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Understanding Aircraft Transponder Modes: Mode A, Mode C, and Mode S

Updated: 4 days ago

Aircraft transponders play a critical role in modern air traffic control by allowing radar systems to identify aircraft, determine altitude, and maintain safe separation. While most pilots are familiar with using a transponder, fewer fully understand what each transponder mode actually does and why it matters.


This article breaks down the three primary transponder modes used in civil aviation—Mode A, Mode C, and Mode S—and explains how each one works, when it’s required, and how it affects ATC operations.



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What Is a Transponder?

A transponder is an onboard radio transmitter that responds to interrogations from ground-based radar systems. When interrogated, the transponder sends back specific information that helps air traffic controllers identify and track aircraft.


Depending on the selected mode, a transponder can provide:

  • Aircraft identification (squawk code)

  • Pressure altitude

  • Unique aircraft address and additional data


Mode A: Identity Only

Mode A is the most basic transponder mode and provides aircraft identification only.


What Mode A Transmits

  • A four-digit octal squawk code (e.g., 1200, 7500, 7600, 7700)

  • No altitude information


How ATC Uses Mode A

Mode A allows controllers to:

  • Identify a specific aircraft on radar

  • Associate the aircraft with a flight plan or radio call sign


Common Mode A Squawk Codes

  • 1200 – VFR flight (U.S.)

  • 7500 – Hijacking

  • 7600 – Radio failure

  • 7700 – Emergency


Limitations of Mode A

  • Does not provide altitude information

  • Requires controllers to rely on primary radar or pilot reports for altitude


Because of these limitations, Mode A alone is rarely sufficient in controlled airspace today.


Mode C: Altitude Reporting

Mode C builds on Mode A by adding altitude information.


What Mode C Transmits

  • The selected squawk code

  • Pressure altitude, reported in 100-foot increments


Altitude data comes from an altitude encoder, not directly from the altimeter displayed in the cockpit.


Why Mode C Is Important

Mode C allows ATC to:

  • See aircraft altitude directly on radar

  • Maintain vertical separation between aircraft

  • Provide traffic advisories and safety alerts


Where Mode C Is Required

In the United States, Mode C is required in:

  • Class A, B, and C airspace

  • Within the Mode C veil (30 NM around Class B airports)

  • Above 10,000 feet MSL (excluding airspace below 2,500 AGL)


Benefits Over Mode A

  • Enables altitude-based traffic separation

  • Supports automated collision-avoidance systems like TCAS


Mode S: Selective and Advanced Surveillance

Mode S (Selective) is the most advanced transponder mode and is now standard on many aircraft.


What Mode S Transmits

  • Squawk code and altitude (like Mode C)

  • A unique 24-bit aircraft address

  • Aircraft identification (call sign or tail number)

  • Additional data, depending on system capability


Why Mode S Is Different

Unlike Modes A and C, Mode S:

  • Responds only to specific interrogations

  • Reduces frequency congestion and interference

  • Enables direct aircraft-to-aircraft communication


Mode S and ADS-B

Many Mode S transponders support ADS-B Out, which broadcasts:

  • GPS-derived position

  • Ground speed and track

  • Altitude and identification


This data is more precise than traditional radar and improves situational awareness for both ATC and pilots.


Benefits of Mode S

  • Improved radar accuracy

  • Reduced signal clutter

  • Enhanced traffic information

  • Required capability for ADS-B Out compliance (1090ES)


Choosing the Right Transponder

For most modern operations:

  • Mode A only is largely obsolete

  • Mode C is the minimum for most controlled airspace

  • Mode S is preferred and often required for ADS-B compliance


Aircraft owners should consider not only regulatory requirements but also operational flexibility and future-proofing when upgrading transponder equipment.


Final Thoughts

Understanding transponder modes helps pilots appreciate how their aircraft is seen and managed by air traffic control. From the basic identity reporting of Mode A to the advanced surveillance capabilities of Mode S, transponders are a key link in the aviation safety system.


As airspace becomes more complex and surveillance technology continues to advance, Mode S transponders are increasingly becoming the standard—bringing greater accuracy, efficiency, and safety to the skies.




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