The Basics of ADS B Decoding

This guide presents an overview of Mode-S message decoding. It is organized as a flowchart to understand what information can be extracted from each message. For a technical understanding of Mode-S messages, visit The 1090MHz Riddle.

Downlink Format

The 1090MHz Riddle Message Structure
pyModeS pyModeS.df(msg)

The downlink format indicates message the message type.

DF	Type
0	Short air-air surveillance (ACAS)
1 - 3	Reserved
4	Surveillance, altitude reply
5	Surveillance, identify reply
6 - 10	Reserved
11	All-call reply
12-15	Reserved
16	Long air-air surveillance (ACAS)
17	Extended squitter
18	Extended squitter / non transponder
19	Military extended squitter
20	Comm-B, altitude reply
21	Comm-B, identify reply
22	Reserved for military use
23	Reserved
24	Reserved Comm-D (ELM)

ICAO

The 1090MHz Riddle ICAO Address
pyModeS pyModeS.icao(msg)

All aircraft are assigned a unique twenty-four bit address. All messages originating from an aircraft will this to associate message data with the aircraft. The function pyModeS.is_icao_assigned(msg) verifies that the ICAO code embeded in a message falls within a range of assigned codes.

Cyclic Redundancy Check

The 1090MHz Riddle ADS-B Checksum
pyModeS pyModeS.crc(msg)

Mode-S messages use Cyclic Redundancy Check (CRC) to verify data integrity. If the data is valid, the CRC remainder will be 0.