Welcome Guest. Sign in or Signup

1 Answers

What is a mode S transponder?

Asked by: 4000 views Aircraft Systems

What's the distinguishing feature of a mode S compared to a mode C transponder?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

1 Answers



  1. John D Collins on Jul 22, 2015

    A mode A/C transponder replies to a pulse sequence with a thirteen bit message that is essentially the transponder code or altitude code, depending on the arrangement of the interrogation pulses. Interrogations are made on 1030 MHz and replies are made on 1090 MHz. The mode A/C only broadcasts when it is interrogated.

    A mode S transponder is uniquely addressable. Each mode S transponder can be interrogated by using a 24 bit address that is assigned to the transponder. In the US, there is an algorithm that relates the address to your N number on a 1 to 1 basis. A mode S transponder can receive a message in the 56 bit message field. This can be used by TCAS to negotiate with another mode S transponder TCAS system to determine the action to take (one aircraft climbs and the other descends) during an RA. The reply can also carry more data about the state of the aircraft. Also the mode S transponder can squitter, that is broadcast a message without being interrogated. There is also an extended squitter (ES) that is double the data block size of the 56 bit squitter, or 112 bits. This is used to broadcast the aircraft position and other data used by ADS-B. You may have heard of the term 1090 ES. So in simple terms, mode S is both addressable and can exchange substantial size messages with a ground station or secondary radar mode S beacon system. It is a mini data link.

    +4 Votes Thumb up 4 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.