Aircraft Transponders: Which Type do You Need (And What’s the Difference?)

Updated for 2026: Transponders have remained a major topic in general aviation since the FAA’s ADS-B Out airspace rule took effect in 2020. Many pilots upgraded around 2019, but if you’re buying a used aircraft or re-outfitting one that hasn’t flown in a while, the same questions still come up: What kind of transponder do I need? Do I need ADS-B? Which transponders are ADS-B Out capable?

This guide combines the fundamentals of aircraft transponders with the latest 2026-relevant updates, including evolving technical standards and international ADS-B mandates. By the end, you’ll know exactly what equipment you need—and why—based on where and how you fly.

What’s new for 2026? The FAA airspace rules themselves haven’t changed, but newer ADS-B performance standards and international requirements now influence which equipment makes sense for long-term compliance, especially if you plan to fly outside the U.S.

Table of Contents

• What is a Transponder?
• Types of Aircraft Transponders
  • Mode A
  • Mode C
  • Mode S
• Is ADS-B the Same as a Transponder?
• Mode S vs ADS-B
• Do ADS-B Devices Replace Transponders?
• International ADS-B Requirements (2026)
• What Transponder Do I Need?
• Airspace-Based Requirements
• Best ADS-B Transponder Options
• Emergency Squawk Codes
• Surveillance Phraseology
• Frequently Asked Questions

What is a Transponder?

An aircraft transponder is an electronic device that responds to radar interrogation signals by transmitting identifying information about an aircraft. This allows air traffic control to identify aircraft on radar displays and maintain safe separation.

Types of Aircraft Transponders

There are three primary categories of aircraft transponders: Mode A, Mode C, and Mode S. Each adds additional information and capability.

Mode A Transponder

Mode A transponders are the most basic. They transmit only a four-digit identification code—commonly called a squawk code—assigned by ATC.

Mode C Transponder

Mode C transponders transmit the squawk code plus pressure altitude. ATC uses this altitude data to maintain vertical separation.

The FAA requires Mode C or better transponders in certain airspace, including Class A, B, and C, and above 10,000 feet MSL.

Mode S Transponder

Mode S is the most advanced transponder type. It transmits squawk code, altitude, and aircraft identification, and supports collision-avoidance systems such as TCAS.

Many Mode S transponders include ADS-B Out capability using a 1090 MHz extended squitter (1090ES).

Is ADS-B the Same as a Transponder?

No. ADS-B (Automatic Dependent Surveillance–Broadcast) is a broadcast system that continuously transmits GPS-derived position, altitude, velocity, and identification without radar interrogation.

Some Mode S transponders include ADS-B Out, but ADS-B can also be provided by a standalone device paired with an existing transponder—if the full system meets FAA requirements.

What Is the Difference Between Mode S and ADS-B?

Mode S transponders respond to interrogation and rely on multilateration to estimate aircraft position. ADS-B Out broadcasts precise GPS position continuously, allowing ATC and nearby aircraft with ADS-B In to see traffic directly.

Do ADS-B Devices Replace Transponders?

An ADS-B Out installation typically meets or exceeds requirements in airspace where a Mode C transponder is required. However, ADS-B Out must be part of an approved installation to replace traditional transponder functions.

International ADS-B Requirements (2026 Update)

In the U.S., ADS-B Out compliance can be met with either 978 UAT (below FL180) or 1090ES systems.

Canada: Canada has implemented ADS-B Out using 1090ES. As of 2024, ADS-B Out is required in Class A and Class B airspace, with expansion into additional airspace planned no earlier than 2028. A 978 UAT-only installation does not meet Canadian requirements.

If international flying is in your future, a 1090ES-capable Mode S transponder is the most future-proof option.

What Type of Aircraft Transponder Do I Need?

Equipment requirements depend on where the aircraft operates—not the type of aircraft.

Airspace-Based Requirements

Class A

Mode C transponder and ADS-B Out required.

Class B

Mode C transponder and ADS-B Out required within the airspace, above the ceiling, and within the Mode C veil.

Class C

Mode C transponder and ADS-B Out required within the airspace and above the ceiling within lateral boundaries.

Class D

No transponder or ADS-B Out requirement. Two-way radio communication required.

Class E

No equipment requirement unless above 10,000 feet MSL (excluding below 2,500 feet AGL) or certain offshore areas.

Class G

No transponder or ADS-B Out requirement below 10,000 feet MSL.

Best ADS-B Transponder Options

All-in-one Mode S ADS-B Out transponders are the simplest path to compliance. Products like the uAvionix tailBeaconX provide 1090ES ADS-B Out with integrated GPS in a low-impact installation.

Emergency Squawk Codes

• 7500 – Hijacking or unlawful interference
• 7600 – Radio failure
• 7700 – General emergency

Surveillance Phraseology

• “Squawk ident” – Activate IDENT feature
• “Identified” – Aircraft identified on radar
• “Stop squawk” – Cease transmitting code
• “Resume squawk” – Resume assigned code

Frequently Asked Questions

• Do I need ADS-B Out in 2026?

  Only if you operate in ADS-B rule airspace. The requirement is location-based, not aircraft-based.

• Is Mode C still acceptable?

  Yes, but only in airspace where ADS-B Out is not required.

• Should I choose 978 UAT or 1090ES?

  978 UAT works for most U.S. GA aircraft below FL180. 1090ES is required above FL180 and for international operations.

• Are more mandates coming?

  There are ongoing discussions about expanded surveillance requirements, but no new FAA mandates beyond ADS-B Out are in effect as of 2026.

Have questions about transponders or ADS-B upgrades? Leave a comment below and we’ll help you choose the right setup for your aircraft.