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What is a mode S transponder?

Asked by: 4001 views Aircraft Systems

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

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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.

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