# 1.1. Introduction¶

## 1.1.1. Message structure¶

An ADS-B message is 112 bits long, and consists of 5 parts:

+--------+--------+-----------+--------------------------+---------+
|  DF 5  |  ** 3  |  ICAO 24  |          DATA 56         |  PI 24  |
+--------+--------+-----------+--------------------------+---------+


This table lists the key bits of a message:

nBits Bits Abbr. Name
5 1 - 5 DF Downlink Format (17 or 18)
3 6 - 8 CA Capability (additional identifier)
24 9 - 32 ICAO ICAO aircraft address
56 33 - 88 DATA Data
[33 - 37] [TC] Type code
24 89 - 112 PI Parity/Interrogator ID

Example:

Raw message in hexadecimal:
8D4840D6202CC371C32CE0576098

-----+------------+--------------+-------------------------------+--------------
HEX  | 8D         | 4840D6       | 202CC371C32CE0                | 576098
-----+------------+--------------+-------------------------------+--------------
BIN  | 10001  101 | 010010000100 | [00100]0000010110011000011011 | 010101110110
|            | 000011010110 | 10001110000110010110011100000 | 000010011000
-----+------------+--------------+-------------------------------+--------------
DEC  |  17    5   |              | [4] .......................   |
-----+------------+--------------+-------------------------------+--------------
DF    CA     ICAO          [TC] ------ DATA ----------    PI


Any ADS-B must start with the Downlink Format 17 or 18 (10001 or 10010 in binary code) for the first 5 bits. Bits 6-8 are used as an additional identifier, which has different meanings within different types of ADS-B message.

In each ADS-B message, the sender (originating aircraft) can be identified using the ICAO address. It is located from 9 to 32 bits in binary (or 3 to 8 in hexadecimal). In the example above, it is 4840D6 or 010010000100.

An unique ICAO address is assigned to each Mode-S transponder of an aircraft. Thus this is a unique identifier for each aircraft. You can use the query tool (World Aircraft Database) from mode-s.org to find out more about the aircraft with a given ICAO address. For instance, using the previous ICAO 4840D6 example, it will return the result of a Fokker 70 (wow, it must be one of last in its kind in operation) with registration of PH-KZD.

To identify what information is contained in an ADS-B message, we need to take a look at the Type Code of the message, indicated at bits 33 - 37 of the ADS-B message (or first 5 bits of the DATA segment).

Following are the relationships between each Type Code and its information contained in the DATA segment:

TC Content
1 - 4 Aircraft identification
5 - 8 Surface position
9 - 18 Airborne position (w/ Baro Altitude)
19 Airborne velocities
20 - 22 Airborne position (w/ GNSS Height)
23 - 31 Reserved for other uses

ADS-B uses a cyclic redundancy check to validate the correctness of the received message, where the last 24 bits are the parity bits. The following pseudo-code describes the CRC process:

GENERATOR = 1111111111111010000001001

MSG = binary(8D4840D6202CC371C32CE0576098)    # 112 bits

for i from 0 to 88:                           # 112 bits - 24 parity bits
if MSG[i] is 1:
MSG[i:i+24] = MSG[i:i+24] ^ GENERATOR

CRC = MSG[-24:]                               # last 24 bits of result

IF CRC not 0:
MSG is corrupted


For the implementation of CRC encoder in Python, refer to the pyModeS library function: pyModeS.util.crc()

A comprehensive documentation on Mode-S parity coding can be found:

Gertz, Jeffrey L. Fundamentals of mode s parity coding. No. ATC-117.
MASSACHUSETTS INST OF TECH LEXINGTON LINCOLN LAB, 1984. APA