10-2GTN 625/635/650 Pilot’s Guide 190-01004-03 Rev. PForewordGettingStartedAudio &Xpdr CtrlCom/NavFPLDirect-ToProcWpt InfoMapTrafficTerrainWeatherNearestServices/MusicUtilitiesSystemMessagesSymbolsAppendixIndex10.2 GPS Altitude for TerrainGPS altitude is derived from satellite measurements. To require an accurate3-D fix (latitude, longitude, altitude), a minimum of four operating satellitesmust 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'sposition and altitude• Calculate the aircraft's flight path in relation to surrounding terrain andobstacles• Predict hazardous terrain conditions and issue alerts10.2.1 GSL Altitude & Indicated AltitudeThe GTN converts GPS altitude data to GSL altitude (i.e., the geometricaltitude relative to MSL) for use in terrain functions. All Terrain page depictionsand elevation indications are in GSL.Variations between GSL altitude and the aircraft's corrected barometricaltitude (or indicated altitude) are common. As a result, Terrain page altitudedata may differ from current altimeter readings. Both GSL altitude and indicatedaltitude represent height above MSL, but differ in accuracy and reliability.Altitude Type FeaturesGSL• Highly accurate and reliable geometric altitude source• Does not require local altimeter settings to determine heightabove MSL• Not subject to pressure and temperature variations• Affected primarily by satellite geometryIndicated• Barometric altitude source corrected for pressure variations• Requires frequent altimeter setting adjustment to determineheight above MSL• Subject to local atmospheric conditions• Affected by variations in pressure, temperature, and lapse rateTable 10-2 GSL and Indicated Altitude Features
