GNS 430 Pilot’s Guide and Reference
11-6
SECTION 11
TERRAIN
TERRAIN Destination Airport
The TERRAIN destination airport is considered to
be the next airport in the flight plan while adhering to
the flight plan path. If no flight plan is entered, or if the
aircraft is not adhering to the entered flight plan path, the
terrain destination airport is estimated using the Nearest
Airport Criteria as described in the section above.
GPS Position and GPS-MSL Altitude
Garmin TERRAIN uses information provided by the
GNS 430 GPS receiver to provide a horizontal position
and altitude. GPS altitude is derived from satellite
measurements. GPS altitude is converted to a Mean
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 variations in pressure and temperature
that normally affect pressure altitude devices. GPS-
MSL altitude does not require local altimeter settings to
determine MSL Altitude. Therefore, GPS altitude provides
a highly accurate and reliable MSL altitude source to
calculate terrain and obstacle alerts.
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
11.3.
Baro-Corrected Altitude versus 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.
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