Taws-B - Garmin Cirrus Perspective SR20 Pilot's Manual

HAZARD AVOIDANCE

6.7 TAWS-B

Do not use TAWS information for primary terrain avoidance. TAWS is intended only to enhance
WARNING:
situational awareness.
The data contained in the TAWS databases comes from government agencies. Garmin accurately
NOTE:
processes and cross-validates the data but cannot guarantee the accuracy and completeness of the data.
Terrain data is not displayed if the aircraft is outside of the installed terrain database coverage area.
NOTE:
TAWS-B (Terrain Awareness and Warning System - Class B) is an optional feature to increase situational awareness
and aid in reducing controlled flight into terrain (CFIT). TAWS-B provides visual and aural annunciations when
terrain and obstacles are within the given altitude threshold from the aircraft. The displayed alerts and warnings
are advisory in nature only.
TAWS-B satisfies TSO-C151b Class A requirements for certification. TAWS-B requires the following to operate
properly:
• A valid terrain/obstacle/airport terrain database
• A valid 3-D GPS position solution
TAWS-B uses terrain and obstacle information supplied by government sources. Terrain information is based
on terrain elevation information in a database that may contain inaccuracies. Individual obstructions may be
shown if available in the database. The data undergoes verification by Garmin to confirm accuracy of the content,
per TSO-C151b. However, the displayed information should never be understood as being all-inclusive and data
may be inaccurate.
TAWS-B uses information provided from the GPS receiver to provide a horizontal position and altitude. GPS
altitude is derived from satellite measurements. GPS altitude is then converted to the height above geodetic sea
level (GSL), which is the height above mean sea level (MSL) calculated geometrically. GSL altitude is used to
determine TAWS-B alerts. GSL altitude accuracy is affected by satellite geometry, but is not subject to variations
in pressure and temperature that normally affect pressure altitude sensors. GSL altitude does not require local
altimeter settings to determine MSL altitude. It is a widely-used MSL altitude source. Therefore, GSL altitude
provides a highly accurate and reliable MSL altitude source to calculate terrain and obstacle alerts.
The terrain and obstacle databases used by TAWS-B are referenced to MSL. Using the GPS position and GSL
altitude, TAWS-B 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 GSL altitude are used to calculate and "predict" the
aircraft' s flight path in relation to the surrounding terrain and obstacles. In this manner, TAWS-B can provide
advanced alerts of predicted dangerous terrain conditions.
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 atmospheric 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 GSL altitude. This variation results in the aircraft' s GSL altitude differing
from the baro-corrected altitude.
Cirrus Perspective
384
by Garmin – Pilot's Guide for the SR20/SR22SR22T
™
190-00820-08 Rev. A