Transponder modes A, C and S explained: what an aircraft transmits and why it matters for tracking

You are standing at the fence, the live map shows the approach, and next to it altitude, speed, registration and squawk. None of this data comes from a camera or a flight plan, but from an inconspicuous box in the avionics bay: the transponder. If you understand what Mode A, Mode C and Mode S transmit, you also understand why some aircraft appear perfectly on the flight tracker, others flit across the screen without a position, and still others never show up at all. This guide explains the three modes, their extra functions and what all of it means for you as a spotter.

Transponders in one sentence

A transponder is an onboard radio that automatically replies to interrogations from secondary radar on the ground – and in its modern form also transmits unprompted. The interrogation arrives on 1030 MHz, and the reply goes back on 1090 MHz. What is in that reply depends on the mode: an identification code for Mode A, the altitude for Mode C, and for Mode S a unique aircraft address plus a whole range of further data.

The difference from classic primary radar is important. That only measures an echo reflected by the aircraft’s metal skin – it knows something is there, but not who or at what altitude. Secondary radar, on the other hand, “asks” the aircraft, and the transponder supplies the answer. The ground station still determines bearing and distance itself; identity and altitude come from the reply.

For you as a spotter, the second value is the decisive one: everything transponders transmit on 1090 MHz can be heard by anyone with suitable hardware. This is exactly what flight tracker services and private receivers are built on.

Mode A: the squawk

Mode A is the oldest civil mode. In response to a Mode A interrogation, the transponder replies with a four-digit code, the squawk. Each digit only ranges from 0 to 7 because the code is octal – which gives 4,096 combinations from 0000 to 7777. You will never see an 8 or 9 as a squawk on any radar map.

The code is assigned by air traffic control, usually along with the route clearance before departure. On the controller’s radar screen, the system links the squawk to the flight plan data, and a dot becomes a labeled flight. That has two consequences:

• The squawk identifies the flight, not the aircraft. The same aircraft gets a different code on its next rotation, and the same code can be assigned to a different flight elsewhere at the same time.
• Codes are scarce. With just over 4,000 possibilities and dense traffic in Europe, they are not enough for unambiguous assignment across large areas. That was one of the reasons for Mode S.

Some codes have a fixed meaning that is the same everywhere: 7700 stands for a general emergency, 7600 for radio failure, 7500 for a hijacking. In Europe, aircraft flying VFR without an assigned code usually set 7000, in the US 1200. What is behind the emergency codes and how to interpret them on the flight tracker is explained in the article Squawk codes 7500, 7600 and 7700.

Mode A also includes the ident function: when the pilot presses the ident button at the controller’s request, the transponder appends an additional pulse to its reply for a few seconds. The target then flashes on the controller’s screen – a simple way to identify a specific aircraft beyond doubt.

Mode C: the altitude

Mode C adds the pressure altitude to Mode A. In response to a Mode C interrogation, the transponder replies with the flight altitude in 100-foot steps. It is measured against the standard air pressure of 1013.25 hectopascals – regardless of what the pilot has currently set on the altimeter. That way all controllers and all aircraft see the same reference, and air traffic control converts to the actual altitude above sea level itself when needed.

In practice, Mode A and Mode C always run together: the ground station interrogates both modes alternately and thus receives squawk and altitude. That is why people often speak of a Mode A/C transponder for short. On a flight tracker, the altitude is usually shown as a flight level or in feet; for very low values on the ground, it often shows as “0” or not at all.

The limits of Mode A/C

As simple as the method is, it quickly hits its limits. Every ground station in range receives replies to interrogations made by a completely different station. When two aircraft fly close together, their replies overlap and become unreadable – experts call this garbling. And because Mode A/C has no unique address, all transponders in the beam always reply, whether they are meant or not. In dense airspace like over Central Europe, this is a real problem.

Mode S: ask selectively, answer uniquely

Mode S stands for “Selective” – and that is exactly the core of it. Every Mode S transponder has a fixed 24-bit address that identifies the aircraft uniquely worldwide. The ground station can thus address a specific aircraft selectively instead of asking all of them at once. This reduces the radio load on 1090 MHz and eliminates many overlaps.

Mode S transponders remain backward compatible: they still reply to Mode A and Mode C interrogations. A modern airliner therefore transmits both the squawk and the Mode S data.

The hex code: your most important spotter value

The 24-bit address is usually written as a six-character hexadecimal value – hence the name hex code or ICAO address. The state of registry assigns it to the aircraft, and it normally stays with the aircraft as long as it is registered there. Each state has its own address block for this. How the blocks are divided and how to infer the country of origin from an address is shown in the article ICAO 24-bit address (hex code).

For flight tracking, the hex code is the key: it links an anonymous radio signal to an entry in the database – registration, type, operator. The callsign, by contrast, changes with every flight, and the squawk does anyway. If you want to keep an eye on a specific aircraft over several days, you therefore search by registration or hex code, not by flight number. The difference between callsign and flight number is explained in Callsign and flight number, and the practical search is described in Tracking an aircraft by registration.

Elementary and Enhanced Surveillance

Mode S can do far more than address and altitude. In Europe, two levels of equipment are distinguished:

Feature	Mode A/C	Mode S ELS	Mode S EHS
Identification	Squawk (4 digits)	Squawk plus fixed 24-bit address	same as ELS
Altitude	100-foot steps	down to 25-foot steps	same as ELS
Callsign	no	yes, automatic	yes
Ground/air status	no	yes	yes
Selected target altitude	no	no	yes
Heading, speed, climb rate	no	no	yes, as onboard parameters
Selective interrogation	no	yes	yes

Elementary Surveillance (ELS) provides the basic functions: address, callsign, finer altitude, status on the ground or in the air and a report on the transponder’s capabilities. Enhanced Surveillance (EHS) adds so-called Downlinked Aircraft Parameters, meaning data straight from the onboard electronics: the selected altitude in the autopilot, magnetic heading, ground speed, indicated airspeed and Mach number, roll angle and vertical speed. The selected altitude is especially valuable for the controller, because he can see immediately whether the crew has set the clearance correctly.

Under EU law, aircraft operating under instrument flight rules in general air traffic must in principle carry at least Mode S ELS. For larger or faster fixed-wing aircraft – over 5.7 tonnes maximum takeoff mass or with a maximum true cruising airspeed above 250 knots – EHS and ADS-B Out are added. There are exceptions, for example for older aircraft, and special rules apply to state aircraft such as military planes.

Squitter and ADS-B: when the transponder transmits unprompted

A Mode S transponder does not just wait for questions. It also regularly sends short messages on its own, so-called squitters. The simple squitter essentially contains the address and tells ground stations and the collision avoidance systems of other aircraft: “Here I am.”

The extended form is called extended squitter – and it is the basis of ADS-B in commercial aviation. Here the transponder packs data into the longer Mode S messages that it does not measure itself: the position from satellite navigation, the ground speed, the climb or descent rate and the callsign. These messages go out several times per second without any interrogation. ADS-B stands for Automatic Dependent Surveillance – Broadcast: automatic, dependent on the onboard navigation, broadcast to everyone.

Besides the 1090 MHz method, the US has a second ADS-B variant on 978 MHz, the UAT. It is intended for aircraft that stay below 18,000 feet and occurs mainly in general aviation. You will find a separate overview under UAT on 978 MHz. In Europe, UAT plays practically no role.

What this means for flight tracker services

Now it gets exciting for you. Whether an aircraft appears on the map of Flightradar24, FlightAware or ADS-B Exchange (a direct comparison of the first two is provided by Flightradar24 vs. FlightAware) depends directly on what its transponder transmits. Roughly, there are four cases:

Equipment	What receivers hear	What you see on the flight tracker
Mode S with ADS-B	Hex code, callsign, altitude, GPS position, speed	precise position almost in real time, full data set
Mode S without ADS-B	Hex code, altitude, sometimes callsign	Position only via MLAT, if enough receivers hear the signal
Mode A/C only	Squawk and altitude, no address	usually nothing at all, since no unambiguous matching is possible
Transponder off or no transponder	nothing on 1090 MHz	invisible, at most estimated from flight plan data

The services solve the middle case with multilateration (MLAT): several receivers with a precise time base hear the same Mode S signal, and the position is calculated from the tiny differences in arrival times. This only works when enough stations are in range, and usually better at higher altitude than on final approach. How MLAT works in detail is covered in the article MLAT/multilateration explained.

Mode A/C signals can hardly be used this way because they lack the unique address. A receiver does hear “squawk 7000, 3,500 feet”, but cannot match that reply to a specific aircraft when several are nearby. That is why many older light aircraft are missing from the common maps. Glider pilots often use entirely different systems such as FLARM, which you can receive with your own equipment – more on that under FLARM and Open Glider Network.

Further reasons why an aircraft is missing – operator block lists, ADS-B switched off on military aircraft, or simply no receivers nearby – are summarized in Why an aircraft doesn’t show up on the flight tracker. If you specifically watch military traffic, you will find tips under Tracking military aircraft online.

Transponders and collision avoidance

Transponders do not just serve air traffic control on the ground. The onboard collision avoidance system TCAS (internationally ACAS) itself interrogates the transponders of other aircraft nearby and calculates from that whether a collision threatens. The full version TCAS II, which gives resolution advisories up or down, requires Mode S – the systems of two aircraft coordinate their maneuvers with each other over this link. With EHS, active resolution advisories are also reported to the ground.

How important the interplay between controllers and TCAS is was shown by the Überlingen collision, which permanently changed how resolution advisories are handled. You can read the background in the article The 2002 Überlingen mid-air collision. More on the role of air traffic control in Germany can be found under DFS Deutsche Flugsicherung (German air traffic control).

Military modes

Besides the civil modes, there are military identification friend or foe (IFF) modes. Mode 1 and Mode 2 transmit military mission and unit codes respectively, Mode 3/A is identical to the civil Mode A, and Mode 4 and the newer Mode 5 work encrypted. Mode 5 can also contain a secured position report. Civil receivers cannot read the encrypted modes. When you see a military aircraft on the flight tracker, the data comes from its Mode S or ADS-B operation, which many state aircraft use in normal air traffic but are not required to use.

Listening to the transponders yourself

The nice thing about 1090 MHz: you don’t need a subscription to listen in. With an SDR stick, a small antenna and a Raspberry Pi or PC, you receive the signals of all aircraft in range and see them on your own map with software like dump1090 or tar1090 – the setup is described in Installing readsb and tar1090, and suitable hardware is compared in SDR sticks for ADS-B. What applies legally when receiving and sharing the data is covered in Receiving ADS-B data: the legal situation. Along the way you quickly learn what the theory looks like in practice:

How the big services differ and which one suits you is compared in the article Flight tracker apps compared. The unfiltered community approach is explained in ADS-B Exchange explained, and an introduction to live tracking at the spot in Flight tracking with ADS-B for spotters.

Common misconceptions

• “The squawk is the aircraft’s identifier.” No, it only applies to the current flight. The permanent identifier is the hex code.
• “Transponders transmit GPS data.” Only if ADS-B is active and connected to a position source. Mode A, C and simple Mode S do not transmit a position.
• “The altitude on the flight tracker is the height above ground.” What is reported is the pressure altitude against standard air pressure. Depending on the weather, it differs noticeably from the actual altitude above sea level, and it has even less to do with the height above ground.
• “If it’s not on Flightradar, it’s not flying.” Many aircraft don’t transmit ADS-B or aren’t displayed. An empty screen doesn’t mean an empty sky.

Conclusion

Modes A, C and S are not competing systems but stages of development of the same idea: the aircraft answers the question that a radar echo leaves open. Mode A delivers the squawk, Mode C the altitude, Mode S the unique address and, with ADS-B, finally the position from satellite navigation. For flight tracker services – and thus for you at the spot – it is exactly this last step that makes the difference between a precise dot on the map, an MLAT estimate and an aircraft that only your eye sees.