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Air Traffic Control (ATC) Transponder


To maintain safe separation of aircraft,either on ground or in air a system is required by ground controller, the system used is called as Air traffic control (ATC) system. The system is based on the principle of Secondary Surveillance Radar (SSR).  Secondary radar was developed during the Second World War to differentiate between friendly aircraft and ships: this system was called Identification Friend or Foe (IFF).

Basic Details

There are two types of radars Primary Radar and Secondary Radar. The Primary Radar send a wave and the same wave is reflected back, however in case of a Secondary Radar the wave is received processed and then send back.

In the case of ATC primary radar, the energy is reflected from the aircraft’s body to provide range and azimuth measurements, which are displayed on the Plan Position Indicator (PPI) as targets. This system is called as Primary Surveillance Radar (PSR).

The disadvantages of using PSR are: 

  • one of which is that the amount of energy being transmitted is very large compared with the amount of energy reflected from the target.

  • Small targets, or those with poor reflecting surfaces, could be difficult to detect as the reflected energy is reduced.

  • Natural and man-made obstacles such as mountains and wind farms also shield the radar signals.


In the case of ATC secondary radar a specific low energy signal (the interrogation) is transmitted to a known target. This signal is analysed by a transponder and a new (or secondary) signal, i.e. not a reflected signal, is sent back (the reply) to the origin. This system is called as Secondary Surveillance Radar (SSR).

In the air traffic control system, the primary and secondary radar antennas are mounted on the same rotating assembly, thereby providing a coordinated system.

Air traffic control system  overview.jpg


The ATC system operates on two frequencies within the L-band of radar:


  • Interrogation codes on a 1030 MHz carrier wave

  • Reply codes on a 1090 MHz carrier wave.


Since the Primary Surveillance Radar (PSR) system works on the principle of reflection, all the aircrafts would look similar on the Plan Position Indicator (PPI), depending upon the amount of reflected energy. To overcome this issue the Secondary Surveillance Radar (SSR) is used with the PSR, the SSR transponder system, allocates a unique four-digit code (allocated by ATC for each flight) which can be used to identify each icon differently on the PPI. By using SSR the unwanted reflections (from tree, building and hits etc.) are not displayed on the PPI.


With an uncluttered screen, and each aircraft readily identified, more aircraft can be allowed into the controlled airspace. The combined PSR/SSR system basic block diagrams is illustrated below.


With the developments of the ATC transponder system have provided additional functionality allowing details such as flight number and altitude to be displayed on the controller’s screen.


Emergency codes can be sent in the event of radio failure or hijacking. The reader will appreciate that it is essential for an aircraft operating in controlled airspace to be equipped with an ATC transponder.

Combined PSR and SSR.jpg


ATC transponder modes

SSR systems have the following modes of operation:

  • Mode A

  • Mode C

  • Mode S​


Mode A

When the pilot selects the four-digit code on the ATC control panel prior to each flight, the aircraft's azimuth is displayed in the form of an icon on the controller's screen. However if two aircraft icons and in close proximity and needs to be distinguished the ATC will request one of the aircraft to push its identity code on the control panel and the aircraft will be highlighted on the controller's screen. 


Since each aircraft is allocated with a unique code, only one icon per aircraft will be highlighted; this unique identification is referred to as a squawk code. Each of the four digits ranges from 0 to 7, these are then coded as octal numbers for use by the transponder.

Mode C

In this mode three-dimensional information is provided with the help of the pressure altitude which is displayed on the controller’s screen, adjacent to the aircraft icon. Altitude can be taken from the pilot’s altimeter from an encoder that sends parallel data (in Gillham/Gray code) to the transponder. This coded data is in 100-foot increments. Aircraft with air data computers will send altitude to the transponder in serial data form, typically ARINC 429.

Mode S (Select)

There is a problem associated with the Mode A and Mode C transponders, that if there are too many aircraft together and all of them sending a their replies together, it will create more confusion to the controller. Instead if there is a system which will select a specific aircraft to reply it will increases the efficiency of the ATC resources. Hence the Mode S transponders are used, as in addition to the basic identification and altitude information, Mode S includes a data linking capability to provide a cooperative surveillance and communication system.

The Mode S system has a number of advantages:

  • Increased traffic densities

  • Higher data integrity

  • Efficient use of the RF spectrum

  • Reduced RF congestion

  • Alleviation of Mode A and C code shortages

  • Reduced workload for ground controllers

  • Additional aircraft parameters available to the ground controller.

Mode S transponders only send a reply to the first interrogation signal; the ground station logs this aircraft’s address code for future reference. Aircraft equipped with Mode S transponders are also able to communicate directly with the Mode S transponders fitted to other aircraft; this is the basis of the traffic alert and collision avoidance system (TCAS)

ATC System Equipment

The ATC System consists of the following:

  • Two ATC antennas 

  • A control panel

  • Two transponders


Since the ATC system and distance measuring equipment (DME) operates in the same frequency range, a mutual suppression circuit is utilised to prevent simultaneous transmissions.


Control panel

Air Traffic Control (ATC) and Traffic Alert and Collision Avoidance System (TCAS) have a combined control panel. The four-digit aircraft identification code is selected by either rotary switches or push buttons, and displayed in a window. Altitude reporting for Mode C transponders can be selected on or off. When requested by ATC, a momentary make switch is pressed; this transmits the selected code for a period of approximately 15 to 20 seconds.

Control Panel .jpg



The aircraft transponder provides the link between the aircraft and ground stations. The ground station SSR antenna is mounted on the antenna of the primary radar surveillance system, thereby rotating synchronously with the primary returns. The airborne transponder receives interrogation codes on a 1030 MHz carrier wave from the ground station via one of two antennas located on the airframe. These signals are then amplified, demodulated and decoded in the transponder. The aircraft reply is coded, amplified and modulated as an RF transmission reply code on a 1090 MHz carrier wave. If the transponder is interrogated by a TCAS II equipped aircraft, it will select the appropriate antenna to transmit the reply. This technique is called antenna diversity; this enhances visibility with TCAS-equipped aircraft flying above the host aircraft.


Altitude encoder

Early altitude encoders were optical-mechanical devices integrated into the barometric altimeter; the data output was a 10-bit parallel bus. Modern encoders on large aircraft are integrated into an air data computer (ADC), with serial data output, e.g. Arinc 429. General Aviation (GA) aircraft typically have a separate solid-state encoder, with options for an RS-232 serial bus and/or parallel Gillham coded bus output.


A practical encoder will have an altitude range from -1000 to +30,000 feet, with some products  extending up to 42,000 feet.

System operation

SSR has many advantages over primary radar, however due to the smaller antenna's radiation pattern contains substantial side-lobes. These side-lobes can generate false returns, and so a method of coding the interrogation signals via pulse techniques is employed.


The solution is to superimpose an omnidirectional pattern from a second antenna onto the directional beam.

A given aircraft’s transponder will receive maximum signal strength each time the ground station’s directional beam passes, i.e. once per revolution. Since P2 is transmitted from the fixed omnidirectional antenna, it is received with constant signal strength; but with lower amplitude than P1/P3.


When the aircraft’s transponder receives the maximum P1/P3 signal strength, i.e. when the rotating antenna is directed at the aircraft, they are received at higher amplitude than P2.


An aircraft not within the main-lobe of the directional beam would receive a P2 pulse from the omnidirectional antenna at higher amplitude than the P1/P2 pulses.


The transponder recognises this as a side-lobe signal and suppresses any replies until 25 to 45 μs after P2 is received. This is called side-lobe suppression (SLS), a technique ensuring that only the main lobe of the rotating antenna is being replied to and not a side-lobe.

Side-lobe suppression (SLS).jpg


Mode A and C interrogation

Interrogation is based on a three-pulse format, each pulse is 0.8 μs wide. Two pulses (P1 and P3) are transmitted on the rotating antenna thereby producing a directional signal. A third pulse (P2) is transmitted on the fixed antenna that radiates an omnidirectional signal. The purpose of the P2 pulse is described in the Mode A reply section.


P1 and P2 have a 2 μs interval; P3 is transmitted at an interval of 8 μs for Mode A and 21 μs for Mode C interrogations.


This spacing between P1 and  P3 therefore determines the type of interrogation signal (Mode A or C). The pulse repetition frequency (PRF) of interrogation signals is unique to each ground station; a typical PRF is 1200 interrogation signals per second. Replies are sent by the aircraft at the same PRF.

Pulse format for Mode A and Mode C inter


Mode A reply

The Mode A reply is the ATC code allocated to that flight, formed into a series of pulses. This reply is framed between two pulses (F1 and F2) that have a time interval of 20.3 μs. Data to be transmitted is coded by twelve pulses (plus an unused ‘spare’ pulse in position X) at 1.45 μs intervals within F1 and F2.

The final part of the aircraft reply is a pulse that is sent after F2; this pulse occurs 4.35 μs after F2, but it is only sent when an ‘ident’ is requested by ATC. The flight crew send this special position identity (SPI) pulse by pressing a momentary make switch on the control panel. The SPI pulse is sent for a period of 15 to 20 seconds after the switch has been pressed; this highlights the aircraft icon on the controller’s screen.

Mode A:C reply pulse train (ATC code or


The twelve pulses are grouped into four groups of three; each group represents an octal code. Each of the four groups is labelled A, B, C and D; single pulses within the group carry a numerical weighting of 1, 2 and 4.


When a pulse occurs in group A, this represents the value 1, 2 or 4 depending on the position of the pulse. When a pulse is not transmitted in the allocated time frame, this represents a value of zero. With four groups of data, the octal numbers between 0000 and 77778 can be transmitted; this corresponds to the ATC code allocated to the flight and selected by the crew on their ATC control panel. (4096 codes are possible using these four octal digits.)

Illustration of Group A pulses.jpg


Mode C reply

The aircraft’s altitude is encoded by the transponder and transmitted as binary coded octal (BCO) (in 100 foot increments) as described for Mode A replies. The reply will also contain a code representing the aircraft’s altitude; this is referenced to standard pressure, 1013.25 mB if the aircraft is above the transition altitude.


Since all SSR transmissions are on the same frequencies (interrogation on 1030 MHz and replies on 1090 MHz), problems can occur when aircraft are within range of two or more ground stations. Several replies could be sent by an aircraft to each ground station that sends an interrogation signal; these undesired replies are known as non-synchronised garble, or false replies from unsynchronised interrogator transmissions (FRUIT). Note that FRUIT is sometimes written as false replies uncorrelated in time.


When interrogations are received simultaneously, the transponder will reply to as many ground stations as possible.  If two or more aircraft are in close proximity, e.g. in a holding pattern, and within the ground

station’s directional antenna beamwidth, it is possible that their individual replies overlap at the ground station’s computer.


The situation where replies are received from two or more interrogators answering the same interrogation is referred to as synchronized garbling. To resolve this, the controller can request the flight crew on a specific aircraft to provide an ident pulse. The problem is that ATCRBS for Modes A/C requires many interrogations to determine the position (range and azimuth) of an aircraft; this requires increased capacity of the ATCRBS infrastructure.


The increasing density of aircraft within a given air space leads to false replies as the ground station saturates with garbling conditions. The solution to this is the Mode S (select) system.

Mode S operation

Although Mode S communication is very different to that of Modes A/C, both types of equipment operate on the same frequencies. The system is two-way compatible in that aircraft equipped with Mode A/C transponders will respond with ident and altitude data if interrogated by a Mode S ground station.


Individual interrogations are sent to specific aircraft; only the transponder on this aircraft sends a reply. This reply contains additional information, e.g. selected altitude and flight number.


Directional and omnidirectional beam patterns are transmitted as illustrated in Figure. Unlike ATCRBS, Mode S uses a monopulse SSR; this reduces the number of interrogations required to track a target. In theory, monopulse radar only requires one reply to determine the target’s azimuth (direction and range).

Mode S antenna pattern.jpg


Mode S Interrogations

Two interrogation uplink formats (UF) are transmitted, these are the all-call and roll-call interrogations.


The two interrogations are transmitted on an alternating basis and differentiated by the width of a P4 pulse; this is either 0.8 μs or 1.6 μs. The shorter pulse is used to solicit replies from Mode A/C transponders; they reply with their ATC code and altitude as before. Mode S transponders will not reply to this interrogation.


When the P4 pulse is 1.6 μs, Mode S equipped aircraft will reply with their unique address. These replies are stored by the Mode S system as unique identifiers for each specific aircraft.



The Mode S discrete addressed interrogation uplink format (UF). Pulse P1 and P2 both have the same amplitude and are part of the directional antenna’s main-lobe. This pair appears as suppression pulses to Mode A/C transponders, so they do not reply.


Mode S transponders then seek the start of the P6 data pulses; this is formed by a pattern of phase reversals that form a series of logic 1/0.


Phase-shift keying (PSK) is a modulation technique that shifts the phase by minus 90 degrees for a logic one, and +90 degrees for a logic zero.


Each data pulse’s duration is 0.25 μs; the pulse’s phase is sampled at these intervals. A reference pulse of 1.25 μs duration is used to indicate the start of the data word. The word length of P6 (56 or 112 bits) depends on the transponder type.

Mode S discrete interrogation signal upl


Mode S reply

The Mode S reply is sent via the 1090 MHz carrier wave, this contains a four-pulse preamble, starting with two pairs of synchronising pulses followed by a block of data pulses (either 56 or 112 bit blocks). Using pulse position modulation (PPM), each data bit is allocated a 1 μs time interval, divided into two halves.


If the first half of this interval contains a pulse, this represents logic 1; if the second half of the interval contains a pulse this represents logic 0. Note that both states are indicated by the presence of a pulse.

Mode S reply downlink format.jpg


Each Mode S-equipped aircraft has a unique address allocated to it by ICAO via the individual national registration authorities; the aircraft address (AA) is a 24-bit code that cannot be changed. Each national authority allocates a header code within the 24 bits. A 24-bit code of all zeros is not valid; all ones are used for the all-call interrogation.

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