into  the  INS  and  displaying  navigation and  system  status  information.    Refer  to  chapter  4  for  a complete   description   and   operating   instructions   for   the ASE/ACS. The   INS   system   receives   AC   power   from   a   10- ampere INS PWR, a 5-ampere INS HIR POWER, and a 5-ampere  INS  FAN  PWR  circuit  breaker,  located  on  the mission   power   cabinet   The   INS   system   receives   DC power  from  a  5-ampere  INS  CONTROL  circuit  breaker located on the overhead circuit breaker panel (fig.  2-7). (1)   Navigation   unit.     The   navigation   unit   (NU) contains    an    inertial    reference    unit    (IRU),    a    digital computer    unit    (DCU),    and    an    inertial    reference    unit electronics   (IRUE).      All   INS   attitude,   navigation,   and steering information is determined in the NU. (a)   Inertial     reference     unit.         The     inertial reference   unit   (IRU)   is   a   gyro-stabilized   platform   and gimbal   assembly   which   is   electronically   controlled   to provide a local horizontal azimuth reference regardless of aircraft attitude. (b)   Battery    unit.        The    battery    unit    (BU) provides auxiliary DC power to initiate INS turn-on and to supply  essential  power  to  maintain  INS  operation  should 115  volt  primary  power  be  interrupted  after  INS  turn-on. The  battery  unit  will  sustain  operation  of  the  INS  for  15 minutes. (c)   Inertial   reference   unit   electronics.     The inertial  reference  unit  electronics  (IRUE)-includes  power circuits, temperature control circuits, and circuits required for   interface   between   the   inertial   reference   unit   (IRU), computer, mode selector unit,(MSU), aircraft survivability equipment/avionics   control   system   (ASE/ACS),   battery unit, and for malfunction warning signals. b. Controls,   Indicators,   and   Functions,   INS   Mode Selector Unit (fig.  3-28). (1)   READY NAV lamp.   The READY NAV (ready to navigate) lamp illuminates when high-accuracy alignment  status  of  the  INS  has  been  attained.    When alignment has been accomplished with the mode selector set to ALIGN, the ready NAV lamp will remain illuminated until   NAV   is   selected.      The   READY   NAV   lamp   will illuminate momentarily during transition from state 9 to 8 and   when   alignment   is   accomplished   with   the   mode selector at the NAV position. (2)   BAT lamp.      The battery (BAT) lamp illuminates   only   when   the   INS   has   automatically   shut down   due   to   low   battery   unit   voltage.      The   INS   will automatically    shut    down    before    the    battery    unit    is completely  discharged.    A  shutdown  due  to  low  battery unit voltage can only occur while the INS is operating on battery unit power.  The INS operates on battery power for a short period during coarse leveling (alignment state 8). If  the  battery  unit  charge  is  below  the  required  minimum level  during  this  period,  the  INS  will  shut  down  and  the BAT lamp will illuminate to indicate the reason. (3)   Mode selector knob.  The mode selector knob controls the operating modes of the INS.  The knob must be    pulled    out    for    rotation    across    mechanical    stops between STBY and ALIGN and between NAV and ATT. 3-66