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1 GENERAL DESCRIPTION
1.1 Introduction
This manual presents the mechanical and electrical installation requirements for installing the Garmin
GTX 23 Mode S Transponder.
1.2 Equipment Description
The Garmin GTX 23 remote-mounted Mode S Transponder is a radio transmitter and receiver that
operates on radar frequencies. It receives ground radar and TCAS interrogations at 1030 MHz, and it
transmits a coded response of pulses at 1090 MHz. The GTX 23 replies to Mode A, Mode C, and Mode S
interrogations. Mode A replies consist of framing pulses and any one of 4,096 codes, which differ in the
position and number of pulses transmitted. Mode C replies include framing pulses and encoded pressure
altitude. Mode S replies can contain various information such as the Mode A code, Mode C altitude,
ICAO address, and Elementary Surveillance and Enhanced Surveillance data described below.
Features of the GTX 23 include support for ADS-B, TIS-A, mutual suppression, IDENT functionality, SI
codes, elementary surveillance (ELS), and enhanced surveillance (EHS). ADS-B, TIS-A, and mutual
suppression are discussed in following sections. IDENT functionality activates a Special Position
Identification (SPI) pulse for 18 seconds in Mode A replies. This helps the air traffic controller confirm
your position on their radar screen. SI codes, ELS, and EHS better facilitate meeting various requirements
for operation in European airspace. SI codes uniquely identify ground radar with overlapping coverage.
Supporting SI codes enables reduction in ground infrastructure complexity. ICAO Annex 10, Volume IV,
Amendment 77, paragraph 2.1.5.1.7.1 mandated SI code support. Elementary and Enhanced Surveillance
provide additional aircraft parameters to ground radar systems (for ELS, see JAA TGL 13 rev 1 dated
June 01, 2003; for EHS, see AMC 20-13, which supersedes JAA NPA 20-12a). Elementary Surveillance
provides a detailed transponder capability report and aircraft identification data (e.g. Flight ID or radio call
sign). Additionally, TCAS II compatible transponders supporting ELS can provide resolution advisory
data if the aircraft is equipped with TCAS II. (ELS is Mode S BDS registers (1,0), (2,0), and (3,0).)
Enhanced Surveillance provides selected vertical intention, track and turn reporting, and heading and
speed reporting. (EHS is Mode S BDS registers (4,0), (5,0), and (6,0).)
1.3 ADS-B Capabilities
The GTX 23 provides the capabilities of Automatic Dependent Surveillance-Broadcast (ADS-B)
technology, which improves situational awareness and flight safety. With ADS-B capabilities, position,
velocity, and heading information are automatically transmitted to other aircraft and ground stations. The
current air traffic control system depends on a transponder request for pertinent aircraft information.
ADS-B provides automatic transmission of aircraft information without a request.
TSOA GPS data from a Garmin GNS 400W/500W-series WAAS enabled unit,
GTN 6XX series, or GTN 7XX series unit is required to support ADS-B
transmissions. If a GNS 400W/500W-series unit is used, note that it must use
main software version 3.20 or later.
GTX 23 Installation Manual
190-00906-01
NOTE
Page 1-1
Revision A
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