TM  55-1510-221-10 disregarded.  Left  data  dis- play  is  blank. 9.   INSERT/ADVANCE   push- button -  Press.  Observe TACAN  station  channel number,  in  whole  numbers, will  appear  in  left  data  dis- play;  degree  symbol  and  dec- imal  point  should  be  disre- garded.  Right  data  display  is blank. NOTE If  INSERT/ADVANCE  pushbutton  is pressed,   the   normal   coordinates   indicated in  step  -3-  will  be  displayed. NOTE Waypoint   thumbwheel   may   be   moved   at any  time  and  normal  coordinates  for  new TACAN  station  will  be  displayed. 10.  Data  selector  -  Momentarily to  any  position  other  than L/L  WY  PT,  UTM  WY  PT or   DIS/TIME.   (Returns   INS to   normal   operation.) (u.)   Magnetic   heading. 1. Data   selector   -   HDG/DA. Observe   true   heading   to nearest   tenth   degree   appears in  right  data  display. 2. Keys  “3“  and  “9”  -  Press simultaneously and   hold. Observe  magnetic  heading  to nearest  tenth  of  a  degree appears  in  left  data  display. Drift  angle  continues  to  be displayed  in  right  data  dis- play. 3. Keys  “3“  and  “9”  -  Release. Observe  left  data  display reverts  to  true  heading. (12.)   INS   updating: (a.)  Normal  geographic  present  posi- tion check and update: 1. Data   selector   -   L/L   POS. Observe  latitude  and  longi- 2. tude   of   present   position appear  in  left  and  right  data displays,   respectively. Illuminated   HOLD   pushbut- ton  -  Press.  Observe  latitude and  longitude  in  data  dis- plays  freeze  at  values  present when   HOLD   pushbutton   is pressed. NOTE While   HOLD   pushbutton   light   is   illumi- nated,   TACAN,   GPS   and   data   link updates   are   inhibited. 3.  Keys  -  Press  in  sequence  to load  latitude  of  position  ref- erence,  starting  with  “N“  or “S”  to  indicate  north  or south. Example:  42°54.0' north    =    N    4    2    5    4    0. Observe INSERT/ ADVANCE  pushbutton  light illuminates  when  first  key  is pressed,  and  latitude  appears in   left   data   display   as   keys are  pressed. 4.  INSERT/ADVANCE  push- button -  Press.  Observe INSERT/ADVANCE    push- button   light   remains   illumi- nated,  and  previous  value  of latitude   reappears. 5. Keys  -  Press  in  sequence  to load  longitude  of  position reference,  starting  with  “W” or  “E“  to  indicate  west  or east.  Example:  87°54.9’  west = W 8 7 5 4 9. Observe lon- gitude   appears   in   right   data display  as  keys  are  pressed. 6.  INSERT/ADVANCE  push- button -  Press.  Observe INSERT/ADVANCE and HOLD   pushbutton   lights remain   illuminated.   North position  error  and  east  posi- tion  error,  in  tenth  of  a  nau- tical  mile,  will  appear  in  left and   right   data   displays, respectively. 3-66