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Functional block diagram

Transponder

Receiver

Decoder

Transmitter

Coder

Interrogator

Coder

Transmitter

Decoder

Receiver

PPI-screen

Synchronizer of
Primary Radar

Interrogation Path
(Uplink)

Replay Path
(Downlink)

Figure 1: Block diagram of a secondary radar

Transponder

Receiver

Decoder

Transmitter

Coder

Interrogator

Coder

Transmitter

Decoder

Receiver

PPI-screen

Synchronizer of
Primary Radar

Interrogation Path
(Uplink)

Replay Path
(Downlink)

Figure 1: Block diagram of a secondary radar

In the interrogator on the ground:

The secondary radar unit needs a synchronous impulse of the (analogous) primary radar unit to the synchronization of the indication.

• The chosen mode is encoded in the Coder. (By the different modes different questions can be defined to the airplane.)
• The transmitter modulates these impulses with the RF frequency. Because another frequency than on the replay path is used on the interrogation path, an expensive duplexer can be renounced.
• The antenna is usually mounted on the antenna of the primary radar unit and turns synchronously to the deflection on the monitor therefore.

In the aircrafts transponder:

A receiving antenna and a transponder are in the airplane.

• The receiver amplifies and demodulates the interrogation impulses.
• The decoder decodes the question according to the desired information and induces the coder to prepare the suitable answer.
• The coder encodes the answer.
• The transmitter amplifies the replay impulses and modulates these with the RF reply-frequency.

Again in the interrogator on the ground:

• The receiver amplifies and demodulates the replay impulses. Jamming or interfering signals are filtered out as well as possible at this.
• From the informations „Mode” and „Code” the decoder decodes the answer.
• The monitor of the primary radar represents the additional interrogator information. Perhaps additional numbers must be shown on an extra display.