TM 1-1510-224-10 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   '"waypoint   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 HTR 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.  2-9). (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    Iamp.          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. (a) OFF.  Deactivates INS. (b) STBY.  The standby  (STBY)  position is  used  only  during  ground  operation.    Setting  the  mode selector to STBY from OFF: Starts fast warm-up in the navigation unit (NU). 3-67