30
Section 1
Introduction
TERRAIN Alerting using the GPS Position/GPS-MSL Altitude
The Garmin TERRAIN system utilizes terrain and obstacle databases that are referenced to Mean Sea
Level (MSL). Using the GPS Position and GPS-MSL Altitude, Garmin TERRAIN portrays a 2D picture of
the surrounding terrain and obstacles relative to the position and altitude of the aircraft. Furthermore, the
GPS Position and GPS-MSL Altitude are used to calculate and ‘predict’ the aircraft’s flight path in relation
to the surrounding terrain and obstacles. In this manner, the Garmin TERRAIN system can provide
advanced alerts of predicted dangerous terrain conditions. Detailed alert information is given in Section 3.
Baro-Corrected Altitude verses GPS-MSL Altitude
Baro-Corrected Altitude (or Indicated Altitude) is derived by adjusting the altimeter setting for the
local atmospheric conditions. The most accurate Baro-Corrected Altitude can be achieved by frequently
updating the altimeter setting to the nearest reporting station along the flight path. However, because
actual atmosphere conditions seldom match the standard conditions defined by the International Standard
Atmosphere model (where pressure, temperature, and lapse rates have fixed values), it is common for the
Baro-Corrected Altitude (as read from the altimeter) to differ from the GPS-MSL Altitude. This variation
results in the aircraft’s True Altitude differing from the Baro-Corrected Altitude.
Premature Descent Alerting (PDA) using GPS-MSL Altitude
The Premature Descent Alert (PDA) is annunciated when the aircraft is below the recommended
minimum altitude for an approach (see page 40 for details). The aircraft’s GPS-MSL altitude is compared
to a calculated Above-Ground-Level (AGL) altitude along the estimated path to the airport. When
the aircraft descends below the calculated minimum altitude for the estimated path, the PDA alert
is annunciated. PDA alerting is provided for both non-published visual approaches and published
instrument approaches to airports.
