Decoding ACARS Messages: Understanding Components and Values

ACARS (Aircraft Communications Addressing and Reporting System) is a digital datalink system for the transmission of short messages between aircraft and ground stations via radio or satellite. Decoding ACARS messages involves understanding their structure, components, and the values they convey. This article will guide you through the process of deciphering ACARS messages, from identifying their components to interpreting their values.

1. ACARS Message Structure

An ACARS message typically consists of several key components:

• Header: Contains information about the message origin and routing.
• Label: A two-character code indicating the type of message.
• Block Identifier: Indicates the sequence of the message blocks.
• Data: The actual content of the message.
• Checksum: Ensures data integrity.

Let’s break down each component in detail.

2. Header

The header provides essential information about the transmission, including:

• Aircraft Registration: The unique identifier of the aircraft, usually in the format of a tail number.
• Flight Number: The flight’s designated number, helping to identify the specific journey.
• Timestamp: The time when the message was sent, often in UTC (Coordinated Universal Time).
• Mode: Indicates the communication mode, such as VHF, HF, or SATCOM.

Example:

Q0DFR 1234 123456 2021-10-01 12:34:56 VHF

3. Label

The label is a crucial part of the ACARS message, as it identifies the type of message being transmitted. It consists of two alphanumeric characters.

Common labels include:

• Q0: Out, Off, On, In (OOOI) times.
• H1: Weather information.
• M1: Maintenance messages.
• D0: Air traffic control (ATC) clearance.

Example:

Q0

4. Block Identifier

The block identifier helps in sequencing multiple message blocks. It is particularly useful when messages are too long to be sent in a single transmission and need to be split into multiple parts.

Example:

1/3

This indicates that this is the first block in a sequence of three.

5. Data

The data section contains the actual content of the message. The format and content of this section vary depending on the message type (as indicated by the label).

For example, an OOOI message might look like this:

OUT123456OFF123456ON123456IN123456

This represents the times the aircraft left the gate (OUT), took off (OFF), landed (ON), and arrived at the gate (IN).

6. Checksum

The checksum is used to verify the integrity of the message. It ensures that the message has not been altered or corrupted during transmission. The checksum is usually calculated using a specific algorithm, such as CRC (Cyclic Redundancy Check).

Example:

CHK1234

7. Decoding Process

Now that we understand the components, let’s walk through the decoding process step by step:

1. Capture the Message: Use an SDR (Software-Defined Radio) or ACARS decoder software to capture the raw message.

   Q0DFR 1234 123456 2021-10-01 12:34:56 VHF Q0 1/3 OUT123456OFF123456ON123456IN123456 CHK1234

2. Extract the Header: Identify and extract the header section.

   Q0DFR 1234 123456 2021-10-01 12:34:56 VHF

3. Identify the Label: Extract the label to determine the message type.

   Q0

4. Sequence the Blocks: Use the block identifier to sequence the message blocks if there are multiple parts.

   1/3

5. Parse the Data: Extract and interpret the data section based on the message type.

   OUT123456OFF123456ON123456IN123456

6. Verify the Checksum: Calculate and compare the checksum to ensure message integrity.

   CHK1234

8. Interpreting Values

Interpreting the values within the data section depends on the message type. Here are a few examples:

• OOOI Times (Label Q0):
• OUT: Time the aircraft left the gate.
• OFF: Time the aircraft took off.
• ON: Time the aircraft landed.
• IN: Time the aircraft arrived at the gate. Example:

  OUT123456OFF123456ON123456IN123456

• Weather Information (Label H1):
• Temperature, wind speed, and direction, visibility, etc. Example:

  TEMP15C WIND27015KT VIS10KM

• Maintenance Messages (Label M1):
• Status reports, fault codes, and other maintenance-related information. Example:

  ENG1OILTEMP85C

• ATC Clearance (Label D0):
• Instructions from air traffic control, such as altitude changes, route adjustments, etc. Example:

  CLIMB FL350

9. Tools and Resources

To facilitate the decoding process, several tools and resources are available:

• Software:
• acarsdec: An open-source ACARS decoder.
• JAERO: A software for decoding ACARS messages received via satellite.
• PlanePlotter: A comprehensive tool for aircraft tracking and ACARS decoding.
• Databases:
• Airframes.io: A platform providing detailed information about aircraft and their communications.
• ACARS Label Dictionary: A reference for identifying and interpreting ACARS labels.

Conclusion

Decoding ACARS messages involves understanding their structure, identifying components, and interpreting the values they convey. By following the steps outlined in this article and utilizing the available tools and resources, you can effectively decode and make sense of ACARS messages. Whether you’re an aviation enthusiast, a researcher, or a professional in the field, mastering ACARS decoding can provide valuable insights into aircraft operations and communications.