Taws-B - Garmin G900X Pilot's Manual

6.8 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 when the aircraft is outside of the installed terrain database coverage
NOTE:
area.
Terrain Awareness and Warning System - Class B (TAWS-B) is an optional software feature that provides audio
and visual warning messages to alert the flight crew of potential conflicts between the current flight path and
surrounding terrain and obstacles. The TAWS-B messages are advisory in nature only.
TAWS-B satisfies TSO-C151b Class B 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 from 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. Garmin verifies the data, 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
190-00726-00 Rev. J
Garmin G900X Pilot's Guide
HAZARD AVOIDANCE
443