5-36 Pilot’s Guide 190-02488-01 Rev. B
Hazard Awareness
GPS Altitude for Terrain
FEATURE REQUIREMENTS
GPS altitude is derived from satellite measurements. To acquire an accurate 3-D fix
(latitude, longitude, altitude), a minimum of four operating satellites must be in view
of the GPS receiver antenna.
The terrain system uses GPS altitude and position data to:
• Create a 2-D image of surrounding terrain and obstacles relative to the
aircraft’s position and altitude
• Calculate the aircraft’s flight path in relation to surrounding terrain and
obstacles
• Predict hazardous terrain conditions and issue alerts
GSL ALTITUDE & INDICATED ALTITUDE
The unit converts GPS altitude data to GSL altitude (i.e., the geometric altitude
relative to MSL) for use in terrain functions. All Terrain page depictions and elevation
indications are in GSL.
Variations between GSL altitude and the aircraft’s corrected barometric altitude (or
indicated altitude) are common. As a result, Terrain page altitude data may differ
from current altimeter readings. Both GSL altitude and indicated altitude represent
height above MSL, but differ in accuracy and reliability.
GSL ALTITUDE INDICATED ALTITUDE
• Highly accurate and reliable
geometric altitude source
• Does not require local altimeter
settings to determine height
above MSL
• Not subject to pressure and
temperature variations
• Affected primarily by satellite
geometry
• Barometric altitude source
corrected for pressure variations
• Requires frequent altimeter
setting adjustment to determine
height above MSL
• Subject to local atmospheric
conditions
• Affected by variations in pressure,
temperature, and lapse rate
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