Transponder and ADS-B Out

The transponder is the aircraft's "ID card plus broadcasting health chart": ground secondary radar asks, it answers; another aircraft's TCAS asks, it answers — what your aircraft is on the controller's screen, and on a neighbour's TCAS, is entirely up to this box. In the Mode S era it learned two new tricks: reporting itself every second without being asked (squitter / ADS-B Out), and acting as the "message office" for TCAS (all collision-avoidance coordination messages go through it). This article covers how the ID card is written, what is in the broadcast, the cool facts hidden in the panel logic, and why it must "fall silent" on the day QNH exceeds 1050 hPa.

1. Three modes and an ID card that never duplicates

Per FCOM DSC-34-20-10-10:

The aircraft has two ATC transponders (XPDR) which are controlled by a control panel (ATC/TCAS) on the center pedestal. Only the selected XPDR operates.

One active, one cold standby — unlike VOR/DME with both online, only one transponder speaks at a time (else the same aircraft gives two replies and the radar sees a formation). It answers two examiners. Per FCOM DSC-34-20-10-10:

The XPDR automatically responds to requests: ‐ From the ATC, to ensure effective air traffic surveillance ‐ From another aircraft that have a TCAS, to ensure that traffic alerts are triggered.

Frequency and mode. Per AMM 34-52-00:

The ATC system operates with two frequencies: - interrogation: 1030 MHz - reply: 1090 MHz

Mode A answers a four-digit identity code, Mode C answers barometric altitude, and Mode S's revolution is "calling by name." Per AMM 34-52-00:

Each (ATC Mode S-equipped) aircraft has its own Mode S address. This address (24-bit) is included in all Mode S transmissions, so that every interrogation can be directed to a specific aircraft, preventing multiple replies.

2²⁴ ≈ sixteen million codes, one per aircraft worldwide (painted on the airframe, burned into the transponder). Legacy ATCRBS "broadcast roll-call" caused a many-thousand-reply howl in busy airspace; Mode S turned it into a one-to-one private conversation. One more anti-fooling design. Per AMM 34-52-00:

If the interrogation is sent by the side lobe of the radar, a characteristic signal is sent allowing the transponder to disregard the interrogation.

Replying to a side-lobe interrogation would put your bearing in the wrong place; better not to answer.

2. EHS — you think nobody knows, but it is all in the broadcast

The transponder has enhanced surveillance (EHS) above ELS, and the transmit list is worth taking whole, because it hides the fact of a "transparent cockpit." Per FCOM DSC-34-20-10-10:

The XPDR is capable of elementary surveillance (ELS) and enhanced surveillance (EHS). It transmits the following data to the ATC center: ‐ The aircraft 24 bit address, the aircraft altitude, the flight number, the RA report ‐ The indicated airspeed, the Mach number, and the barometric vertical speed that are all supplied by the ADRs ‐ The magnetic heading, the roll angle, the ground speed, the track angle, the track angle rate, and the inertial vertical speed, that are all supplied by the IRs ‐ The selected altitude and barometric reference settings supplied by the FCUs.

The last line is the key of keys: the altitude and QNH set in the FCU window are visible to the controller. The European controller's "confirm selected altitude" is not telepathy but EHS; a wrong altitude window is a public event in the radar room. "RA report" is the downlink of a TCAS resolution advisory (the ground knows an RA is commanding you). The source side echoes the "bus fatalism" of the overview article: XPDR 1 eats ADR 1, XPDR 2 eats ADR 2, switching to ADR 3 with the AIR DATA rotary on failure. Per AMM 34-52-00:

In normal configuration, each ATC receives the altitude information from its corresponding ADIRU 1 or 2. In case of ADIRU 1/2 failure, the pilot can select the altitude information from the ADIRU 3. This selection is through the AIR DATA selector switch installed on the center pedestal.

This chain is the basis for the "ATC/XPDR to SYS 1/SYS 2" action in the dual-ADR procedure: the altitude source died, so switch to the transponder that still has a live one.

3. Squitter and the diversity antenna — an unrequested introduction

Per AMM 34-52-00:

Approximately every second, a Mode S transponder makes a transmission known as squitter. These transmissions alternate between the top and bottom Mode S antennas. Squitter is coded with the aircraft Mode S address and equipment capabilities.

The squitter is an "I am here" heartbeat — ground stations and neighbouring TCAS hear it to discover you have entered the surveillance ring (the start of TCAS target acquisition). The point of top and bottom antennas. Per AMM 34-52-00:

The diversity antenna allows selection of signal receptions from either the top or the bottom antenna based on the characteristics of the received interrogation signals.

In a turn the fuselage shadows one face; a TCAS coordination message must never be lost to a bank angle — one top, one bottom, use whichever hears best — a hard prerequisite of TCAS II compatibility. The "message office" relationship with TCAS. Per AMM 34-52-00:

The communication between two TCAS-equipped aircraft occurs from the TCAS antenna of one aircraft to the Mode S transponder antenna of the other aircraft. Aboard the TCAS-equipped aircraft, data received by the Mode S transponder is passed to a TCAS computer, and the TCAS computer determines the content of reply messages.

So the TCAS article shows: dual transponder failure = TCAS dead (TCAS is the first line of the NAV ATC/XPDR 1+2 FAULT INOP list) — the collision-avoidance system's mouth and ears are both the transponder's. The Mode S reply also carries the FMGEC-injected flight number — a wrong flight number on the pre-flight INIT page puts the wrong callsign on the controller's screen, the origin of a cockpit-preparation cross-check.

4. Panel logic — three cool facts

Panel elements: XPDR 1/2 selection, a four-digit keypad + CLR, IDENT, ALT RPTG ON/OFF, mode selection (STBY/AUTO/ON) + the TCAS part. Three cool facts.

(1) Not muted on the ground, but only answers by name. Per FCOM DSC-34-20-10-20:

In flight: The selected XPDR operates. On ground: The selected XPDR only operates in mode S (Selective aircraft interrogation mode).

On taxi it does not answer the ground radar's Mode A/C broadcast interrogations (to avoid a sea of replies on the airport surface), but it does answer Mode S by name and keeps squittering — the surface-movement radar and A-SMGCS "see" you by this, which is why the transponder is set early before take-off: not "useful only at take-off."

(2) The old code stands guard during a change. Per FCOM DSC-34-20-10-20:

As long as the four figures of the new code are not entirely written, the previous code remains.

There is no "half-entered odd code" going up — the design blocks the incomplete work for you.

(3) The control box dies, the transponder keeps vigil. Per AMM 34-52-00:

If there is a control unit failure, the transponder continues to transmit the last selected ATC code. No new code selection is possible.

A dead panel does not mean disappearing from the radar — you are still on the controller's screen, you just cannot change the code (report by radio if a change is needed). The ALT RPTG switch alone governs "report altitude or not." Per FCOM DSC-34-20-10-20:

The XPDR sends barometric standard altitude data.

Note it sends standard barometric altitude (1013 datum) — the controller converts to local QNH, so a wrong QNH does not affect the altitude the radar sees (only the flight level you fly). The ALT RPTG OFF knock-on is in the panel note: ECAM shows TCAS STBY — no altitude report, and TCAS cannot play either (a neighbour's TCAS uses your reported altitude to compute an RA).

5. ADS-B Out — from "reply when named" to "broadcast unprompted"

Per FCOM DSC-34-20-10-10:

With the ADS-B OUT capability, the Mode S transponders automatically and continuously transmit surveillance data, without preliminary interrogation, to: ‐ The ATC ground station ‐ Aircraft capable of ADS-B IN function.

The broadcast list (read against the EHS list — the core ADS-B increment is GPS position and integrity). Per FCOM DSC-34-20-10-10:

‐ The latitude and longitude, the Horizontal Integrity Limit (HIL), the difference between barometric altitude and geometric altitude, the ground speed, all supplied by GPS ‐ The barometric altitude supplied by ADIRS ‐ The track, the vertical speed, all supplied by the IRs ‐ The flight number (registered on the ATC flight plan and entered in the FMS during cockpit preparation) supplied by the FMS ‐ The emergency situation indicator ‐ The selected altitude and heading, the barometric pressure setting (QNH/QFE) from FCU.

HIL is the radius of the "integrity umbrella" of the GPS article — in the ADS-B era controllers dare to shrink separation because each aircraft self-reports "my position error is inside this circle." Dual GPS failure → ADS-B Out considered inoperative (a MEL knock-on) follows naturally: without GPS, the broadcast's core content is empty words. The most common operational trap. Per FCOM PRO-NOR-SUP-SURV:

The flight crew cannot switch off the ADS-B automatic data transmission independently of the XPDR.

Therefore, do not switch off the ATC transponder or the altitude reporting, in order to maintain TCAS operations or Air Traffic Controller radar surveillance.

When a controller calls "STOP ADS-B TRANSMISSION" (often because your broadcast data is judged invalid) — there is no such switch; the correct action is to apply the local AIP alternate procedure, not to turn the transponder into "radar-invisible + TCAS self-blinded."

6. QNH above 1050 hPa — the transponder's "high-pressure no-fly line"

A polar anticyclone can push QNH above 1050 hPa. Systems have different pressure tolerances. Per FCOM PRO-NOR-SUP-MISC-B:

The FMS, FCU, ISIS instrument, PFD, and CPCS are capable of operating at QNH/QFE up to 1 100 hPa, or 32.48 inHg.

In addition, the ATC/TCAS operates only up to 1 050 hPa (i.e. as long as the aircraft altitude remains above -1 000 ft standard pressure altitude).

Mechanism: Mode C/S reports standard pressure altitude, whose coding floor is about −1000 ft; at an airport with QNH > 1050, the field elevation converted to standard pressure altitude falls below −1000 ft — the transponder cannot report it, and any report would be wrong. So the supplementary procedure is: before take-off ALT RPTG OFF + notify ATC "altitude reporting unavailable" (with ALT RPTG OFF and TCAS STBY memos on ECAM); climbing through 1000 ft, time permitting, ALT RPTG ON. Per FCOM PRO-NOR-SUP-MISC-B:

As a result, the TCAS automatically reverts to its previous setting

Approach is the reverse: notify before final, then ALT RPTG OFF.

7. The RVSM pairing fact and the memo family

A single transponder failure switched to the other (SYS 1↔2) is an awareness-level action, but RVSM airspace adds one line. Per FCOM PRO-ABN-NAV:

In case of flight in RVSM airspace, select AP1 if SYS1 is used and AP2 if SYS2 is used.

The reason: the XPDR reports altitude from its own-side ADR, and the AP holds altitude on its own-side reference too — make "the you the controller sees" and "the you the AP flies" share a source, so the RVSM 300 m separation lives up to the arithmetic. Memo family: TCAS STBY (amber phase 6 / green phases 2, 7) appears in four cases — manual STBY, dual ATC or dual RA failure, ALT RPTG OFF, triple ADR failure; the panel amber FAULT light = the selected transponder is sick. Per FCOM DSC-34-20-10-20:

TCAS STBY : This memo appears in amber in flight phase 6 when the ATC/XPDR used by the TCAS is faulty.

Key numbers

Self-test

[!note]- Q1. What does each of Mode A/C/S answer, and how do the 24-bit address and the four-digit code relate (do they clash)? A = identity code, C = barometric altitude, S = adds a 24-bit address. They do not clash: the four-digit squawk is what the controller assigns; the 24-bit address is a fixed hardware identity for directed interrogation.

[!note]- Q2. Which three sources feed the EHS list, and how does the controller know your selected FL? ADR (IAS/Mach/baro VS), IR (heading/roll/GS/track/track rate/inertial VS), FCU (selected altitude + baro setting). The FCU selected altitude is transmitted directly.

[!note]- Q3. What is the squitter's content and use, and why must top and bottom antennas alternate? Content: Mode S address + capabilities; use: lets ground/TCAS discover you. Alternating covers the fuselage-shadowed face in a turn so coordination is never lost.

[!note]- Q4. During pushback/taxi, whom does the transponder answer and not answer, and what does that mean for surface surveillance? It answers Mode S selectively and squitters, but not Mode A/C broadcast — so surface-movement radar and A-SMGCS still see it, which is why it is set early.

[!note]- Q5. ATC asks to stop ADS-B — the right and wrong actions, and why? Right: apply the local AIP alternate procedure. Wrong: switch off the transponder or altitude reporting — there is no independent ADS-B switch, and turning it off self-blinds TCAS and radar.

[!note]- Q6. Departing at QNH 1053 hPa — what do you do to the transponder, which two memos on ECAM, and after 1000 ft? ALT RPTG OFF + notify ATC before take-off; ECAM shows ALT RPTG OFF and TCAS STBY. Climbing through 1000 ft, time permitting, ALT RPTG ON (TCAS reverts automatically).

Key takeaways

References

• FCOM DSC-34-20-10-10/-20 — dual transponders, response objects, EHS list, ADS-B Out list, panel logic, ALT RPTG, ground Mode S, memo.
• FCOM PRO-NOR-SUP-SURV — ADS-B cannot be switched off independently + AIP alternate.
• FCOM PRO-NOR-SUP-MISC-B — QNH > 1050 hPa limit and procedure.
• FCOM PRO-ABN-NAV — RVSM XPDR-AP pairing.
• AMM 34-52-00 — 24-bit address, frequency, squitter, diversity, side-lobe, TCAS relay, hold-last-code, ADR 3 switching.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.