TM 55-1510-220-10 b. GPS Controls, Indicators, and Functions. (1) GPS control/display unit (fig 3-25A). (a) Line selection keys. Four line selection keys, located to the left of the CDU display screen, are used t initiate and terminate data entries and to select various system options. (b) Display  screen.    system  information  is shown  on  the  cathode  ray  tube  display  screen.    The  display screen  can  show  four  lines  of  text  with  13  alphanumeric characters on each. (c) Mode    selector    switch.          The    four- position  mode  selector  switch,  placarded  PULL  OFF,  INIT, NAV, and PULL TEST, is used to select the operating mode of the GPS system. (d) Display  brightness  control.    A  control knob  placarded  BRT  is  provided  to  control  the  brightness  of the cathode ray tube display screen.  Clockwise rotation of the control increases brightness. (e) Data   entry   keys   (0   through   9).      The data entry keys are used to enter alphanumeric data. (f) USE   LTR   key.      The   use   letter   key, placarded  USE  LTR,  is  used  to  select  alphabetic  prompt  in free   format   data   entry.      The   USE   LTR   key   terminates alphabetic entry when pressed. (g) Clear  key.      The  clear  key,  placarded CLR,   is   used   to   clear   erroneous   data   entry   and   message displays. (h) Slew   key.      The   slew   key   is   used   to access  additional  pages  within  a  data  display  selected  by  the data  selector  switch.    If  more  than  one  page  is  available,  a double  arrow  is  displayed  in  the  lower  right  corner  of  the display.    Pressing  the  slew  key  will  access  the  next  page. Repeated pressing the slew key will return the display to the first page after the last page has been accessed. (i) Data   selector   switch.      For   all   data selector  switch  positions  there  are  two  modes  of  displayed data: NOTE Pressing the WP key switches the CDU between the two modes. 1. Destination mode  (active waypoint as destination) 2. Waypoint   (WP)   examine      mode (any waypoint) The 10 position data selector switch is used to select the type information to be displayed on the CDU as shown in Table 3- 5A. Table 3-5A.  Data Selector Switch Positions Position Function POS Position data is displayed. MSN Mission data is displayed. OPT Option data is displayed.  Six pages of information pertaining to the GPS receiver are made available when the OPT position is selected. STAT Status data is displayed. VAR-DTM Magnetic variation and map datum data is displayed. ERR Error data is displayed. WIND Wind data is displayed. DIS-TG Distance and time to go data is displayed. TRK-GS Track and ground speed data is displayed. DTA-VA Desired track and vertical angle data is displayed. BTO3617 Change 5       3-76A