Garmin GTS 855 - Manual (Page 12)

8 Garmin GTS 8XX Series Traffic Advisory System 190-00587-02 Rev. A
opERATIoN
gTs 8XX series Traffic
The Garmin GTS 8XX Series unit is a Traffic Advisory System (TAS - GTS 800
and GTS 820) or Traffic Alert and Collision Avoidance System (TCAS I - GTS 850).
The GTS 8XX Series enhances flight crew situational awareness by displaying traffic
information for transponder-equipped aircraft. The GTS 8XX Series also provides
visual and aural traffic alerts including voice announcements to assist in visually
acquiring traffic. The GTS 800 and GTS 820 are TAS systems, the GTS 850 is a
TSO-approved TCAS I system. The GTS 8XX uses active interrogations of Mode
A/C/S transponders to provide Traffic Advisories.
Theory of operation
When the GTS 8XX Series is in Operating Mode, the unit interrogates the
transponders of intruding aircraft while monitoring transponder replies. The
GTS 8XX Series uses this information to derive the distance, relative bearing, and
if reported, the altitude and vertical trend for each aircraft within its surveillance
range. The GTS 8XX Series then calculates a closure rate to each intruder based
on the projected Closest Point of Approach (CPA). If the closure rate meets the
threat criteria for a Traffic Advisory (TA), visual and aural alerting is provided.
TAS/TCAS Surveillance Volume
The GTS 8XX Series surveillance system monitors the airspace within ±10,000
feet of own altitude. Under ideal conditions, the unit scans transponder traffic
up to 40 NM (GTS 820 and GTS 850) or 12 NM (GTS 800) in the forward
direction using top and bottom-mounted antennas similar to TCAS II systems.
In areas of greater transponder traffic density or when TCAS II systems are
detected, the GTS 820 and GTS 850 systems automatically reduce interrogation
transmitter power (and therefore range) in order to limit potential interference
from frequency congestion. The GTS 800’s range is 12 NM in the forward direction
and the maximum interrogation transmitter power remains constant. There is
also a minimum alerting threshold for range to allow alerting in situations where
traffic is slowly converging. Alerting may be caused by closure rate or minimum
distance to the intruder.