29Section 1IntroductionFigure 1-1: The Garmin TERRAIN system offers manyof the features of Garmin TAWS but is non-TSO-C151b-certified.PART THREETERRAIN INTERFACESection 1: IntroductionOverviewGarmin TERRAIN is a non-TSO-C151b-certified terrain awareness system incorporated into 400/500Series units to increase situational awareness and aid in reducing controlled flight into terrain (CFIT).TERRAIN functionality is a standard feature found in 400 Series units with main software version 5.01or above, and in 500 Series units with main software version 6.01 or above, along with appropriatehardware upgrades.Operating CriteriaThe Garmin TERRAIN system requires the following to operate properly:• The system must have a valid 3D GPS position solution• The system must have a valid terrain/obstacle databaseGPS Position and GPS-MSL AltitudeGarmin TERRAIN uses information provided by the 400/500 GPS receiver to provide a horizontalposition and altitude. GPS altitude is derived from satellite measurements. GPS altitude is converted to aMean Sea Level (MSL)-based altitude (GPS-MSL Altitude) and is used to determine terrain alerts.GPS-MSL altitude accuracy is affected by factors such as satellite geometry, but is not subject to variationsin pressure and temperature that normally affect pressure altitude devices. GPS-MSL altitude does notrequire local altimeter settings to determine MSL Altitude. Therefore, GPS altitude provides a highlyaccurate and reliable MSL altitude source to calculate terrain and obstacle alerts.
