1 Background Display
_ (Ref. Fig. 020) The background display is computed from the aircraft altitude with respect to the terrain data in the digital elevation matrix overlays. Where terrain data are available and sufficiently close to the aircraft altitude, they are presented in background color dot patterns reflecting the projected separations shown in this illustration. (Ref. Fig. 019) Different dot density patterns and colors are used to represent terrain altitude bands with respect to the aircraft. Areas with no terrain data available are painted with the low density magenta. Known terrain sufficiently below the aircraft altitude is black.
 NOTE : Terrain is not shown if more than 2000 ft. below
____
 reference altitude

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 AND/OR
 Terrain is not shown if terrain elevation is within 400
 ft. of runway elevation nearest the aircraft.
 2_  Terrain Caution Alert
R  (Ref. Fig. 021A)
 When the conditions have been met to generate a terrain
 caution alert, a specific audio alert and light output is
 triggered and the background image is enhanced to highlight
 the terrain caution threats.
 At the start of a terrain caution alert, the terrain awareness
 function triggers the caution audio alert phrase TERRAIN
 AHEAD. The phrase is repeated after seven seconds if still
 within the terrain caution envelope.
 During a terrain caution alert the PULL UP legend of
 pushbutton switches 4WZ and 5WZ is on.
 During a terrain caution alert, areas where terrain violates
 the terrain caution envelope along the aircraft track, and
 within plus or minus 90 deg. of the aircraft track, are
 painted with the caution color 100 per cent yellow.
 3_  Terrain Warning Alert
R  (Ref. Fig. 021A)
 When the conditions have been met to generate a terrain
 warning alert, a specific audio alert and light output is
 triggered and the background image is enhanced to highlight
 the terrain caution and warning threats.
 At the start of a terrain warning alert, the terrain awareness
 function triggers the warning audio alert phrase TERRAIN
 AHEAD, PULL UP. The phrase is repeated continuously while
 within the terrain warning envelope.
 During a terrain warning alert the PULL UP legend of
 pushbutton switches 4WZ and 5WZ is on.
 During a terrain warning alert, areas where terrain violates
 the terrain warning envelope along the aircraft track, and
 within plus or minus 90 deg. of the aircraft track, are
 painted with the warning color 100 per cent red.
 NOTE : -Terrain may only be displayed on ND's when FCU mode____
 ARC or ROSE is selected.
 -When an alert occurs (caution or warning) and the FCU
 mode is not in a correct mode (ARC or ROSE), the
 message TERR. CHANGE MODE is displayed on ND's.
 -When an alert occurs (caution or warning) and the FCU
 range selected is 160 or 320Nm the message TERR. REDUCE
 RANGE is displayed on ND's.

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 Enhanced GPWS - Warning and Caution Display on ND R Figure 021A
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 (e)
 Terrain Data Base (Ref. Fig. 016A) Local terrain processing extracts and formats local topographic terrain data from the Enhanced GPWS terrain data base for use by the terrain threat detection and display processing functions. This terrain data base divides the earth surface into grid sets referenced horizontally on the geographic (latitude/longitude) coordinate system of the WGS-84. Elements of the grid sets record the highest terrain altitude (above MSL) in that element respective area. Grid sets vary in resolution depending on geographic location. Higher resolution grids are used around airports. Lower resolution grids are used outside of airport areas where aircraft altitude enroute makes hazardous conditions unlikely and for which detailed terrain features are not important to the flight crew. Digital Elevation Models (DEM) are available for most of the airports around the world today. In cases where the data are not currently available, DEMs are generated in-house from available topographic maps, sectional charts, and airline approach plates. The process of acquiring, generating, assembling, and updating the data base is governed by strict configuration controls to ensure the highest level of data integrity. DEMs from external sources are inputs to this process and are checked and formatted for generation of the Enhanced GPWS terrain data base. The global Enhanced GPWS terrain data base is organized in a flexible manner. Using digital compression techniques, the complete data base is stored in non-volatile memory within the LRU. Updates and additions are easily accomplished by inserting a single PCMCIA card in the card slot on the LRU front panel. Status LEDs on the LRU front panel allow the operator to monitor the data base load progress and completion.
　
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