Gps Altitude For Terrain - Garmin GTN 625 Pilot's Manual Upgrade Supplement

10.2
Foreword
GPS altitude is derived from satellite measurements. To require an accurate
Getting
3-D fix (latitude, longitude, altitude), a minimum of four operating satellites
Started
must be in view of the GPS receiver antenna.
Audio &
Xpdr Ctrl
The terrain system uses GPS altitude and position data to:
• Create a 2-D image of surrounding terrain and obstacles relative to the
Com/Nav
aircraft's position and altitude
FPL
• Calculate the aircraft's flight path in relation to surrounding terrain and
obstacles
Direct-To
• Predict hazardous terrain conditions and issue alerts
Proc
10.2.1
Wpt Info
The GTN converts GPS altitude data to GSL altitude (i.e., the geometric
altitude relative to MSL) for use in terrain functions. All Terrain page depictions
Map
and elevation indications are in GSL.
Variations between GSL altitude and the aircraft's corrected barometric
Traffic
altitude (or indicated altitude) are common. As a result, Terrain page altitude
data may differ from current altimeter readings. Both GSL altitude and indicated
Terrain
altitude represent height above MSL, but differ in accuracy and reliability.
Weather
Altitude Type
Nearest
Services/
GSL
Music
Utilities
System
Messages
Indicated
Symbols
Appendix
Index
10-2

GPS Altitude for Terrain

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
Table 10-2 GSL and Indicated Altitude Features
GTN 625/635/650 Pilot's Guide
Features
190-01004-03 Rev. N