Air traffic surveillance is not based only on ground radar reflecting energy from the metal skin of an aircraft. Modern airliners also carry transponders that listen for coded interrogations and transmit structured replies containing identity, altitude and other information. Mode S — short for Mode Select — is the most capable form of conventional secondary surveillance transponder used by transport aircraft. The FAA describes Mode S as a cooperative secondary-radar system that identifies and tracks transponder-equipped aircraft in terminal and en-route airspace and provides aircraft position and altitude information to surveillance systems. [1]
The most important difference between Mode S and older Mode A/C surveillance is selectivity. A Mode S-equipped aircraft is assigned a unique 24-bit aircraft address, allowing ground stations and other airborne systems to interrogate or exchange data with a specific aircraft rather than requiring every transponder in range to answer every interrogation. FAA AC 20-151C identifies the Mode S transponder as a required component of TCAS II and explains that the unique address also supports air-to-air data exchange between collision-avoidance systems. [2]
Why primary radar is not enough
Primary surveillance radar transmits radio energy and listens for reflections. It can detect an object without cooperation from the aircraft, but the reflection alone does not automatically tell ATC the aircraft’s identity or pressure altitude.
Secondary surveillance adds cooperative data. A ground interrogator transmits a coded request, and the aircraft transponder sends a stronger, structured response. This improves identification and altitude reporting and reduces dependence on interpreting a weak reflected echo.
Mode A: the four-digit squawk
Mode A transponders reply with the four-digit code selected by the crew. Because each digit ranges from 0 to 7, there are 4,096 possible Mode A codes. ATC assigns a code so radar automation can associate the aircraft with its flight plan.
Mode A answers the question “which code is this target using?” but does not inherently provide altitude.
Mode C adds pressure altitude
Mode C supplements the Mode A identity code with encoded pressure altitude from the aircraft air-data system. This gives ATC a vertical dimension for separation and conflict detection.
Pressure altitude is transmitted relative to the standard pressure reference rather than the local altimeter setting, allowing surveillance computers to apply the appropriate corrections consistently.
Why Mode A/C becomes inefficient in dense traffic
Traditional ATCRBS interrogations can cause many aircraft in the beam to reply. In busy airspace, overlapping replies — known as garble — can make decoding more difficult. Unnecessary interrogations also add radio-frequency congestion.
Mode S was designed to improve this by selectively addressing individual aircraft once they have been acquired.
The unique 24-bit address
Every Mode S aircraft is assigned a discrete 24-bit address by its state of registration. FAA AC 20-151C explains that this ICAO address uniquely identifies the aircraft and incorporates allocation associated with the country of registration. [3]
Twenty-four bits provide more than 16 million possible binary combinations, though portions of the address space are allocated systematically to states and uses.
Why the address must match the aircraft
If a transponder is programmed with the wrong 24-bit address, surveillance and collision-avoidance systems can associate data with the wrong identity. FAA guidance has documented cases in which duplicate addresses occurred after registration changes and instructs operators to verify one-to-one correspondence. [3]
Programming the address is therefore an airworthiness and maintenance task, not just an administrative detail.
Selective interrogation
After a Mode S radar identifies an aircraft, it can send an interrogation addressed specifically to that 24-bit code. Other Mode S aircraft recognise that the message is not for them and remain silent.
This reduces unnecessary replies and improves surveillance performance in congested airspace. It is the core meaning of “Mode Select”.
All-call acquisition
A ground sensor still has to discover new aircraft entering its coverage. Mode S therefore includes acquisition processes in which aircraft respond to appropriate all-call interrogations. Once the ground system learns the discrete address, it can transition to selective surveillance.
The exact interrogation formats and lockout mechanisms are defined in Mode S technical standards rather than normal pilot procedures.
Mode S is compatible with older surveillance
Mode S was designed to coexist with ATCRBS Mode A/C equipment. A Mode S transponder can perform the conventional functions required by existing secondary-radar systems while adding selective and digital capabilities.
This allowed airspace infrastructure to transition gradually rather than requiring every radar and aircraft to change simultaneously. FAA AC 20-151C explicitly describes Mode S as compatible with ATCRBS surveillance. [2]
Aircraft identification
Mode S can transmit an aircraft identification associated with the flight, typically derived from the callsign or flight identification entered into the flight-management or transponder system. ATC automation can use this to correlate surveillance with the filed flight plan.
The FAA flight-plan equipment codes distinguish Mode S installations that provide aircraft identification, pressure altitude, enhanced surveillance and extended squitter. [4]
Pressure altitude through Mode S
Mode S transponders can report pressure altitude just as Mode C systems do, but within the Mode S digital message structure. Ground surveillance then combines identity, altitude and measured bearing/range to create the controller’s track.
Not every possible Mode S equipment code includes altitude reporting, which is why flight-plan designators distinguish capabilities.
Enhanced surveillance
Some Mode S installations can downlink additional aircraft-derived parameters, known as enhanced surveillance. Depending on regional implementation and equipment, these can include selected altitude, heading, speed or other data from onboard avionics.
FAA flight-plan guidance defines enhanced surveillance as the ability to downlink aircraft-derived data via the Mode S transponder. [5]
Why ATC values selected-altitude data
If the surveillance system receives the altitude selected in the aircraft’s mode-control panel or flight-guidance system, automation can compare that value with the controller’s clearance and detect some potential misunderstandings earlier.
The exact use of enhanced data depends on the air navigation service provider and avionics implementation.
TCAS requires Mode S
TCAS II does not merely detect replies from nearby aircraft. It also needs a data link to coordinate resolution advisories between two TCAS-equipped aircraft. FAA AC 20-151C states that a Mode S transponder is required for TCAS II operation. [2]
The Mode S link allows the two collision-avoidance systems to agree complementary manoeuvres rather than both independently commanding the same vertical response.
Coordinated resolution advisories
If two TCAS II aircraft predict a collision threat, one system can coordinate with the other through Mode S data exchange. One aircraft may receive a climb command while the other receives a descend command, depending on the geometry and system logic.
This coordination is one of the most safety-critical air-to-air uses of Mode S.
TCAS surveillance itself
TCAS interrogates nearby transponders and analyses their replies to estimate range, bearing and altitude relationships. Mode S improves the process because individual aircraft can be selectively addressed and data can be exchanged digitally.
Hybrid surveillance on newer TCAS installations can also use ADS-B information to reduce active interrogations while retaining the certified collision-avoidance function.
Mode S and ADS-B are not the same thing
ADS-B Out broadcasts the aircraft’s position and other data periodically without waiting for a radar interrogation. Mode S surveillance traditionally involves interrogations and replies.
However, the two technologies overlap because 1090 MHz Extended Squitter ADS-B is transmitted through a Mode S transponder architecture. FAA ADS-B guidance identifies 1090ES as a Mode S transponder with Extended Squitter. [6]
Extended Squitter
A “squitter” is an unsolicited Mode S transmission. Extended Squitter provides a longer message format capable of carrying ADS-B information such as position, velocity and identity.
Unlike a normal selective radar reply, the aircraft broadcasts these messages automatically so compatible ground stations and aircraft can receive them.
1090 MHz
Mode S and 1090ES operate around 1090 MHz for aircraft replies and broadcasts, with interrogations transmitted on 1030 MHz. This frequency pair is also used by conventional secondary surveillance and TCAS.
Because many systems share the spectrum, interrogation management and selective addressing are important to limiting channel congestion.
Why high-altitude U.S. aircraft use 1090ES
The FAA requires aircraft operating at and above Flight Level 180 in ADS-B-required U.S. airspace to use 1090ES rather than the 978 MHz Universal Access Transceiver option. [6]
This aligns high-altitude and international transport operations with the globally used Mode S/1090 MHz surveillance environment.
Monopulse Mode S
Mode S ground radars commonly use monopulse techniques to estimate bearing accurately from a single reply. This reduces dependence on multiple overlapping beam responses and improves target position measurement.
The FAA identifies monopulse detection as part of Mode S surveillance capability. [1]
Why the transponder must remain on while taxiing
Modern surface-surveillance systems can use Mode S and ADS-B data to identify aircraft on airport movement areas. FAA AIM guidance therefore instructs pilots to operate transponder and ADS-B equipment appropriately on the ground as well as in flight. [7]
This helps controllers and automated systems distinguish one aircraft from another on congested taxiways.
The IDENT function
When ATC asks a pilot to “IDENT”, pressing the transponder IDENT control causes a special indication in the surveillance reply for a short period. The controller’s display highlights that target, making identification easier.
IDENT does not change the aircraft’s 24-bit Mode S address or squawk code permanently. It is a temporary attention flag.
Emergency codes still matter
Mode S aircraft continue to support conventional emergency squawk codes such as 7500, 7600 and 7700. The four-digit code remains operationally useful even though the aircraft also has a unique Mode S address.
The discrete address identifies the aircraft; the squawk can convey an ATC-assigned code or emergency state. They serve different purposes.
Why a 24-bit address is not the flight number
The aircraft address belongs to the airframe registration allocation and generally remains tied to that registration state assignment. The flight identification can change every sector.
An airliner can therefore fly under different airline flight numbers while retaining the same Mode S address. Ground systems correlate the two data elements.
Transponder altitude source
The transponder receives pressure-altitude data from the aircraft’s air-data system or an encoding source. On modern integrated avionics, digital buses carry the data.
A transponder can be functioning as a radio while still receiving incorrect altitude from another system, which is why surveillance discrepancies can require troubleshooting beyond the transponder itself.
Diversity antennas
Large transport aircraft may use top and bottom transponder antennas so surveillance and TCAS coverage remains reliable for both ground and airborne interrogators. Antenna switching or diversity logic selects the appropriate path.
This is particularly important because TCAS threats can be above or below, while ground Mode S stations illuminate the aircraft from below.
Why antenna cable condition matters
At 1030/1090 MHz, cable losses, connector condition and antenna bonding affect transmitted power and receiver sensitivity. Maintenance therefore tests the complete installed system rather than assuming the transponder box alone determines performance.
A degraded antenna can reduce surveillance range even though cockpit self-test appears normal.
Mode S failure and dispatch
A failed Mode S transponder can affect ATC surveillance, TCAS capability and ADS-B Out where the transponder provides 1090ES. Whether dispatch remains allowed depends on airspace, aircraft equipment redundancy and the approved Minimum Equipment List.
Many airliners carry two transponders so one can remain available after a unit failure.
Two transponders do not use two identities
Dual installations are programmed with the same correct aircraft address because both belong to the same airframe. The crew selects which transponder is active according to procedures.
Improper simultaneous transmissions or incorrect programming can create surveillance problems, so installation logic manages the active unit carefully.
Mode S ground infrastructure remains widespread
The FAA reported 137 operational Mode S radar systems in the National Airspace System as of its September 2025 update, with additional support systems. [8]
ADS-B has changed surveillance architecture, but Mode S remains deeply integrated into ATC and airborne collision avoidance.
Why Mode S is still needed in an ADS-B world
ADS-B provides extremely useful broadcast position information, but TCAS II coordination, legacy radar compatibility and conventional transponder requirements still depend on Mode S functions.
The technologies have therefore converged rather than one simply replacing the other. A 1090ES transponder can serve Mode S radar, TCAS and ADS-B simultaneously.
A digital identity inside every reply
The key advance of Mode S is that surveillance became individually addressable. Instead of treating every transponder reply as an anonymous burst distinguished mainly by a four-digit squawk, Mode S gives the aircraft a unique digital identity and a structured data link.
That lets ground radar selectively interrogate one aircraft, allows ATC to receive richer surveillance information, gives TCAS-equipped aircraft a channel for coordinating collision-avoidance manoeuvres and provides the foundation for 1090ES ADS-B. The small transponder panel in the cockpit therefore sits at the centre of several different surveillance systems operating at once.
Verified Sources / References
- Federal Aviation Administration — Radar Surveillance Terminology. FAA description of Mode S surveillance, aircraft identification, altitude information and monopulse capability.
- FAA AC 20-151C — TCAS II and Associated Mode S Transponders. Active FAA airworthiness guidance covering Mode S and TCAS II integration.
- FAA AC 20-151B — Mode S Technical Background. FAA explanation of discrete aircraft addresses and TCAS air-to-air coordination.
- FAA Aeronautical Information Manual — Aircraft Surveillance Equipment Designators.
- FAA Flight Plan Appendix — Mode S and Enhanced Surveillance Capabilities.
- FAA ADS-B Installation. FAA explanation of 1090ES Mode S transponders and ADS-B equipment requirements.
- FAA AIM — Transponder and ADS-B Out Operation.
- FAA — The Mode S Team. Current FAA overview of Mode S ground infrastructure.
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Editorial Notice: This article was prepared using information considered reliable and publicly available at the time of publication. Every reasonable effort has been made to ensure accuracy; however, aviation requirements, technical standards and operational guidance may change as further information or revised regulation becomes available. This article is for general aviation education and reporting and is not a substitute for approved aircraft manuals, operator procedures, regulatory material or professional training. Cockpit King does not allege fault or responsibility against any person or organisation unless confirmed by an authoritative source. If you believe any material is inaccurate, misleading, improperly attributed or should be reviewed for amendment or removal, please contact us with the article title, the specific passage concerned and supporting evidence. We will assess legitimate requests promptly and, where appropriate, correct, clarify, update or remove the material.