Transponder Mode A, C and S: the technology behind radar identification and ADS-B

On the flight radar map, next to the Airbus on approach it says “Squawk 2353,” a few lines further down “Hex 3C6xxx” – and the business jet next to it has an altitude but no position. Behind all of this is a device hardly bigger than a car radio: the transponder. If you understand what transponder Mode A, C and S do, you read flight radar apps and ADS-B receivers with very different eyes. This guide explains the technology behind radar identification step by step, without losing you in pulse diagrams, and shows what of it is really useful at the spot.

Transponders in one sentence

A transponder is the reply station on board: it receives an interrogation from secondary radar on 1030 MHz and sends a coded reply back on 1090 MHz. What is in this reply depends on the mode. Mode A provides an identity, Mode C the altitude, Mode S a unique aircraft address along with further data. ADS-B, finally, is a Mode S transmission that the transponder sends out entirely on its own, without any interrogation.

Primary radar and secondary radar

To understand transponders, a quick look at the two types of radar helps. Primary radar works like an echo: the antenna sends a pulse, part of it is reflected by the aircraft’s skin and comes back. Position follows from the travel time and antenna direction. The aircraft doesn’t have to do anything for this, but also provides no information about who it is or how high it is flying.

Secondary radar (Secondary Surveillance Radar, SSR) reverses the principle. The ground station asks, the aircraft actively answers. Because the reply comes back with its own transmit power, secondary radar reaches far, and the reply can contain data. The roots lie in the military identification friend or foe (IFF) of World War II. In the 1950s, ICAO began to standardize the method for civil aviation.

Air traffic control today mostly combines both. How controllers in Germany work with it is explained in the article on the DFS German Air Navigation Service Provider; a look back at the beginnings is provided by the history of air traffic control.

Mode A: identity via squawk

Mode A is the oldest civil mode. The reply contains a four-digit code that the pilot sets on the transponder and that air traffic control assigns by radio – the famous “squawk two-three-five-three.” Each digit runs from 0 to 7 because it is an octal code. Digits like 8 or 9 therefore don’t exist in a squawk. Four octal digits give 4,096 combinations.

For controllers, the squawk is the link between radar target and flight plan: the system knows which code was assigned to which flight and displays the matching callsign. Because there are so few codes, they are reassigned again and again regionally. A squawk is therefore not a permanent identifier of an aircraft.

Some codes are permanently assigned worldwide and especially interesting for spotters:

Code	Meaning	What you can read from it
7700	general emergency	The crew has declared an emergency, often with a diversion
7600	radio failure	No voice radio connection, the aircraft follows set procedures
7500	unlawful interference	Hijacking – very rare in practice and often a mistaken entry
7000	VFR without an assigned code (Europe)	typical for small aircraft in VFR operation
1200	VFR without an assigned code (USA)	the US counterpart to 7000
2000	no code assigned by air traffic control, e.g. when entering from an area without radar coverage	shows up e.g. on flights from oceanic airspace

What is behind the three emergency codes and how flight radar sites highlight them is covered in detail under Squawk 7500, 7600 and 7700. Mode A also includes the ident function: when the controller asks for “squawk ident,” the pilot presses a button, and the radar target is briefly highlighted on the screen.

Mode C: altitude is added

Mode C adds the barometric pressure altitude. An altimeter or a separate altitude encoder passes the altitude to the transponder, which transmits it in 100-foot steps. The altitude always refers to standard pressure of 1013.25 hPa – regardless of what the pilot has currently set on the altimeter. The conversion to actual altitude is handled by the radar system on the ground.

In practice, Mode A and Mode C run together: the ground station alternates between the two interrogation types, which differ in the spacing between two pulses – 8 microseconds for Mode A, 21 microseconds for Mode C. That is why people usually simply speak of a Mode A/C transponder.

For you at the spot, this means: the altitude on the flight radar map is generally a flight level or pressure altitude, not a figure in meters above ground. In strong high or low pressure, an aircraft on final approach therefore sometimes looks higher or lower than it actually is.

Mode S: each aircraft individually

Mode S stands for “Select.” The mode was developed from the 1970s in the US and the United Kingdom and standardized by ICAO in 1987. It uses the same frequencies as Mode A and C and is backward compatible with them, so a Mode S transponder also answers classic interrogations.

The core is a 24-bit address permanently assigned to each transponder. ICAO allocates address blocks to states, and the aviation authorities assign addresses from them to individual aircraft. The address is usually written as a six-digit hexadecimal number, hence the name hex code. German addresses, for example, start with 3C to 3F. How to derive the state of registry and registration from the hex code is explained in the guide to the ICAO 24-bit address.

With this address, the radar can address an aircraft specifically. It finds new targets through an all-call interrogation, then interrogates each aircraft individually. That reduces overlaps considerably. At the same time the reply becomes a data channel: a Mode S transponder can provide the callsign, flight status (airborne or on the ground) and – depending on equipment – an altitude in 25-foot steps.

Elementary and Enhanced Surveillance

In Europe, two levels of capability are distinguished:

• Elementary Surveillance (ELS): address, callsign, altitude, flight status and the transponder’s capability report. This is the basic equipment for instrument flight in the EU.
• Enhanced Surveillance (EHS): In addition, the aircraft provides so-called downlinked aircraft parameters, such as the target altitude set by the pilot, magnetic heading, indicated airspeed and Mach number, ground speed, roll angle and climb or descent rate.

The selected target altitude in particular is valuable for air traffic control: the controller sees whether the crew has entered the cleared altitude correctly before the aircraft reaches it. This data, incidentally, also appears in some detail views of flight radar sites – when it says “Selected Altitude” there, the value comes from Mode S. What the other fields and symbols mean is explained in the guide to Symbols and colors on flight radar maps.

Squitter: when the transponder transmits unasked

A Mode S transponder doesn’t just wait for questions. At regular intervals it sends short messages with its address on its own, so-called squitters. That way ground stations and the collision warning systems of other aircraft can detect it without having to search for it first.

The extended squitter (1090 ES) is the long variant – and that is exactly ADS-B as it is common in Europe and on airliners worldwide. These messages contain the position from the satellite navigation receiver on board, altitude, speed, heading and callsign. Because nobody has to ask, anyone with a suitable receiver can listen in. That is the foundation of all flight radar apps and of every self-built ADS-B receiver – more in the basics article Flight tracking with ADS-B for spotters. Because the messages are radiated omnidirectionally, even satellites can pick them up, as the article on space-based ADS-B shows.

Feature	Mode A	Mode C	Mode S	ADS-B (1090 ES)
Trigger	interrogation from radar	interrogation from radar	targeted interrogation or squitter	transmits on its own
Identity	squawk (changing)	–	24-bit address, callsign	24-bit address, callsign
Altitude	–	pressure altitude, 100-ft steps	pressure altitude, 25-ft steps depending on the unit	pressure altitude, often GNSS altitude as well
Position	only via the radar	only via the radar	only via radar or MLAT	reported by the aircraft itself
Usable for spotter receivers	barely	barely	hex code, altitude, squawk	fully, with position

In the US, besides 1090 ES there is a second ADS-B method for general aviation, UAT on 978 MHz. Technically it has nothing to do with Mode S; more under UAT on 978 MHz.

Who has to carry what on board

Equipment requirements depend on airspace, flight rules and aircraft class. The key points:

• EU: Implementing Regulation (EU) No 1207/2011 and its amendments require Mode S transponders for flights under instrument flight rules. Aircraft with more than 5,700 kg maximum takeoff weight or more than 250 knots maximum cruising speed additionally need Enhanced Surveillance and ADS-B Out via 1090 ES. Special rules apply to state aircraft.
• USA: Since January 1, 2020, ADS-B Out has been mandatory in most airspace where a transponder was already required before, such as Classes A, B and C – with 1090 ES or UAT.
• Small aircraft in Europe: Whether a transponder is needed depends on the airspace, for example in transponder mandatory zones (TMZ) around regional airfields. In VFR flight without an assigned code, the squawk is then set to 7000. Which airspaces require which equipment is explained in the overview of airspace classes A to G.

For you this means: airliners and most business jets practically always appear with a position in Europe. For small aircraft, helicopters, older aircraft and military, the picture is much more varied.

What you actually see at the receiver

An ADS-B receiver on 1090 MHz hears everything traveling on this frequency: extended squitters, short squitters and the replies to radar interrogations. Typical cases result:

• Aircraft with position: The transponder sends ADS-B. You see hex code, callsign, position, altitude and speed.
• Aircraft without position but with hex code and altitude: A Mode S transponder without ADS-B position data. Networks with many receivers can locate such aircraft via multilateration (MLAT) – provided at least four stations hear the signal.
• Only squawk and altitude, no hex code: A pure Mode A/C transponder. Because Mode A/C replies contain no address, they can hardly be assigned to a specific aircraft and generally cannot be shown on maps.
• Nothing at all: Transponder off, no transponder on board or outside your reception range. Common causes are collected in the article Why an aircraft doesn’t appear on Flightradar.

Transponders and collision avoidance

Transponders aren’t only there for air traffic control. The onboard collision warning system TCAS (or ACAS) itself interrogates the transponders of nearby aircraft and calculates distance, altitude and closure from the replies. When two aircraft with TCAS II meet, the systems coordinate their avoidance recommendations with each other over the Mode S data channel so that both don’t evade in the same direction. The role that conflicting instructions from TCAS and the controller played in one of the worst accidents in German airspace is described in the article on the midair collision at Überlingen in 2002.

The hex code as a tool for spotters

For spotters, the 24-bit address is the most useful legacy of Mode S. It generally stays the same for an aircraft as long as it is registered in the same state. If the registration changes through a move to another country, the aircraft usually also gets a new address from the new state’s block. With the hex code you can therefore search for a specific airframe, match older sightings or find an aircraft whose callsign you don’t know. How this works in practice is shown in the guide Tracking an aircraft by registration or hex code; how registrations themselves are built is explained in Understanding aircraft registrations.

A note for context: in the US, the FAA’s Privacy ICAO Address program makes it possible for certain aircraft to use a temporary address that is not publicly assigned. An unknown hex code there is therefore not automatically an error. Military aircraft also sometimes don’t transmit at all or with restricted data – more under Tracking military aircraft online.

What to keep in mind

• Don’t mistake the squawk for an identifier. The same code shows up on the same day on completely different flights. For identity, hex code and registration count.
• Read emergency codes with caution. A briefly flashing 7500 is often an input error while changing codes. An emergency is confirmed only when it is displayed for longer or air traffic control communicates it.
• Put approach altitudes in perspective. Pressure altitude and true altitude differ depending on air pressure. On approach, the time remaining to landing matters more than the exact figure.
• Keep the legal side in mind. Receiving ADS-B is unproblematic in many countries, but passing on data and radio traffic has rules of its own. You can find overviews under Receiving and sharing ADS-B data and Listening to aviation radio: legal situation.

Conclusion

Mode A, Mode C and Mode S are three stages of development of the same idea: the aircraft answers the radar and reveals more than a mere echo. Mode A brought the identity, Mode C the altitude, Mode S the unique address and the data channel. The Mode S squitter eventually became ADS-B – and with it the technology that shows you at the spot which aircraft is turning onto final approach. Anyone who knows how it all fits together also understands why some aircraft are missing on the map or show up only via MLAT.