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Part Three: Section 2
TAWS Operation
Section 2: TAWS Operation
TAWS Alerting
TAWS uses information provided from the 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 TAWS alerts. GPS-MSL altitude accuracy is
affected by factors such as satellite geometry, but it 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.
TAWS utilizes terrain and obstacle databases that
are referenced to mean sea level (MSL). Using the GPS
position and GPS-MSL altitude, TAWS displays a 2-D
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, TAWS can provide advanced alerts of predicted
dangerous terrain conditions. Detailed alert modes are
described later in this section.
Baro-Corrected Altitude
Baro-corrected altitude (or indicated altitude) is de-
rived by adjusting the altimeter setting for local atmo-
spheric 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
32
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190-00356-30 Rev H
defined by the International Standard Atmosphere
(ISA) model (where pressure, temperature, and lapse
rates have fixed values), it is common for the baro-cor-
rected 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-cor-
rected altitude.
Power Up
During power-up of the 500W Series unit, the
terrain/obstacle database versions are displayed along
with a disclaimer to the pilot. At the same time,
TAWS self-test begins. TAWS gives the following aural
messages upon test completion:
• "TAWS System Test, OK", if the system passes the test
• "TAWS System Failure", if the system fails the test
A test failure is also annunciated visually for TAWS,
as shown in the TAWS Alert Summary table.
TAWS Page
The TAWS Page is in the NAV group of pages (see
the 500W Series Pilot' s Guide for detailed information
regarding page groups and pages).
The TAWS Page
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