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R  (b) Local Terrain Processing 
R  The local terrain processing block extracts and formats local 
R  topographic data and terrain features from the related data bases 
R  creating a set of digital elevation matrix overlays for use by 
R  the terrain threat detection and display processing functions. 
R  Additionally, data for the nearest runway are also extracted for 
R  use by the terrain threat detection and display processing 
R  functions. Processing for each topographic data base and the 
R  runway data base are described in the following sub-sections. 
R  1_  Terrain Surface Data
R  Local terrain processing of topographic surface data updates a 
R  set of digital elevation matrix overlays that are positioned 
R  with respect to aircraft position. Each matrix element 
R  contains the highest terrain altitude with respect to mean sea 
R  level in that element area. Elements where terrain data are 
R  not available are marked invalid. These unknown data are 
R  displayed in magenta low density on ND whatever the altitude. 
R  2_  Obstacle Data
R  In addition to terrain surface data, the terrain database 
R  contains obstacle data. The obstacle data are displayed as 
R  terrain data (same coloring scheme) and cause visual 
R  indications of warning and caution alerts as terrain. The 
R  current obstacle database is obtained from N.O.A.A. (National 
R  Oceanic and Atmospheric Administration). It includes obtacles 
R  in the United States and part of Canada, Mexico, the Bahamas 
R  and Europe. Obstacle alerting and display is activated via a 
R  specific programming pin. 
R  3_  Nearest Runway Data
R  Data for the nearest runway are extracted and processed for 
R  use by the terrain threat detection and display processing 
R  functions. Data are extracted from the same airport data base 
R  used by the Terrain Clearance Floor functions (Ref. para. 
R  D.(2)). This data base contains data on all hard-surface 
R  runways 3500 ft. or more in length with published coordinates. 
R  The contents of the data base are processed by the local 
R  terrain processing into nearest runway center position, 
R  nearest runway threshold position, and nearest runway altitude 
R  for use by the Enhanced GPWS. These data are updated when the 
R  terrain threat detection and display processing functions are 
R  performed. 

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R  (c) Terrain Threat Detection 
R  The terrain threat detection and display processing block 
R  performs the threat analysis on the terrain data within computed 
R  caution and warning envelope boundaries below and forward of the 
R  aircraft path. Results of these threat assessments are combined 
R  with background terrain data and data for the nearest runway and 
R  formatted into a terrain display image which can be displayed on 
R  Navigation Display in place of the weather image. In the event of 
R  terrain caution or warning conditions, a specific audio alert is 
R  triggered and the terrain display image is enhanced to highlight 
R  each of the types of terrain threats. 
R  1_  Terrain Caution and Warning Envelopes
R  (Ref. Fig. 018) 
R  a_  Caution Altitude Floor
R  The caution altitude floor (or terrain floor) is computed 
R  as a function of aircraft altitude with respect to nearest 
R  runway altitude and range to the nearest runway threshold 
R  position. This parameter represents a distance below the 
R  aircraft. The relationship to the nearest runway threshold 
R  location prevents undesired alerts when the aircraft is 
R  taking off or landing at an airport. The system is 
R  compatible with terrain clearances allowed for by 
R  regulatory approach and departure design criteria. 
R  b_  Caution Look Ahead Distance
R  The caution look ahead distance is computed from aircraft 
　
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