Also known as: Mode S, Mode-S

Mode S (mode select) is the selective-addressing aviation transponder protocol on 1090 MHz that carries each aircraft’s unique 24-bit ICAO address and forms the foundation of ADS-B. Messages are 56- or 112-bit frames protected by a CRC-24.¹

A Mode S frame: pulse preamble, then a CRC-protected data block keyed by the aircraft's 24-bit address.

Overview

ADS-B is carried in extended squitter (DF17/18) Mode S frames whose payload includes identity, position, and velocity. GopherTrunk decodes these (via a BEAST upstream or native demod) into tracked aircraft — see the ADS-B page.

Sources

1. Secondary surveillance radar — Mode S — Wikipedia, for the Mode S selective-addressing transponder protocol, its 24-bit ICAO address, frame sizes, and CRC. ↩

See also

• ADS-B
• Compact position reporting (CPR)
• Cyclic redundancy check (CRC)
• ICAO

Learn it step by step

• Other signals you'll meet

From the blog

• SDR Internals, Part 10: Protocol decoders & state machines

Related links

• GopherTrunk ADS-B decoder