Transponder

A transponder is an aircraft device that automatically transmits signal when queried by radar. Through a panel in the cockpit, the pilot enters the assigned transponder code, enabling air traffic control to identify the aircraft. Upon interrogation, the transponder transmits its identity and altitude information. The aircraft’s position is determined based on the signal round-trip time and the direction of the radar antenna. When an aircraft’s transponder is disabled, no time can be obtained about that aircraft and communication cannot be established.

The term transponder is derived from the English words transmitter and responder in the 1940s.

History

The aviation transponder was first developed during the Second World World War by the American and British armed forces as a means of distinguishing friendly from hostile aircraft on radar, known as Identification Friend or Foe (IFF).

The same system was adapted by civil air traffic control in the 1950s for use in secondary surveillance radar (beacon radar) systems to provide traffic services for general and commercial aviation.

Transponder Types

Many different RF (Radio frequency) communication protocols have been standardized for civil aviation transponders. Depending on the interrogation mode, transponders provide identification codes, airplane position, pressure altitude, call name, and other data.

Definitions of different transponder modes:

• Mode A: The equipment transmits only an identification code.
• Mode C: Provides the identification code and aircraft pressure altitude.
• Mode S: Provides information in multiple formats for selective interrogation. Designed to avoid excessive interrogation and enable automatic collision avoidance.

The most widely used transponder mode is Mode S.

Transponder Settings

A transponder switch typically has multiple positions:

• OFF: Turns off the transponder.
• STBY: Powers on the transponder and prepares it for flight. (Warm-up)
• ON: Transmits only basic information such as code and position to radar.
• ALT: Used with Mode S or Mode C; additionally transmits altitude information.
• TA: Provides traffic advisory.
• TA/RA: Provides traffic advisory and resolution advisory.

Example of transponder settings

• IDENT: All Mode A, C, and S transponders include an "IDENT" (identification) special function button that assists air traffic controllers in locating an aircraft. The IDENT button is used only upon request by ATC.

IDENT button

Transponder Codes

The transponder transmitter generally uses different codes to identify flights. Squawk codes consist of four-digit numbers. The dials on the transponder range from 0 to 7 reader.

Specific squawk codes are assigned for particular situations or flights.

• 7700: "Emergency on board."
• 7600: "Radio failure."
• 7500: "Aircraft hijacked."

Transponder Operating Principle

Since a transponder is designed to respond to queries from a place based radar system, it typically operates as follows:

1. Radar Query: A ground-based radar system sends a query to a transponder to track an aircraft in the airspace. This query requests specific information from the aircraft.

2. Signal Transmission: The transponder responds to the radar query by transmitting the aircraft’s information. This information is encoded within a radio signal sent by the transponder.

3. Radar Reception: The ground-based radar system receives and decodes the signal from the transponder to gather data about the aircraft.

4. Display Output: The ground-based radar system converts the information received from the transponder into a visual display for air traffic control operators.