TM 1-1510-224-10 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 con-trolled 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 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. (5) Pattern steering using INS. Pattern steering provides a box or straight line pattern which may be   used   to   fly   the   aircraft   in   a   specific   pattern.      The pattern   will   originate   at   the   current   TO   waypoint.      The length  of  the  base  leg,  true  bearing  of  the  pattern,  offset distance  of  the  return  leg,  and  direction  of  the  turns  are loaded  into  the  computer  via  the  ASE/ACS.    Should  the operator  specify  a  zero  offset  distance  for  the  return  leg, the  direction  of  the  turns  will  alternate  at  each  end  of  the pattern. 3-69