Transponder Modes A, C, and S: Squawk Codes, ADS-B, TCAS, and the Airspace Rules
- Nathan Hodell
- Dec 17, 2025
- 13 min read
Updated: 5 days ago
Aircraft transponders are a critical link in modern air traffic control — they let radar identify aircraft, determine altitude, and maintain safe separation, and they're the foundation of the collision-avoidance systems that have made midair collisions rare. Most pilots use a transponder without fully understanding what each mode actually does, why the airspace rules are written the way they are, and how the transponder connects to TCAS and ADS-B. Understanding transponder modes turns a knob you twist before takeoff into a system you actually comprehend — which matters for using it correctly and for answering the questions examiners reliably ask.
This post covers transponder modes in practical depth: how a transponder works with radar, Modes A, C, and S explained, the squawk codes (including the ones beyond the emergencies), where transponders are required and where they aren't, ADS-B Out and In, how Mode S drives TCAS collision avoidance, the IDENT and phraseology, and the transponder-inoperative provisions.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What a Transponder Is and How Radar Uses It
A transponder ("transmitter-responder") is an onboard radio that responds to interrogations from radar systems, sending back information that helps controllers identify and track the aircraft.
Primary vs. secondary radar:
Primary radar: sends out a pulse and detects the reflection off the aircraft skin. It shows a target but no identity or altitude. Works without any aircraft equipment.
Secondary surveillance radar (SSR): interrogates the aircraft's transponder, which replies with coded data. This provides identity and altitude.
The transponder works with SECONDARY radar
Secondary radar is how ATC gets identity and altitude
Why secondary radar matters:
Primary radar alone shows blips with no identification
The transponder reply adds the squawk code and altitude
This lets ATC identify and separate aircraft efficiently
The modern ATC system relies on transponder replies
The interrogation-reply cycle:
The ground radar transmits an interrogation
The transponder receives it and replies on a different frequency
The reply contains the mode-dependent data
This happens continuously as the radar sweeps
What a transponder can provide (by mode):
Aircraft identification (squawk code)
Pressure altitude (Mode C and S)
Unique aircraft address and identification (Mode S)
The data depends on the mode
Mode A: Identity Only
Mode A is the most basic mode, providing identification only.
What Mode A transmits:
A four-digit octal squawk code (each digit 0-7, e.g., 1200, 4567)
No altitude information
Just the identity code
The octal detail:
The squawk code is octal (base 8) — each digit is 0 through 7
This gives 4,096 possible codes (8⁴)
That's why you never see an 8 or 9 in a squawk code
4,096 discrete codes available
How ATC uses Mode A:
Identify a specific aircraft on radar
Associate the target with a flight plan or call sign
A discrete code assigned to an aircraft tags its target
Basic identification
The limitations:
No altitude information
ATC must rely on pilot-reported altitude or primary radar
Insufficient for modern altitude-based separation
Mode A alone is rarely adequate in controlled airspace today
Mode C: Altitude Reporting
Mode C adds altitude to the identification, and it's the practical minimum for most controlled airspace.
What Mode C transmits:
The squawk code (like Mode A)
Pressure altitude in 100-foot increments
Identity plus altitude
The altitude encoder:
The altitude comes from an altitude encoder (a "blind encoder"), NOT the cockpit altimeter display
The encoder reports PRESSURE altitude (referenced to 29.92)
ATC's computer adjusts it to the local altimeter setting for display
This is why your Mode C altitude is correct to ATC even if your altimeter setting differs
Why pressure altitude:
The encoder always reports pressure altitude (29.92 reference)
ATC applies the local altimeter setting to display your actual altitude
Standardized regardless of local pressure
The system handles the correction
Why Mode C is important:
ATC sees altitude directly on the scope
Enables vertical separation between aircraft
Supports traffic advisories and safety alerts (like low-altitude and conflict alerts)
The foundation of altitude-based separation
Benefits over Mode A:
Altitude-based traffic separation
Supports TCAS (collision avoidance needs altitude)
Automated safety alerts
Essential for modern operations
Where Mode C (a Transponder) Is Required
The transponder/Mode C airspace requirements (14 CFR 91.215) are heavily tested, so here's the complete picture.
Mode C (or Mode S) is required:
In Class A airspace
In Class B airspace (and within the Mode C veil)
In and above Class C airspace (up to 10,000 MSL)
Above 10,000 feet MSL (excluding airspace at and below 2,500 feet AGL)
Within the Mode C veil: 30 NM around a Class B primary airport, from surface to 10,000 MSL
The Mode C veil:
A 30-NM ring around the primary Class B airport
Transponder with Mode C required within it (surface to 10,000 MSL)
Depicted on charts (a magenta ring)
Even outside the Class B shelves, within the veil you need Mode C
The 10,000 MSL rule with the exception:
Above 10,000 MSL, Mode C is required
EXCEPT at and below 2,500 feet AGL
So over high terrain, you might be above 10,000 MSL but within 2,500 AGL (no transponder required there)
The exception accounts for high-elevation areas
Where a transponder is NOT required:
In Class G and much of Class E airspace below 10,000 MSL (outside the veil and Class B/C)
Aircraft without electrical systems (gliders, balloons, certain vintage aircraft) have exceptions
Below the floor of Class C (in some configurations)
VFR at lower altitudes away from busy airspace
The no-electrical-system exception:
Aircraft certificated without an electrical system (many gliders, balloons, vintage aircraft)
May operate without a transponder in some of the airspace that would otherwise require one
Specific provisions apply (e.g., outside Class A/B/C, below certain altitudes)
A limited exception
Mode S: Selective and Advanced
Mode S (Selective) is the most advanced mode and the modern standard.
What Mode S transmits:
Squawk code and altitude (like Mode C)
A unique 24-bit aircraft address (permanently assigned to the airframe)
Aircraft identification (call sign or tail number)
Additional data depending on capability
The 24-bit address:
Every Mode S transponder has a unique 24-bit address
Assigned to the specific aircraft (like a hardware serial number)
Allows selective interrogation (addressing one specific aircraft)
16 million+ possible addresses
Why Mode S is different — selective interrogation:
Modes A/C reply to every interrogation (from every radar in range)
Mode S can be interrogated SELECTIVELY (a radar addresses a specific aircraft)
This reduces the number of replies (less congestion)
Reduces frequency congestion and mutual interference ("garble")
The congestion problem Mode S solves:
In busy airspace, Mode A/C transponders reply to every radar constantly
This floods the frequency (1090 MHz) with replies
Mode S's selective interrogation reduces unnecessary replies
Cleaner, more efficient surveillance
Enhanced data:
Mode S can exchange more data (aircraft identification, and via extensions, more)
Supports datalink capabilities
Enables direct aircraft-to-aircraft communication (for TCAS coordination)
More than just a code and altitude
How Mode S Drives TCAS Collision Avoidance
A critical connection the basic treatment underdevelops: Mode S is what makes modern collision avoidance work.
What TCAS is:
Traffic Alert and Collision Avoidance System (TCAS), also called ACAS internationally
An onboard system that detects nearby transponder-equipped aircraft
Provides traffic alerts and, in TCAS II, resolution advisories (climb/descend commands)
Required on larger aircraft (airliners, larger turbine aircraft)
How it uses transponders:
TCAS interrogates the transponders of nearby aircraft
It uses their replies (altitude and range) to detect conflicts
It tracks the closure and computes collision risk
The other aircraft's transponder is what TCAS "sees"
Why Mode S is essential for TCAS II:
TCAS II provides Resolution Advisories (RAs) — commands to climb or descend
When two TCAS II aircraft conflict, they must COORDINATE (one climbs, one descends)
This coordination happens via Mode S datalink between the aircraft
Mode S enables the aircraft to agree on complementary maneuvers
Without Mode S coordination, both might maneuver the same way
The coordination:
Two conflicting TCAS II aircraft communicate via Mode S
They agree: one gets "climb," the other "descend"
This complementary maneuvering resolves the conflict
Mode S is the communication link that makes it work
The traffic advisory (TA) vs. resolution advisory (RA):
TA: "Traffic, traffic" — alerts the pilot to nearby traffic (situational awareness)
RA: "Climb, climb" or "Descend, descend" — a command to maneuver (TCAS II)
TAs build awareness; RAs command action
Pilots must respond to RAs promptly (and ATC expects it)
Why it matters:
TCAS has dramatically reduced midair collisions
It relies entirely on transponders
Mode S enables the coordinated RAs
Your transponder makes you visible to TCAS-equipped aircraft (transponder ON is a safety issue)
Mode S and ADS-B
Mode S is the foundation for ADS-B, the modern surveillance technology.
What ADS-B is:
Automatic Dependent Surveillance-Broadcast
The aircraft broadcasts its GPS-derived position (and other data) automatically
More precise than radar
The backbone of modern surveillance (NextGen)
ADS-B Out (required):
Broadcasts the aircraft's position, altitude, velocity, and identification
Required since January 2020 in the same airspace where transponders are required (91.225)
One-way (the aircraft broadcasts out)
Enables ATC and other aircraft to see the aircraft precisely
ADS-B In (optional):
RECEIVES ADS-B data from other aircraft and ground stations
Provides traffic (TIS-B) and weather (FIS-B) in the cockpit
Optional (not required)
Gives pilots a traffic and weather display
The Out vs. In distinction:
Out: broadcasts your data (required in rule airspace)
In: receives others' data and services (optional, adds cockpit displays)
Out is the mandate; In is the bonus
Many aircraft have both
The two link options — 1090ES vs. 978 UAT:
1090ES (Extended Squitter): uses the Mode S transponder at 1090 MHz. Required for aircraft operating above 18,000 feet (Class A) and internationally.
978 UAT (Universal Access Transceiver): a separate 978 MHz link. Only usable below 18,000 feet (U.S. only). Offers FIS-B weather.
1090ES is transponder-based (Mode S); UAT is a separate box
Aircraft that fly high or internationally need 1090ES
Why the link choice matters:
1090ES works everywhere and uses the Mode S transponder
UAT is U.S.-only, below 18,000 feet, but provides free weather (FIS-B)
The choice depends on the aircraft's operations
Many GA aircraft use one or the other
Squawk Codes: Beyond the Emergencies
Pilots know the emergency codes, but there are others worth knowing.
The emergency/special codes (memorize these):
7500: Hijacking (unlawful interference)
7600: Communication (radio) failure
7700: Emergency
The memory aid: "7500 taken alive (hijack), 7600 nix on the comms (radio), 7700 going to heaven (emergency)"
The VFR code:
1200: VFR flight (United States)
The default VFR squawk
Used when not receiving a discrete code from ATC
Other specific codes:
1201: VFR in the vicinity of certain operations (regional)
1202: Gliders (not in contact with ATC)
1255: Firefighting aircraft
1277: Search and rescue aircraft
0000: Generally not used (military/special)
These have specific purposes
Discrete codes:
When ATC provides services (flight following, IFR), they assign a discrete code
A unique code (not 1200) that tags your specific target
"Squawk 4573"
Set the assigned code
The codes to never squawk inadvertently:
Avoid passing through 7500, 7600, 7700 when changing codes
Rapidly changing from, say, 1200 to 7700 might briefly show 7500 or 7600
Change codes deliberately to avoid a false emergency indication
Be careful dialing
Transponder Operation and Phraseology
Using the transponder correctly involves specific settings and ATC phraseology.
The transponder modes/settings (on the control panel):
OFF: transponder off
SBY (Standby): powered but not replying (warming up, on the ground sometimes)
ON (Mode A): replying with the code, no altitude (rarely used now)
ALT (Mode C): replying with code and altitude (the normal in-flight setting)
GND: ground mode (some transponders, for ADS-B on the ground)
The normal setting:
Fly with the transponder in ALT (Mode C/altitude reporting)
This gives ATC your altitude
The standard in-flight position
"Altitude" mode is normal
IDENT:
The transponder has an IDENT button
Pressing it makes the aircraft's target "bloom" or highlight on the controller's scope
Used when ATC says "squawk ident" to positively identify the aircraft
Press IDENT only when instructed (or when appropriate)
Common ATC phraseology:
"Squawk 4573": set code 4573
"Squawk ident": press the IDENT button
"Squawk altitude": ensure Mode C (ALT) is on
"Squawk standby": set to SBY (standby)
"Squawk VFR": set 1200
"Stop squawk" / "Squawk normal": return to normal operation
"Recycle transponder": turn off and back on (or SBY and back)
The "squawk ident" purpose:
Positively identifies your target among others
Confirms which blip is you
Don't press ident unless instructed (it's a specific request)
A tool for the controller to find you
When the Transponder Is Inoperative
What to do when the transponder fails, and how to still fly in airspace that requires one.
The requirement to operate it:
If the aircraft has an operable transponder, it must be ON (in ALT/Mode C) in controlled airspace (91.215)
You can't just turn it off
Operate it when you have it
Transponder inoperative — the provisions:
If the transponder fails, ATC MAY authorize operation in airspace requiring one
Request authorization from ATC
ATC can accommodate (using primary radar, etc.)
For flight INTO airspace requiring a transponder with an inoperative unit, request the authorization
The advance-notice provision:
To fly in transponder-required airspace with an inoperative transponder
Request may need to be made in advance (for some airspace, at least 1 hour before)
ATC can grant a deviation
Plan ahead if the transponder is known inoperative
The immediate failure:
If the transponder fails in flight, advise ATC
They can often accommodate you
Don't panic — request continued service
ATC has options
Why the provisions exist:
Equipment fails
The system has flexibility for inoperative equipment
ATC can use primary radar or other means
The rules allow for accommodation
Choosing and Upgrading Transponder Equipment
Practical considerations for aircraft owners.
The current landscape:
Mode A only: obsolete
Mode C: the older minimum, but ADS-B now required in the same airspace
Mode S with 1090ES ADS-B Out: the modern standard for most operations
Mode S is increasingly the practical choice
The ADS-B mandate driving upgrades:
ADS-B Out is required in most controlled airspace (since 2020)
This drove widespread transponder upgrades
A Mode S transponder with 1090ES satisfies both transponder and ADS-B Out (for many operations)
Upgrading often means going to Mode S/ADS-B
Considerations:
Regulatory requirements (where you fly)
1090ES vs. UAT (based on altitude/international operations)
ADS-B In capability (for cockpit traffic/weather — optional but valuable)
Future-proofing
Integration with other avionics
The practical bottom line:
Most modern aircraft need Mode S with ADS-B Out (1090ES) for the airspace they fly in
ADS-B In adds valuable traffic and weather (worth considering)
Mode A/C alone no longer meets the ADS-B mandate
Upgrade to meet current requirements
Common Misconceptions
"Mode C altitude comes from my altimeter."
No — it comes from a separate altitude encoder reporting PRESSURE altitude. ATC applies the local setting to display your actual altitude.
"Squawk codes can contain any digits."
No — they're octal (0-7 only). You'll never see an 8 or 9. There are 4,096 possible codes.
"ADS-B Out and In are the same thing."
Out broadcasts your data (required in rule airspace); In receives others' data and weather (optional).
"A transponder works with primary radar."
It works with SECONDARY radar (which interrogates the transponder). Primary radar detects skin reflection without the transponder.
"If my transponder fails, I can't fly in Class C."
ATC may authorize operation with an inoperative transponder (sometimes with advance notice). Request the authorization.
"I should press IDENT to help ATC whenever."
Press IDENT only when ATC requests it ("squawk ident"). It's a specific identification tool.
"1090ES and UAT are interchangeable."
1090ES (Mode S-based) works everywhere and above 18,000 feet; UAT is U.S.-only below 18,000 feet. The choice depends on your operations.
Final Thoughts
Transponder modes are a key part of how your aircraft is seen and managed by air traffic control. Mode A provides identity, Mode C adds the altitude that makes modern separation possible, and Mode S brings selective interrogation, unique addressing, and the datalink that powers TCAS resolution advisories and ADS-B. Understanding the modes — along with the squawk codes, the airspace requirements, the ADS-B Out/In distinction, and the phraseology — turns the transponder from a knob you set into a system you comprehend.
As airspace grows more complex and surveillance technology advances, Mode S with ADS-B has become the standard, bringing greater accuracy, efficiency, and safety. Your transponder is what makes you visible — to ATC and to the collision-avoidance systems aboard other aircraft — which is why operating it correctly (in ALT, with the right code) is a genuine safety responsibility.
On the Written Test and Checkride
Transponders appear on tests and checkride orals. The most commonly tested topics:
The transponder modes (A: identity, C: +altitude, S: advanced)
The emergency codes (7500, 7600, 7700) and 1200 (VFR)
Where a transponder/Mode C is required (Class A/B/C, Mode C veil, above 10,000 MSL)
The Mode C veil (30 NM around Class B)
ADS-B Out requirements and the 1090ES/UAT distinction
Transponder phraseology (squawk, ident, altitude)
Quick Reference
How It Works:
Transponder replies to SECONDARY radar interrogations
Primary radar: skin reflection (no ID/altitude)
Secondary radar: interrogates transponder (ID + altitude)
The Modes:
Mode | Provides |
A | Identity (squawk code) only |
C | Identity + pressure altitude |
S | Identity + altitude + 24-bit address + ID + datalink |
Squawk Codes (octal, 0-7, 4,096 total):
1200: VFR (US)
7500: Hijacking
7600: Radio failure
7700: Emergency
1202: Gliders
1255: Firefighting
1277: Search and rescue
Discrete codes: assigned by ATC
Mode C Altitude:
From an altitude ENCODER (not the altimeter display)
Reports PRESSURE altitude (29.92)
ATC applies local setting to display actual altitude
Where a Transponder/Mode C Is Required (91.215):
Class A, B, C
Mode C veil (30 NM around Class B, surface-10,000 MSL)
Above 10,000 MSL (except at/below 2,500 AGL)
Exceptions: no-electrical-system aircraft (gliders, balloons)
Mode S:
Selective interrogation (reduces congestion)
Unique 24-bit address
Enables TCAS coordination and ADS-B
TCAS (uses transponders):
TA (Traffic Advisory): "Traffic, traffic" (awareness)
RA (Resolution Advisory): "Climb/Descend" (TCAS II, command)
Mode S coordinates complementary RAs between aircraft
ADS-B:
Out: broadcasts position/altitude/ID (REQUIRED in rule airspace, since 2020)
In: receives traffic (TIS-B) and weather (FIS-B) (optional)
1090ES: Mode S-based, works above 18,000 ft and internationally
978 UAT: US-only below 18,000 ft, provides FIS-B weather
Transponder Settings:
OFF / SBY (standby) / ON (Mode A) / ALT (Mode C — normal) / GND
Fly in ALT (altitude reporting)
Phraseology:
"Squawk 4573": set code
"Squawk ident": press IDENT
"Squawk altitude": Mode C on
"Squawk standby": SBY
"Squawk VFR": 1200
Inoperative Transponder:
Operate it if you have it (91.215)
ATC may authorize flight in required airspace without it (sometimes 1-hour advance notice)
Key Principle:
Mode A gives identity, Mode C adds pressure altitude (from an encoder, corrected by ATC), and Mode S adds selective interrogation, a unique address, and the datalink behind TCAS resolution advisories and ADS-B. A transponder/Mode C is required in Class A/B/C, the Mode C veil (30 NM around Class B), and above 10,000 MSL. ADS-B Out is now required in that same airspace — via 1090ES (Mode S) or UAT.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.