Mode S transponders

by Fred Van Driel, 2nd year avionics student, December 2011.
(SAIT Polytechnic, Calgary, Alberta, Canada)

(Disclaimer: the author is not an expert on Mode S transponders. No promise of accuracy is made. The sources consulted don't all agree on the details of how exactly Mode S works. Use appropriate manuals, aviation authorities, textbooks, manufacturers' data, or authoritative websites.)


A Mode S transponder is a aircraft radio receiver & transmitter that, when it receives an interrogation signal from a ground-based secondary surveillance radar (SSR) or another aircraft, replies with a specific digitally-encoded data burst, allowing the interrogating station to know the identity, location, heading and other information about the answering aircraft. Even though the SSR is called a radar, it is really a two-way radio system whose ground antenna is a highly directional one that is mounted on top of the the primary surveillance radar (PSR), and rotates with it. The PSR and SSR outputs are combined by computer and the information gathered by them is displayed together on air traffic controllers’ “radar” screens.

Figure 1. Becker Mode-S transponder, model BXP 6401


Transponders date back to World War II, when radar first came into use. Germany was the first to produce transponders so its radar could identify friendly aircraft. But the British learned how the system worked, so Mosquito nightfighters could interrogate a German aircraft, causing it to betray its position [14].

The Americans had no such system, with tragic consequences. The Japanese aircraft on their way to attack Pearl Harbor on 7 Dec 1941 were detected by a radar station on Hawaii at a distance of over 200 kilometres[15], but a untrained officer on watch believed them to be American B-17s, so no defensive action was taken [11].

Modes A and C transponders came into common use in the 1950s to help air traffic control keep track of aircraft.

Staff of the Lincoln Laboratories of MIT began work on replacing or supplementing Modes A and C in 1968 when they realized that the air traffic control system would soon be overloaded by the existing Modes A and C. Because there was no way to have only 1 aircraft reply to an interrogation from an SSR, increased air traffic would result in radio chaos[2].

One of the key aspects of design was backwards compatibility, because the new system had to use the same radio frequencies[2].

Basic principles of Modes A and C

Only an SSR transmits a Mode A interrogation, and the transponders of all aircraft in the radar beam will reply to a valid Mode A interrogation. The reply is the 4-digit octal code (commonly known as a squawk code) that was assigned to it by air traffic control, or 1200 in North America if the aircraft is on a VFR flight without an assigned code.

A Mode C transponder is very similar with the only difference being that the reply is the aircraft’s altitude instead of the squawk code. Mode C also only responds to SSR interrogations.

Details of the Modes A and C transponders are described elsewhere, and are outside the scope of this report.

The SSR’s interrogation uses precise format and timing in its interrogation transmission to identify whether it is asking for a Mode A or Mode C reply.

A Mode A transponder’s reply consisted of a 12 bit reply (plus control and framing bits) that encodes the aircraft’s squawk code, with precise timing to differentiate it from Mode C.

A Mode C transponder’s reply consisted of a 12 bit reply (plus control and framing bits) that encodes the aircraft’s altitude, with precise pulse timing to differentiate it from Mode A.

Basic principles of Mode S

Because Mode S was developed to be backwards-compatible with Mode A/C, it uses the same 1030 & 1090 MHz radio frequencies. Much effort was put into making the two systems compatible. A Mode S transponder will respond to a Mode A/C interrogation exactly the same as a Mode A/C transponder, with the same information. It is when it receives a Mode S interrogation that it acts very differently. For Mode S, the SSR transmission uses the sidelobe suppression feature of Mode A/C to tell Mode A/C transponders not to respond to a Mode S interrogation.

A Mode A/C interrogation uses three radio pulses, what are called P1, P2, and P3, with precise timing intervals and signal strengths, to send its interrogation. A Mode S interrogation also uses P1, P2 and P3, but adds a P4, P5 and P6 to it.

ICAO address (unique identifier)

Mode A/C squawk codes are assigned by ATC at time of departure of each flight, and may be changed by ATC in mid-flight. One problem is that being a 4-digit octal code, there are only 4096 possibilities. The Mode S system requires that every aircraft that has a Mode S transponder must be registered, and upon registration will be issued at unique 24-bit identity code. Issued by ICAO or a designated national aviation authority on its behalf, this identity is coded into the aircraft’s transponder, and will not change as long at that aircraft is registered in that country. If the transponder needs to be changed, then the new transponder will be programmed with the same code. With a 24-bit address, there are 16 million different codes available, enough for the foreseeable future.

The major advancement with Mode S is that the ground station can interrogate one single aircraft by using that aircraft’s ICAO unique identifier. Also, aircraft can send out Mode S interrogations to other aircraft. This feature is the basis of collision avoidance systems (e.g., TCAS) and non–radar-based traffic control systems such as ADS-B.

Figure 2. Mode S & ADS-B schematic

Mode S interrogations

With an ATC system that uses all three modes, A, C, and S, the system can do a variety of different interrogations:

Mode A/C request

P2 is sent by the omnidirectional transmitter, timing of P3 differentiates between Mode A and Mode C interrogation, there is no P4, P5 or P6.
Replies: All Mode A or C transponders reply as normal. The missing P4 makes all Mode S transponders reply as if they were Mode A or Mode C transponders.

No reply” interrogation

P3 and P4 pulses are both 0.8 μs long, P2 is transmitted by the onmi transmitter as with all Mode A/C interrogations. The timing of the P3 pulse identifies it as either Mode A or Mode C.
Replies: All Mode A or all Mode C transponders reply. The duration of the P4 pulse tells all Mode S aircraft not to reply.

Mode A/C/S all-call interrogation

P1 and P2 are sent as normal for a Mode A/C interrogation, P3 is 0.8 μs long, P4 pulse is 1.6 μs long. The P6 pulse has the station identification (Interrogator Code, or IC) encoded in it.
Replies: All Mode A or all Mode C transponders reply. All Mode S aircraft reply because of the P4 pulse length, and each reply contains the aircraft’s unique ICAO code. The Mode S aircraft will then not reply again to this type of interrogation from this interrogator, as identified by the IC, for 18 seconds (unless an override signal is sent by the same station[4]).

Mode S reply wanted

P2 is send by the directional SSR transmitter at full strength to suppress Mode A/C replies. There is no P4, but there is a P5 and P6. The P6 pulse has the interrogation request, specifying which aircraft is to reply, and what information is wanted.
Replies: Only one aircraft, the one whose identity is in the P6 encoding, will reply. What it sends back depends on the question encoded in the interrogation.

Figure 3. Mode S interrogation, long format

What interrogates what?
In Mode A and C, it is always a ground station that does the interrogation, and always an aircraft that replies. Therefore the terms uplink and downlink have been considered synonymous with interrogation and reply. In contrast, Mode S interrogations can also be from an aircraft, and replies may be intended for, or received by, other aircraft, so the terms uplink and downlink are no longer technically accurate. However, because uplink and downlink have been used for so long, they are still used to mean interrogation and reply.

Mode S interrogation

“The Mode S uplink interrogation format starts with two pulses, P1 and P2, which are solely for the purpose of suppressing existing Mode A/C–only transponders so that they are not aware of the main Mode S information. The Mode S interrogation data contained in the P6 data block is phase modulated. The first phase reversal is the timing point for the subsequent bits (chips) of information. The Mode S interrogation may be short (56 bits) or long (112 bits) format.”[6]

There is no P3 or P4 pulse, and the P5 pulse is sent at the same time as the P6 pulse, of slightly weaker signal strength, only 0.8μs in length, and is out of phase with P6. Depending on whether the interrogator is a ground station or an aircraft, P5 may be sent by an omnidirectional transmitter or the main transmitter, so the relative strengths can be used by the receiver to determine sidelobe suppression in the same manner that P1 and P2 are compared in Mode A/C.

The P6 pulse is encoded using differential phase shift keying (DPSK), and may be 56 or 112 data bits long, resulting in a total P6 pulse duration of 16.25μs or 30.25μs.

Mode S interrogation signal formats

The interrogations formats are labeled “UFx” for “uplink format,” and all the replies are labelled “DF” for “downlink format.” Each DF is a reply to a specific UF. For example, UF4 requests altitude from a specific aircraft, which replies with DF4.[3,10]

Encoded into the P6 pulse is the actual interrogation part of the signal. There are over 20 different interrogations that can be sent, labelled as UF1 to UF24. The information starts with an interrogator identifier (II), then the target aircraft’s ICAO address, followed by the question code, followed by 24 bits of error correcting parity information.

Mode S reply signal formats

Each DF is a reply to a specific UF. For example, UF4 requests altitude from a specific aircraft, which replies with DF4.[3,10]


Details of the each particular UF/DF question/answer pair is beyond the scope of this wiki, but some examples are[3,8,10]:
UF/DF4: Altitude
UF/DF11: (all-call) requests mode S ICAO address.
UF/DF16: air-to-air
UF/DF11: ADS-B squitter
UF/DF17: ADS-B extended squitter


Ground equipment

ATC Mode-S is co-located with the Mode A/C equipment, only the processing equipment needs to be different.

Aircraft equipment

Mode S requires replacing the Mode A/C transponder with a new Mode S one. An ICAO 24-bit address is required, and several antennas will need to be installed. Mode A/C only requires 1 antenna, Mode S when used for ADS-B requires 4, two on top and two underneath the aircraft. The transponder will also have data links to and from avionics systems such as TCAS, INS, and GPS.

Transponder inputs by aircrew

(not necessarily before each flight)
  • squawk code
  • flight number

Inputs by avionics techs

(hard-coded into transponder by avionics technician)
  • 24-bit ICAO ID
  • tail number


Testing with no equipment

There is not a lot that can be done to test Mode S transponders without test equipment. Most new transponders have built-in test equipment (BITE), so the transponder will do a self-test when turned on, and report errors. If air traffic control doesn't complain about an aircraft's transponder, and the transponder itself doesn't report a fault, it is a reasonable assumption that it is working properly. There isn't much more testing that can be done, especially since many of them automatically go into standby mode upon landing to avoid TCAS systems from reacting to planes on the ground.

Ramp testing

Ramp testing of a Mode S transponder is similar to testing a Mode A/C transponder, it requires a specialized test box. A test box allows a qualified technician to test various parameters. Test boxes such as the IFR 6000 will automate the test. The IFR 6000 can test Mode A/C/S, as well as DME, ADS-B and TCAS functions, and give a pass/fail answer, without the user having to initiate each individual test. The user can also run individual tests if desired.

Some of the parameters that it can test are:
Flight ID
Power output
Frequency stability and accuracy
Reply timing
Specific UF/DF formats

Bench testing

Just as with ramp testing, bench testing requires special equipment. A unit such as the IFR 6000 can also be used on the bench.

Future developments

The Mode S transponder is a vital part of TCAS (Traffic Alert and Collision Avoidance System) and ADS-B (Automatic Dependent Surveillance-Broadcast). Therefore it has a bright future. Its original purpose as the airborne component of the air traffic control system has been added to and has become only a part of its job. Even as the ATC system moves away from using SSR, it will use the Mode S transponder as the primary communication tool to, from, and between aircraft.

Sources consulted

1. Becker Avionic Systems. Mode S transponder BXP 6401-1-(XX) Class 1 : installation and operation. Rheinmünster, Germany: Becker Avionic Systems, 2007.

2. Chang, Emily, et al. The story of Mode S., 2000. Accessed 13 Nov. 2011.

3. Clarification: Mode S transponder in an airport/A-SMGCS environment. Brussels : EUROCONTROL, 2005.

4. Principles of Mode S operation and interrogator codes. Brussels : EUROCONTROL, 2003.

5. Radar basics—Mode S uplink formats. Accessed 26 Nov. 2011.

6. Radar basics—Mode S individual interrogation. Accessed 26 Nov. 2011.

7. Radar basics— Mode S - Differential Phase shift Keying (DPSK). Accessed 26 Nov. 2011.

8. Radar basics—Mode S reply encoding. Accessed 26 Nov. 2011.

9. Radar basics—Mode S downlink broadcast. Accessed 26 Nov. 2011.

10. Stamper, Wes. Understanding mode S technology. Defense Electronics, Dec. 2005. Accessed 26 Nov. 2011.

11. "Testimony of Lt. Kermit Tyler before the Navy Court of Inquiry." Pearl Harbor Attack, vol. 32, pp. 341–350. Accessed 26 Nov. 2011.

12. Tooley, Mike, and David Wyatt. Aircraft communications and navigation systems : principles, maintenance and operation. 1st ed. Amsterdam: Elsevier, 2007.

13. Weatherby, Lisa. "Air traffic control systems." Course lectures, Aircraft navigation systems ASYS310. Calgary, Alta.: SAIT Polytechnic, Nov. 2011.

14. Wikipedia, s.v. "Identification friend or foe." Accessed 12 Nov. 2011.

15. Wikipedia, s.v. "SCR-270 radar." Accessed 12 Nov. 2011.

Images used

1. Mode S transponder: Becker USA website. Accessed 09 Dec. 2011.

2. System schematic: ADS-B Technologies, LLC. Accessed 09 Dec. 2011.

3. Radar basics—Mode S individual interrogation. Accessed 26 Nov. 2011.