1.3.2 Interrogator; Transponder - Garmin GTX 327 ATC Maintenance Manual

1.3.2 Interrogator

The ATC ground based interrogator rotates synchronously with the primary surveillance radar and transmits a
sequence of pulses at a frequency of 1030 MHz. The interrogation repeatedly requests all transponders on the
mode being used to reply. The received replies are available for display on the ATC radar screen.
There are five types of interrogation modes, A, B, C, D and S which may be transmitted by the ground station.
Each interrogation mode is distinct from the other and is characterized by the spacing of the transmitted framing
pulses. The GARMIN GTX 327 is designed for Mode A and Mode C operation.
Similar to the ground based interrogator, an airborne TCAS processor transmits interrogation signals searching
for airborne traffic in relatively close proximity. TCAS is capable of detecting the direction and altitude of an
aircraft equipped with a transponder such as the GARMIN GTX 327. Typical TCAS equipment is shown in
figure 1-2.
INTERROGATION AND REPLY
SIGNALS
TCAS EQUIPPED AIRCRAFT
TCAS DISPLAY
Figure 1-2. Typical TCAS Encounter
1.3.3

Transponder

When the interrogation signal is detected by the GTX 327, an appropriate response is generated. This response is
transmitted omnidirectionally by the GTX 327 at a frequency of 1090 MHz. When multiple replies return to the
interrogating system, appropriate equipment processes the identification, altitude, distance and azimuth data to
compute location, speed and direction information for appropriate use.
GTX 327 MAINTENANCE MANUAL Page 1-3
190-00187-05 Rev. B
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