TM  1-1520-240-10 3-3-28            (3) Position update can be effected by depress- ing the ENT key. The computer updated the present posi- tion at the time the KYBD key was depressed by using the stored destination coordinates, and adding to them the distance traveled between the time the KYBD key was depressed and the ENT key was depressed. In addition, if an associated variation for the stored destination ex- ists, the present position variation is also updated. (4) If a present position update is unnecessary (as  indicated  by  an  appropriately  small  value  of  DIS- TANCE to go on overflying the destination), set the DIS- PLAY switched to some other position - this action aborts the update mode. b.  Updating Present Position from a Landmark. NOTE There are two methods for updating present position from a landmark. Method 1 is particu- larly useful if the landmark comes up unex- pectedly and the operator needs time to de- termine the coordinates. Method 2 is useful when a landmark update is anticipated. Method 1 (Unexpected update) (1) Set DISPLAY switch to PP position. (2) Overfly landmark and depress KYBD key. The present position display shall freeze. (3) Compare landmark coordinates with those on display. (4) If the difference warrants an update, enter the landmark coordinates by depressing the KYBD key to blink the field to be changed, enter coordinates, then depress the ENT key. The computer updates the present position (from the time the KYBD key was depressed) to the landmark coordinates, and adds to the updated pres- ent position the distance traveled between the time the KYBD  key  was  depressed  and  the  ENT  key  was  de- pressed. (5) If an update is not desired, set the DISPLAY switch  to  some  other  position.  This  action  aborts  the update mode. Method 2 (Anticipated update) 1.   Set DISPLAY switch to WP/TGT position. 2.   Access P by depressing the LTR LEFT key followed by the 6 key, entering destination 00 then depressing the DEC key, or enter- ing destination 99 then depressing the INC key. 3.   Depress KYBD key. Observe that the dis- play freezes. 4.   Manually  enter  the  landmark  coordinates by  depressing  the  KYBD  key  to  blink  the field to be changed and enter the coordi- nates. 5.   When  overflying  landmark,  depress  ENT key. 6.   If an update is not desired, set the DISPLAY switch to some other position. This action aborts the update mode. 3-3-52.  Fly-To Destination Operation. a.  Initialization of Desired Course. When a fly-to destination is selected such as at the start of a leg, the present position at the time is stored in the computer. A course is then computed between the se- lected point and the destination. If the aircraft deviates from this desired course, the lateral offset or crosstrack distance error is computed. Distance and bearing to destination, actual track angle, and track angle error correction are computed from pres- ent position to destination. See figure 3-3-8 for graphic definition of these terms. b.  Procedure for Selecting One of 100 Possible Fly- To Destinations (Direct Mode). The  Doppler/GPS  navigation  set  has  the  capability  of selecting a fly-to destination from 100 destinations (num- ber 00 through 99). As  an  example,  consider  selecting  Fly-To  destination number 43. (1) Set MODE switch to MGRS (LAT/LONG or GPS LDG may also be used). (2) Set DISPLAY switch to XTK/TKE. Observe e  standard  Cross  Track  (XTK)  and  Track  angle  error (TKE) display. (DIST/BRG/TIME may also be used) (3) To display Fly-To destination 43 depress the INC or DEC keys, or depress keys 4 then 3. This is a direct key entry action. Left-Right Steering Signals There are two methods the pilot may use to fly-to destina- tion,  using  left-right  steering  signals  displayed  on  the computer-display unit. Left-right steering signals may be used  when  flying  the  shortest  distance  to  destination from present position (Method 1) or when flying a ground track from start of leg to destination (Method 2).