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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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