Radar Basics

Transponder

Secondary radar depends on a transponder (short-for Transmitter-responder) in the airplane to respond to interrogations from the ground station to make the aircraft visible and to report additional information like the aircraft's altitude.

Figure 1 shows an older one transponder. The replays code for Mode 1 and 3/A can be choosen with the black hand wheels. The yellow painted edges mark this device as reference unit of a repair shop.

Newer one transponders operate with two antennae and two receivers in diversity mode. One antenna is mounted on top and the other one at the bottom of the airplanes fuselage. Additional informations are derived from the onboard avionics navigation systems.

Altitude Reporting
(basically enabling
Mode C)

Enables TCAS (when
selected, TAs and
RAs are provided

Transponder
Failure
Indicator

Enter
Mode A
Code

Enter
Flight ID

Self Test
(press)

Traffic
(Auto or
Manual)

TCAS
Range

Source of
Altitude (not
selected Alt)

Select
(Enter)

SPI

Clear

Figure 2: Possible view of a main display of a Mode-s transponder

Altitude Reporting
(basically enabling
Mode C)

Enables TCAS (when
selected, TAs and
RAs are provided

Transponder
Failure
Indicator

Enter
Mode A
Code

Enter
Flight ID

Self Test
(press)

Traffic
(Auto or
Manual)

TCAS
Range

Source of
Altitude (not
selected Alt)

Select
(Enter)

SPI

Clear

Figure 2: Possible view of a main display of a Mode-s transponder

The Transponder maintains avionics data in 256 different 56 bit wide Binary Data Store (BDS) Registers that can be loaded with information and read-out by the ground system. Each register contains the data payload of a particular Mode S reply or extended squitter. These BDS registers are also referred to as Ground Initiated Comm B (GICB) registers. They are specified in the ICAO document “Manual on Mode S Specific Services” (Doc 9688). Registers not updated within a fixed period are cleared by the transponder. Registers are identified by a two digit hex number for example BDS 05h (in some publications written as BDS 0,5) is the position squitter. Commonly used registers are shown in Table 1.

down
converter

A/D-
converter

monitor

single-chip-processor

power
amplifier

waveform-
generator

keyboard

external sensors

local
oscillator

Figure 3: Functional Block Diagram of a modern transponder

down
converter

A/D-
converter

monitor

single-chip-processor

power
amplifier

waveform-
generator

keyboard

external sensors

local
oscillator

Figure 3: Functional Block Diagram of a modern transponder

down
converter

A/D-
converter

monitor

single-chip-processor

power
amplifier

waveform-
generator

keyboard

external sensors

local
oscillator

Figure 3: Functional Block Diagram of a modern transponder