GNS 530(A) Pilot’s Guide and Reference 190-00181-00 Rev. E
13-2
SECTION 13
TAWS
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 derived
by adjusting the altimeter setting for 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 (ISA) 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.
Using TAWS
During power-up, the terrain/obstacle database versions
are displayed along with a disclaimer to the pilot. At the
same time, TAWS self-test begins. One of the following
aural messages is generated:
• “TAWS System Test OK”
• “TAWS System Failure”
A test failure is also annunciated for TAWS as shown
in Table 13-4.