TM 1-1510-223-10 c. INS  Modes  of  Operation.   The  INS  can  be operated    in    four    modes:    Standby    (STBY),    Align, navigation   (NAV),   attitude   (ATT).      When   the   INS   is turned   on   it   will   (under   normal   conditions)   proceed through the STBY and ALIGN modes and enter the NAV mode, where it will remain until it is downmoded at the end of the flight.  The only exception will be in the event of  a  malfunction,  in  which  case  the  INS  will  be  shut down or placed in the ATT mode.  In the ATT mode, the computer  is  turned  off  and  the  INS  will  only  provide attitude signals. (1) Standby  mode.    The  characteristics  of the  standby  mode  vary,  depending  upon  whether  the INS  is  being  turned  on  or  is  being  downmoded  from NAV,    ALIGN,    or    ATT.        During    standby    mode    the alignment state number is 9. (a) Standby  mode  (INS  being  turned on).   If  the  INS  is  being  turned  on  by  setting  the  mode selector  to  STBY,  the  navigation  unit  is  brought  up  to operating temperature by fast warm-up heaters and the gyro wheels are brought up to speed.  The INS platform is caged to the aircraft axes and all instrument warning lamps controlled by the INS indicate warning. (b) Standby mode (INS being downmoded).     If  the  INS  is  being  downmoded  from  a higher   mode   by   setting   the   mode   selector   switch   to STBY,  the  INS  platform  retains  its  alignment  with  local horizontal, and all instrument flags controlled by the INS except   ATTITUDE   and   PLATFORM   HEADING   flags, will indicate warning. NOTE Present position, waypoint coordinates, and TACAN station data can be inserted into the INS while in the standby mode. (2) Alignment   mode.       In    the    alignment (ALIGN)  mode  the  INS  computer  progresses  through  a series of submodes called alignment states. (a) Alignment state 8. During alignment  state  8,  the  INS  platform  is  aligned  to  local horizontal   and   the   INS   battery   is   tested.      Attitude warnings are removed at the beginning of state 8, but all other instrument flags controlled by the INS continue to indicate warnings.  The battery unit is tested during the first   12   seconds   of   this   state.      The   INS   spends   a minimum of 51 seconds in state 8. During   the   transition   of   state   8   to   state   7,   the heading   is   set   to   the   stored   value.      If   this   value compares favorably with magnetic heading plus magnetic variation, the status will remain unchanged.  If the comparison is unfavorable, the  status  will  be  set  invalid,  and  the  system  must  be upmoded to NAV before a heading may be stored. NOTE Present position must be loaded before the system will progress from state 8 to state 7. (b) Alignment  states  7  and  6.     The primary   function   of   states   7   and   6   is   to   refine   the relationship  between  the  INS  platform  and  true  north. This    function    continues    unchanged    throughout    both states 6 and 7.  In state 6 the INS compares the latitude of  the  loaded  present  position  with  the  latitude  it  has computed as part of the alignment process, and it uses the results of this comparison (with other information) to update certain self-calibration data.  During these states it is possible to enter the navigation mode if the stored heading  is  valid.    Entry  to  NAV  will  immediately  follow the setting of the mode selector to NAV.  The alignment will    be    entirely    dependent    upon    the    stored    values regardless of the length of time in states 7 or 6. (c) Alignment    states    5    through    0. Entry  into  alignment  state  5  indicates  that  normal  or standard   accuracy   alignment   is   complete.      The   NAV mode  may  be  selected  at  any  time  after  entry  into  this mode, but for precision inertial accuracies, the alignment    should    be    allowed    to    progress    until    the READY   NAV   lamp   is   illuminated,   and   preferably   to alignment state 2 or 1.  A standard accuracy alignment takes     about     20     minutes,     while     a     high     accuracy alignment requires about 45 minutes. Alignment  states  4  through  0  indicate  continuing operating    of    the    self    calibration    process    begun    in alignment    states    6    and    5.        In    addition,    automatic calibration data for the azimuth gyro is now available.  If the  INS  is  left  in  ALIGN,  the  self-calibration  data  will continue   to   be   refined   and   the   alignment   state   will decrease toward 0. (3) Navigation   mode.     In   the   navigation (NAV)   mode,   all   navigation   data   is   computed   and available for display.  All warning flags controlled by the INS  will  disappear  from  view.    The  INS  depends  on  its own   inertial   instruments   for   all   position   and   velocity data.  The present position computed by the INS can be updated  by  TACAN,  GPS,  data  link,  or  manually  when the aircraft passes over a known position reference. (4) Attitude  mode.    In  the  attitude  (ATT) mode,  the  computer  is  shut  down  and  no  navigation information  is  available.    The  ATT  mode  is  entered  at the  discretion  of  the  pilot  if  navigation  and/or  steering data  signals  become  unreliable.    Attitude  signals  and platform heading outputs continue to be available.  The NAV  mode  can  only  be  reentered  from  the  ATT  mode after  the  system  has  been  realigned.    Alignment  can take place only on the ground. 3-68