Transponder : Block Diagram, Working, Types, Codes, & Its Applications

The transponder was invented during World War II by joining the terms transmitter & responder. These devices play a key role in current aviation, satellite communication, and also the medical field. On a satellite, the transponder is a transmitter-receiver device used to transmit signals once it gets fixed signals. This device processes amplifies & retransmits a variety of frequencies to another antenna or location or terminal on the earth. There are different transponders available on the satellite where each transponder is capable of supporting a minimum or more number of communication channels. This article discusses an overview of a transponder, its types, codes, and its applications.

What is a Transponder?

A control device that receives & responds automatically to an incoming signal is known as a transponder. The transponder is a combination of transmitter and responder or receiver so the term ‘Transponder’ is attained by combining the two terms transmitter (trans) & responder (ponder). The function of the transponder is to detect, identify & locate objects, so there are frequently found in military aircraft, civilians, and many more.

Transponders are used in a wide range of applications, including satellite communications, aviation, radar systems, and telecommunications.

The transponder performs both the transmitter & responder functions within a satellite. In space segment subsystems, the transponder is one of the most significant subsystems. The transponder frequency is; it gets interrogation on 1030 MHz from the SSR (Secondary Surveillance Radar) & responds on 1090 MHz frequency.


6/4 GHz Band Transponder Block Diagram

A transponder in a satellite is a set of different elements which performs like a channel for communication. The single conversion 6/4 GHz band transponder block diagram is shown below. The operation of each element within the transponder is discussed below.

Here in the above block diagram, when the signal is transmitted from one end to the other, the 6 GHz signal is converted into 4 GHz. At first, a 6 GHz signal will be received from an antenna and given to an LPF (low noise amplifier) where the signal amplification is performed. Furthermore, this amplified signal is down-converted into 4 GHz with a local oscillator by 2.225 GHz of frequency.

Transponder Block Diagram
                                                          Transponder Block Diagram

After that, the intermediate frequency (IF) bandpass filter (BPF) receives the output signal as 4 GHz & removes the not required frequency signal. Further, the filtered signal experiences amplification through a pre-amplifier. After that, the preamplifier’s output is given to HPA or high-power amplifier. This amplifier levels the phase & amplitude difference of the signal obtained.

So in this manner, the different units in a transponder will down-convert the signal from 6 GHz into 4 GHz & provide the amplified signal output from the high-power amplifier to the transmitting antenna. Therefore, a 6/4 GHz transponder works in this way.

Transponder Working Principle

The transponder works with radio frequencies & responds to wireless monitoring, control device signals & communications. Once a signal is sent, a transponder reacts by simply returning an identified signal. The information within the response changes based on the kind of transponder, although it can contain identifying codes & location. The transponder automatically transmits back a radio signal at a fixed frequency. To transmit & receive signals concurrently, transmitting & receiving signals should be set at dissimilar frequencies.

Transponder Types :

There are several types of transponders used in different communication systems. Here are a few common types:

Satellite Transponder

As mentioned earlier, a satellite transponder receives signals from Earth-based stations, amplifies them, and retransmits them back to Earth. These transponders are used in satellite communication systems to facilitate communication over large distances. We have already seen the above regarding the satellite transponder.

Satellite transponder performs two functions translation of frequency and received input signal amplification. There are two types of satellite transponders like bent pipe and regenerative type which are discussed below.

A bent pipe type transponder is also known as a conventional transponder or repeater. This transponder gets a microwave frequency signal and it changes the input signal frequency to RF frequency after that, it amplifies the signal.

The regenerative transponder is also known as a processing transponder which performs Bent pipe transponder functions like translation & amplification of frequency and also performs the RF carrier demodulation to baseband, signals & modulation regeneration. These transponders are appropriate for only digital signals. These transponders have many benefits like S/N ratio improvement & have more flexibility within the implementation.

Aviation Transponder

Aviation transponders are used in aircraft for air traffic control and identification purposes. They receive signals from ground-based radar systems and transmit a response that includes information about the aircraft’s identity, altitude, and other relevant data. This helps in tracking and monitoring aircraft during flight. An air traffic controller transmits an interrogator signal & gets identifying information over an aircraft. This allows the control tower to follow the aircraft within the nearby airspace & provides other information to assist the pilots in maintaining sufficient distance from another aircraft.

Radar Transponder

Radar transponders are devices used to enhance aircraft visibility on radar systems. They receive a radar signal from a ground-based radar system or another aircraft and immediately transmit a response signal back to the source. This allows the radar system to detect and locate the aircraft more accurately.

Radar transponder is mainly designed to utilize in SAR (search and rescue) operations. This transponder mainly provides the position for any near vessel & aircraft through X-band radar. These are also applicable in liferafts & lifeboats.

Automatic Dependent Surveillance-Broadcast (ADS-B) Transponder

ADS-B transponders are used in aviation for surveillance and tracking purposes. They receive GPS signals and transmit information about the aircraft’s position, altitude, speed, and other data to ground-based receivers and other aircraft equipped with ADS-B systems. This helps in enhancing situational awareness and air traffic management.

Telecommunications Transponder

Telecommunications transponders are used in various communication systems, such as fiber-optic networks, wireless networks, and satellite communications. They receive incoming signals, amplify them, and retransmit them to their destination. These transponders are used to extend the range and improve the quality of the communication signal.

Fiber Optic Transponder

A fiber optic transponder is an element that is used to transmit & receive the optical signal using a fiber. It is normally characterized by its data speed & the highest distance the signal can move. The main functions of fiber optic transponder are; optical and electrical signal conversion, monitoring & controlling, serialization & deserialization.

  • Fiber optic transponders are used within WDM networking.
  • It is used for converting from Multimode to single-mode.
  • Every optical transponder includes a redundant fiber path option used for additional protection
  • Some optical transponders efficiently expand an optical signal for covering the preferred distance.
  • The optical transponder is used to perform wavelength conversion within commercial networks.

RFID Transponders

The transponders which are used in RFID are known as RFID transponders. These are also known as tags which provide unique and specific identification numbers that exist within the chip memory. Smart card transponders are available with extra memory to store a lot of data as compared to a simple ID number. The capacity of this memory is frequently utilized in different payment, time/location access authentication, and other applications that store significant data or that demand safety.

These transponders are divided into two categories based on the communication technique with readers active and passive. Active transponders include a power source to strengthen the transmitting signal between the transponder & reader. These are mainly used for recognizing pallets & railroad cars, allowing reading distances from meters of distance. Passive transponders are used to read the distance from a few centimeters to 15 meters. As compared to active transponders, these transponders have many benefits like less price, longer life, etc.

Sunpass Transponder

It is a portable transponder used in different countries where E-ZPass is accepted. This transponder provides drivers the ease of tolls paying automatically by letting nonstop travel by single toll account. This type of transponder communicates through RF with equipment & toll plaza readers. Whenever your car passes through SunPass-equipped lanes, then toll gate charges are deducted electronically from your prepaid account of toll.

Transponder Types In Controlling Air Traffic:

There are three types of transponders like Transponder A, Transponder C & Transponder S.

  • Transponder A is used to transmit a code back to air traffic control.
  • Transponder C is used to transmit a code, altitude, or flight level information to air traffic control.
  • Transponder S is used to transmit code & altitude information while reporting & getting data to other transponders.
  • All these transponders should have the capacity to produce a 4-digit code with 1 to 7 numbers, which gives 4,096 achievable codes.

Modes of Transponder

A transponder in aircraft can communicate in different modes with ATC & other aircraft known as modes of transponder. For instance, when you imagine an ATC station on the Earth and an aircraft in the air. When the ATC station requests the aircraft for information then the transponder in the aircraft replies to ATC. Here, this reply format can be determined by transponder mode. There are two types of transponder modes like military transponder modes and civilian transponder modes which are discussed below.

Military Transponder Modes

The military transponder modes include Mode-1 to Mode-5 which are discussed below.

Mode 1: This is a type-1 military transponder mode that is used in identifying the task or function of a military flight like air interception, air defense, or training. The aircraft sends a 2-digit code that is allocated by the military to give a general indication regarding its type of mission.

Mode 2: In this type of military transport mode, the military aircraft sends a 4-digit code to serve as an exclusive aircraft identifier. So this code assists ATC & also other military units in recognizing a particular aircraft throughout a mission.

Mode 3/A: This type of military transponder mode is similar to civilian mode-A. So, the military aircraft sends a 4-digit code that is allocated by ATC. This code helps ATC to distinguish between aircraft on their radar displays. This code is used by both military & civilian aircraft.

Mode 4: Mode 4 is a safety mode mainly utilized by military aircraft to verify their individuality by sending an encrypted code. This mode provides a high range of security as compared to the other types of transponder modes because the aircraft’s identity is verified by only the certified receivers by the right decryption key.

Mode 5: This is an advance and much-secured version of Mode S in militaries. In addition to the Mode S features, Mode-5 mainly includes encrypted, secure communication & also very superior anti-jamming capacities. This transponder mode can also support a tactical data link n/w and Link 16 utilized by NATO & linked military forces for real-time data transmitting.

Civilian Transponder Modes

Civilian transponder modes mainly include A, B, C, D & S. Here, only modes A, C & S are used but Mode B & D are not used.

Mode A: This is the most essential transponder mode, This mode sends the 4-digit squawk code. This mode is very helpful in identifying an aircraft & also its location.

Mode C: This is the most broadly utilized civilian transponder mode. This mode is used to provide information above the pressure altitude of the aircraft. By combining mode A with this mode, ATC & other aircraft gets a unique squawk code, altitude & position of aircraft.

Mode S: Mode S (Mode Select) civilian transponder mode that provides very advanced communication abilities as compared to mode-A/mode-C transponders. Once you use this transponder mode, then it sends different valuable data to ATC as well as other aircraft. The critical data sent by this transponder are; unique ICAO address, squawk code, altitude, position, speed & heading, and additional data regarding the flight.

There are two systems that make this transponder mode very valuable TCAS (Traffic Collision Avoidance Systems) and ADS-B (Automatic Dependent Surveillance-Broadcast).

So these two systems will develop situational awareness & avoid mid-air collisions by simply letting aircraft send their locations & other related data.

Codes of Transponder

Transponder codes are usually 4-digit numbers that are transmitted by an aircraft transponder in reply to a secondary surveillance radar interrogation signal to help air traffic controllers through traffic separation. A separate transponder code known as squawk code is allocated by air traffic controllers for recognizing an aircraft exclusively within an FIR (flight information region).

Transponder Codes in Emergency Use:

In an emergency situation, pilots must transmit the below transponder codes suitable to their position:

  • Mode 3A Code 7700 is used in emergency conditions.
  • Mode 3A Code 7600 is used in Comms Failure –
  • Mode 3A Code 7500 is used in Unlawful Interference –

Transponder Requirements

Different classes of airspace include different transponder requirements.

  • In Class A, B & C airspace, all aircraft should be equipped with a Mode-C transponder.
  • Class D Airspace doesn’t need a transponder because, in this class of airspace, pilots need only two-way radio communication.
  • The transponder requirements will vary in Class E Airspace based on the height of the aircraft:
  • A transponder is not necessary below 10,000 feet mean sea level unless the airplane is within 30 sea miles of a Class-B airport.
  • A transponder with height reporting ability is necessary above or at 10,000 feet mean sea level.

Transceiver Vs Transponder

The difference between a transceiver and a transponder includes the following.



A transceiver is a device used to transmit as well as receive signals. A transponder is a device that receives a radio signal & transmits a different signal automatically.
Transceiver electrically interfaces with the host system through a serial interface. It uses a parallel interface.
Transceivers consume less power. Transponders consume more power.
The frequency of this ranges from  4.9 – 5.9 GHz: The frequency of this ranges from  1030 to 1090 MHz.
Examples of transceivers are; nRF, BLE modules, Lora, etc. The examples of transponders are; RFID tags, 433MHz transmitter,etc

Transponder Vs Transducer

The difference between a transponder and a transducer includes the following.



A transponder is a combined unit of a transmitter & a receiver. A transducer is one kind of electromechanical device.
It gets the weak signal and amplify it & transmits the signal again. It changes a particular kind of energy into an electric signal.
Transponders are usually found in both military aircraft & civilian and in car keys.


Transducers are frequently used in automation, control systems, etc where electrical signals are changed to other physical quantities like force, energy, torque, light, position, motion,  etc.
An example of a transponder is an RFID device. An example of the transducer is a speaker.


The advantages of a transponder include the following.

  • It improves functionality.
  • It provides security.
  • It is simple to use and handle.
  • It is much more secure.
  • It improves the efficiency & productivity of motor carriers & drivers.
  • It decreases the usage of diesel & emissions of carbon dioxide.
  • It decreases paperwork for motor carriers & government by keeping records & automated reporting.
  • It enhances the efficiency of enforcement by aiming at high-risk carriers.
  • It improves the integrity, accuracy & credentials verification by sharing the data of commercial vehicles between different states & the FMCSA (Federal Motor Carrier Safety Administration).
  • It permits motor carriers for getting data very quickly from enforcement & governing agencies.
  • It improves Signal to Noise Ratio & has more flexibility within an implementation.


The transponder applications are discussed below.

  • Transponders are used in satellites, civilian & military aircraft.
  • These are used in different technologies like sonar, vehicle keys, aircraft identification, optical communications, electronic toll collection systems, communications satellites, credit cards magnetic labels, lap timing, and many more.
  • It is used as part of an avionic system in an aircraft to provide altitude information.
  • In a single satellite, there are various transponders where every transponder supports above one communication channel.
  • These are used in advanced vehicles to transmit & receive signals
  • It is used for receiving audio & video signals.
  • A sonar transponder is used to measure distance depending on the marking of the underwater location.
  • These are available as small chips within the head of the car keys.
  • These are used in air defense for identifying enemies and objects.

Thus, this is an overview of a transponder, its working, advantages, and its applications. A transponder is a kind of radio transmitter used to transmit signals automatically once it gets particular signals. Here is a question for you, what is a transceiver?