Limitations; Htaws Operation; Htaws Alerting - Garmin GTN 725 Pilot's Manual

11.4.1.3

Limitations

NOTE: The data contained in the terrain and obstacle databases comes
from government agencies. Garmin accurately processes and cross-
validates the data, but cannot guarantee the accuracy and completeness
of the data.
HTAWS displays terrain and obstructions relative to the flight path of the
aircraft. Individual obstructions may be shown if available in the database.
However, all obstructions may not be available in the database and data may be
inaccurate. Never use this information for navigation.
NOTE: Terrain databases do not consistently represent foliage. Some trees
may extend above HTAWS protection limits in some operating modes.
Terrain information is based on terrain elevation data contained in a database
that may contain inaccuracies. Terrain information should be used as an aid
to situational awareness. Never use it for navigation or to maneuver to avoid
terrain.
HTAWS uses terrain and obstacle information supplied by government
sources. The data undergoes verification by Garmin to confirm accuracy of the
content. However, the displayed information should never be understood as
being all-inclusive.
11.4.2

HTAWS Operation

11.4.2.1

HTAWS Alerting

HTAWS uses information provided from the GPS receiver to provide
a horizontal position and altitude. GPS altitude is derived from satellite
measurements. GPS altitude is converted to a mean sea level (MSL)-based
altitude (GSL altitude) and is used to determine HTAWS alerts. GSL altitude
accuracy is affected by factors such as satellite geometry, but it is not subject
to variations in pressure and temperature that normally affect pressure altitude
devices. GSL altitude does not require local altimeter settings to determine MSL
altitude. Therefore, GPS altitude provides a highly accurate and reliable MSL
altitude source to calculate terrain and obstacle alerts.
HTAWS utilizes terrain and obstacle databases that are referenced to mean
sea level (MSL). Using the GPS position and GSL altitude, HTAWS displays a
2-D picture of the surrounding terrain and obstacles relative to the position and
190-01007-03 Rev. D Garmin GTN 725/750 Pilot's Guide
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