Garmin GNC 420(A) Pilot's Manual & Reference page 202

SECTION 11
ADDITIONAL FEATURES
Pilots should be aware of TAS/TCAS system
limitations. If an intruder transponder does not respond
to interrogations due to antenna shading or marginal
transponder performance, it will not be displayed, or
display may be intermittent. Pilots should remain vigilant
for traffic at all times when using TAS/TCAS systems for
non-transponder equipped airplanes or unresponsive
airplanes.
TCAS I Surveillance Volume
Top and bottom mounted antennas allow an active
surveillance range of up to 12 nm (GTS 800) or 40 nm
(GTS 820/850) in the forward direction, and somewhat
reduced ranges to the sides and aft of own aircraft due
to the directional interrogation patterns. Interference
limiting in GTS 820/850 units may automatically reduce
range in high density traffic areas.
TA Alerting Conditions
The GTS 8XX automatically adjusts its TA sensitivity level
(Table 11-5) to reduce the likelihood of nuisance TA alerting
during flight phases likely to be near airports. Sensitivity
Level A (less sensitivity) TA is used when the aircraft' s radar
altimeter (if equipped) indicates own altitude is less than
2000 feet AGL. If no radar altimeter is present, Sensitivity
Level A is active when the landing gear is extended. Level A
is also active when groundspeed is less than 120 knots with
no radar altimeter present in a fixed gear aircraft.
Intruder
Sensitivity
Altitude
Level
Available
A Yes
A No
B Yes
B No
11-12
Intruder closing rate provides less than 20 seconds of vertical and horizontal separation.
Or:
Intruder range is within 0.2 nm and vertical separation is within 600 feet.
Intruder closing rate is less than 15 seconds.
Intruder closing rate provides less than 30 seconds of vertical and horizontal separation.
Or:
Intruder range is within 0.55 nm and vertical separation is within 800 feet.
Intruder closing rate is less than 20 seconds.
Table 11-5 TA Sensitivity Level and TA Alerting Criteria
GNC 420(A) Pilot's Guide and Reference
In all other conditions, Level B (greater sensitivity) TA
sensitivity is used to assess TA threats.
Traffic Symbology
Traffic information from the GTS 8XX is displayed on
the GNC 420 unit using TAS/TCAS symbology (Table
11-4) on a dedicated Traffic page, and on the moving Map
Page. The displayed traffic information generally includes
the relative range, bearing, and altitude of intruder aircraft.
The GTS 8XX also generates aural announcements heard
on the cockpit audio system.
Target altitude relative to own aircraft altitude (relative
altitude) is displayed (in hundreds of feet) for each target
symbol (Figure 11-20). If traffic is above own aircraft altitude
the relative altitude is shown above the target next to a '+'
symbol. If traffic is below own aircraft altitude the relative
altitude is shown below the target next to a '-' symbol.
Altitude trend (Figure 11-20) is displayed as an up
arrow (≥ +500 fpm), down arrow (≤ -500 fpm), or no
symbol if less than 500 fpm rate in either direction.
Traffic
Type
Figure 11-20 Traffic Symbol Components
TA Alerting Conditions
Relative
Altitude
Altitude
Trend
190-00140-20 Rev. K