Garmin GTN 725 Pilot's Manual page 266

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altitude (GSL altitude) and is used to determine Terrain 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.

Terrain utilizes terrain and obstacle databases that are referenced to mean

sea level (MSL). Using the GPS position and GSL altitude, Terrain displays a

2-D picture of the surrounding terrain and obstacles relative to the position and

altitude of the aircraft. In this manner, Terrain Proximity can provide advanced

alerts of predicted dangerous terrain conditions.

Terrain requires the following to operate properly:

• The system must have a valid 3-D GPS position solution.

• The system must have a valid terrain/obstacle database.

Figure 11-1 Terrain Proximity Page Functional Diagram

Garmin GTN 725/750 Pilot's Guide

190-01007-03 Rev. D

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