TM  1-1520-240-10 3-3-20            tered in any of the 100 fly-to destinations. The landing approach is determined by present position and the en- tered touch down altitude, glideslope and inbound ap- proach course. 3-3-46.  CDU Operation. Various required operating data, such an initial present position (if GPS is not valid or Doppler mode is selected). destination  coordinates  with  or  without  GPS  landing data, and magnetic variation can at any time be entered into the CDU via its keyboard, or the dataloader via the preprogrammed  dataloader  cartridge.  In  most  cases, these data will be entered before the aircraft takes off. a. The   GPS   provides   present   positions   to   AN/ ASN-128B. If GPS is not available or Doppler is selected present position can be initialized as follows: (1) The MODE switch should be set to MGRS or LAT/LONG, the WT/TGT display position of the DISPLAY switch is selected, the destination number is set to P and KYBD key is pressed. The coordinates of the initial posi- tion is overflown, the ENT key is pressed. The computer then determines changes from the initial position contin- uously, and the coordinates of the current present posi- tion can be read either by remaining in this configuration or by setting the DISPLAY switch to PP (present position) and the MODE switch to MGRS or LAT/LONG. (2) To  update  present  position  over  a  stored destination, KYBD key is depressed and released when the aircraft overflies this destination. If an update is de- sired, the ENT key is depressed and released and the update is completed. The DISPLAY switch is in the DIST/ BRG/TIME position and the FLY-TO-DEST is set to this destination during this process. The distance-to-go, dis- played while over the stored destination, is the position error of the system at that moment. (3) To update present position over fixed point not  previously  stored  in  the  computer,  the  DISPLAY switch is placed PP and KYBD key is depressed and released as the fix point is overflown. This freezes the display while allowing computation of changes in present position to continue within the computer. If an update is required, the coordinates of the fix point are entered via the  keyboard,  and  ENT  key  is  pressed.  The  position change  which  occurred  since  over-flying  the  fix  point automatically added to the fix point coordinates to com- plete the position update. (4) Magnetic variation can be entered for each destination, and the system will compute present posi- tion magnetic variation. If operation is to occur in a region with relatively constant magnetic variation, the operator enters magnetic variation only for present position and the computer will use this value throughout the flight. If MGRS data are to be entered or displayed, the MGRS datum of operation is also entered. b. Target-of-opportunity can be stored by pressing TGT STR (target store) key when the target is overflown. This operation stores the coordinates of the target in one of ten destination locations in the computer; locations 90 through 99 sequentially incrementing each time the TGT STR key is depressed. The location is displayed in the appropriate display field. The computer can keep track of individual target positions which may include speeds and directions input by the operator. c. Self test of the AN/ASN-128B is accomplished using built-in-test-equipment (BITE) with the RTA, SDC, and CDU units connected an energized for normal op- eration. Self test enables the unit to isolate failures to one of the four main functions (RTA, SDC, CDU or EGR) or to one of the circuit cards in the SDC or CDU. Self test is accomplished as follows:  Self test of the AN/ASN-128B is  accomplished  using  built-in-test-equipment  (BITE) with the RTA, SDC, and CDU units connected an ener- gized for normal operation. Self test enables the unit to isolate failures to one of the four main functions (RTA, SDC, CDU or EGR) or to one of the circuit cards in the SDC or CDU. Self test is accomplished as follows: (1) The CDU (except for the keyboard and dis- play) is checked on a continuous basis, and any failure is displayed by illumination of the MAL indicator lamp on the CDU. If the MODE switch on the CDU is set to the TEST position, identification of the failed circuit card in the CDU is indicated by a code on the display panel. (2) The DRVS and EGR are tested by setting the  MODE  switch  on  the  CDU  to  the  TEST  position. Failure of the DRVS or EGR are displayed on the CDU by illumination of the MAL indicator lamp, and identifica- tion of the failed unit or circuit card is indicated by a code on the display panel of the CDU. (3) Continuous monitoring of the Signal Digital Data  Converter  and  Receiver  Transmitter  Antenna  is provided by the system status indication. The system will not use Doppler velocities in normal operation when fly- ing over glassy smooth water. However, if the system continues to not use Doppler (e.g., using GPS only when combined has been selected) for excessive periods of time (e.g. more than10 minutes) over land or rough wa- ter, then a malfunction may exist in the navigation set and the operator should set the MODE switch to TEST to determine the nature of failure. (4) The display portion of the CDU is tested by illuminating all the lamp segments in each alphanumeric characters in the LAMP TEST mode. (5) Keyboard operation is verified by observing the alphanumeric characters as the keyboard is exer- cised. 3-3-47.  Route Sequencing Modes. The system has the ability to fly a preprogrammed se- quence of waypoints. This sequence can be either con- secutively numbered in which case a start and end way- point are entered or randomly numbered in which case