Garmin GTN 650 Pilot's Guide - Page 187

10.3.3 Computing GPS Altitude for TAWS Foreword TAWS uses information provided from the GPS receiver to provide a horizontal position and altitude. GPS altitude is derived from satellite measurements. GPS Getting Started altitude is converted to a Mean Sea Level (MSL)-based altitude (GSL altitude) and is used to determine TAWS alerts. GSL altitude accuracy is affected by factors Xpdr Ctrl such as satellite geometry, but it is not subject to variations in pressure and temperature that normally affect pressure altitude devices. GSL altitude does Com/Nav not require local altimeter settings to determine MSL altitude. Therefore, GPS altitude provides a highly accurate and reliable MSL altitude source to calculate FPL terrain and obstacle alerts. Direct-To The terrain and obstacle databases used by TAWS are referenced to Mean Sea Level. Using the GPS position and GSL altitude, TAWS displays a 2-D picture of Proc the surrounding terrain and obstacles relative to the position and altitude of the aircraft. Furthermore, the GPS position and GSL altitude are used to calculate Wpt Info and "predict" the aircraft's flight path in relation to the surrounding terrain and obstacles. In this manner, TAWS can provide advanced alerts of predicted Map dangerous terrain conditions. Traffic 10.3.4 Baro-Corrected Altitude Versus GSL Altitude Terrain Baro-corrected altitude (or indicated altitude) is derived by adjusting the altimeter setting for local atmospheric conditions. The most accurate baro- Weather corrected altitude can be achieved by frequently updating the altimeter setting to the nearest reporting station along the flight path. However, because actual Nearest atmospheric conditions seldom match the standard conditions defined by the International Standard Atmosphere (ISA) model (where pressure, temperature, Music and lapse rates have fixed values), it is common for the baro-corrected altitude (as read from the altimeter) to differ from the GSL altitude. This variation results Utilities in the aircraft's true altitude differing from the baro-corrected altitude. System Messages Symbols Appendix Index 190-01004-03 Rev. B Garmin GTN 625/635/650 Pilot's Guide 10-7