Garmin GDL 69A GPS Receiver User Manual


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Part Three: Section 1
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 ight 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ight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xed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 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
190-00356-30 Rev C
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