Purpose Of Equipment; Atcrbs System Operation - Garmin GTX 327 ATC Maintenance Manual

1.3

PURPOSE OF EQUIPMENT

The GARMIN GTX 327 Transponder System provides an air to ground link in the Air Traffic Control Radar
Beacon System (ATCRBS) to support in-flight aircraft identification. It also provides air to air link participation
in the airborne Traffic Alert and Collision Avoidance System (TCAS) for aircraft equipped with Traffic
Avoidance equipment.
A pulsed interrogation from the ground-based secondary radar or airborne TCAS generates a pulsed reply from
the GTX 327. These pulses are processed and displayed on an ATC radar screen on the ground or traffic display
in other aircraft. Normal radar returns from the primary radar system would vary in intensity depending on
aircraft size, distance away and other factors. The Transponder System equalizes returns by providing similar
digital data for all aircraft.
Selected codes enable the GTX 327 to reply with identification information useful to ATC. Special codes may
be selected to indicate conditions such as emergency or VFR traffic.
In addition to Mode A operation for identification, Mode C is available for altitude information when an altitude
encoder is connected to the GTX 327 and altitude replies are selected.
1.3.1

ATCRBS System Operation

There are two classes of radar systems used by ATC, primary and secondary. Primary class includes surveillance
or search radar. The secondary class is comprised of special purpose radar including ATC interrogators and
transponders. Secondary radar is able to obtain data that cannot be determined by primary radar alone. The
secondary radar is officially referred to as the Air Traffic Control Radar Beacon System or ATCRBS. The
essential elements of the ATCRBS are show in figure 1-1.
INTERROGATION ANTENNA
SECONDARY RADAR
INTERROGATION 1030 MHZ
REPLY 1090 MHZ
REPLIES
PRIMARY RADAR
ANTENNA
Figure 1-1. ATCRBS System
1-2 GTX 327 MAINTENANCE MANUAL
Rev B 190-00187-05
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