1. Detailed Structure of an ACARS Message
An ACARS message typically consists of several components that follow a standard format. Each type of message ACARS transmits has a defined structure. Here's an overview of the components:
a. Flight Identifier
The first part of the message is a code that identifies the airline and flight number. This identifier is crucial for routing the message between the aircraft and the airline’s ground operations.
- Format: The airline is coded with the first 3 letters of its ICAO code (e.g., "DAL" for Delta Airlines).
- Example: "DAL123", where "DAL" is Delta Airlines and "123" is the flight number.
b. Message Type
After the flight identifier, the message specifies the type of communication. The type indicates what data the message contains, such as weather reports, operational updates, or maintenance requests.
- Format: Typically a set of letters identifying the type of data.
- Example: "METAR", which indicates the message pertains to a METAR (meteorological report).
c. Ground Station Address
The message includes a code that identifies the ground station (such as an operational center or maintenance facility) to which the message is directed. This can be represented as a numeric address or other reference code.
d. Message Body
The body of the message contains the actual content of the communication. This is usually brief but can include complex information like flight plans, weather reports, maintenance updates, or other operational details.
- Format: This can vary depending on the type of message. The body can be composed of alphanumeric characters, numbers, or symbols.
- Example: A weather message might look like this: "METAR 1230Z KATL" (indicating a METAR report for KATL airport at 12:30 UTC).
e. Message Encoding
The encoding is a crucial aspect that ensures the message is transmitted securely and efficiently. ACARS messages are generally encoded in one of the following formats:
- ASCII (American Standard Code for Information Interchange): Used for plain text messages where each character is represented by a 7-bit binary number.
- BCD (Binary-Coded Decimal): Used for encoding numbers and binary data, such as sensor measurements, in a compact and error-resistant form.
- Proprietary Systems: Some airlines may use proprietary encoding systems that require specific decoding procedures on the ground station.
f. Control and Parity Codes
Many ACARS messages are accompanied by parity codes or a checksum, which helps ensure data integrity during transmission. These error-checking mechanisms are especially useful for radio communications, where transmission reliability may not always be guaranteed.
2. Types of ACARS Messages and How They Are Encoded
ACARS can handle different types of messages, each with its unique structure. Here are some of the primary message types:
a. Meteorological Messages (METAR/TAF)
These messages include weather reports (METAR) or short-term forecasts (TAF) for a given airport. A METAR message, for instance, provides information such as temperature, wind speed, visibility, and sky conditions. An example METAR message might look like this:
METAR KATL 121953Z 25012G18KT 10SM CLR 15/M03 A3030 RMK AO2 SLP278
- METAR: The type of message.
- KATL: The reference airport (Atlanta).
- 121953Z: The date and time of the message (12th December at 19:53 UTC).
- 25012G18KT: Wind from 250° at 12 knots with gusts of 18 knots.
- 10SM: Visibility is 10 statute miles.
- CLR: Clear skies.
- 15/M03: Temperature of 15°C and dew point of -3°C.
- A3030: Altimeter setting (QNH) of 30.30 inches of mercury.
b. Status Messages
These messages provide information on the operational status of the aircraft, such as engine health, fuel levels, and other system statuses. A typical status message might look like this:
STATUS 1234 72% FUEL 25000 FT
- STATUS 1234: The message type and flight identifier.
- 72% FUEL: Fuel level is at 72%.
- 25000 FT: The aircraft is at an altitude of 25,000 feet.
c. Maintenance Messages
ACARS messages may also include diagnostic reports about the aircraft's systems, such as sensors or engine conditions. These messages often contain error codes that need to be interpreted by ground maintenance personnel to initiate any necessary interventions.
3. Advanced Decoding: Use of Software and Tools
Manually decoding an ACARS message requires experience with the message formats and an understanding of the codes used. However, most people don't have direct access to ACARS messages in flight, but they can use automated decoding tools:
ACARS Decoding Software:
- There are several software applications (both commercial and open-source) that receive and decode ACARS messages, such as MULTIPSK, ACARS Decoder or Multimode Decoder.
- These tools typically receive VHF radio signals and then decode them into human-readable formats.
Flight Tracking and Satellite Systems:
- FlightRadar24, PlaneFinder, and other flight tracking systems provide access to a variety of data, including ACARS messages transmitted via satellite or radio.
- These platforms offer real-time information on flights and associated ACARS messages, allowing anyone to decode flight-related communications.
Radio Receivers and Hardware Decoding:
- Aviation enthusiasts may use SDR (Software Defined Radio) receivers or VHF receivers to listen to ACARS frequencies and decode messages in real-time. Software like SDR# or ACARSDeco allows users to receive and decode ACARS data.
4. Security and Confidentiality Considerations
Since ACARS transmits critical flight information, these messages are generally encrypted or protected with integrity checks to prevent unauthorized access. Therefore, not all information is easily accessible to the public. Only authorized personnel, such as flight crews, air traffic control, and airline operators, have full access to these messages.
ACARS message decoding 
