Aircraft Communications Addressing and Reporting System (ACARS) is a carrier sensing, multiple access packet radio system for aircraft communications. ACARS operates in the VHF band, mainly around 130 MHz, using 2400 bps NRZI coded coherent audio frequency MSK (Minimum Shift Keying - a particular form of FSK) on AM to make use of standard aircraft AM communications equipment.

To receive ACARS an omni-directional 108-136 MHz antenna, a VHF AM receiver (scanner) with 13 kHz channel bandwidth and a corresponding AF output is necessary. As the ACARS packets are very short, turn the squelch of the receiver OFF.

Speed selection is not available for ACARS as only one speed is in use (2400 Baud).

Processing of ACARS Messages

It is possible to do some interpretation of ACARS messages. The following options are available:

Reassemble multipart messages

This setting enables the reassembling of bigger messages which are sent in several parts. Instead of showing each part separately, the message is shown as one single message when the last part has been received. The message header is still shown separately for each part.

Parse ADS-C messages

This setting enables the parsing of ADS-C messages (Automatic Dependent Surveillance - Contract). When an ADS-C message is received, it is decoded and the additional information is shown in the output window.

ACARS Frame

Messages may be single or multi-block. The pre-key sequence and the BCS have no parity bits.

ACARS communications are divided in Category A and Category B.

Using Category A, an aircraft may broadcast its messages to all ground stations. This is denoted by an ASCII "2" in the Mode field of the downlink message. The WAVECOM software translates this character to "A".

Using Category B an aircraft transmits its message to a single ground station. This is denoted by an ASCII character in the range "@" to "]" in the Mode field of the downlink message.

The ground station may use either "2" or the range "‘" to "}" in the Mode field. All ground stations support Category A, but may uplink "‘" to "}" in the Mode field.

The WAVECOM software translates the ground station address (also called the Logical Channel Number) into a number in the range 0...29.

A station will transmit after having monitored the HF channel for traffic, otherwise it waits until the channel is clear. If a collision occurs between the packets of two stations transmitting at the same time, they will back-off and new transmission intervals will be set by random interval timers in the radio equipment.

At the receiving end, a block check calculation is made and compared to the calculation appended to the packet by the transmitting station. If the downlink messages contain errors, no response will be given and the transmitting station will retransmit the packet a number of times, until a positive acknowledgement is received and the message can be deleted from storage, or the aircrew be alerted to its non-transmission.

If an uplink message is found in error, the airborne equipment will generate a negative acknowledgement (NAK), which triggers an uplink retransmission. Retransmission is also triggered by timeout.

Positive acknowledgement from the aircraft consists of the transmission of the Uplink Block Identifier of the correctly received block. Positive acknowledgement from the ground station consists of a similar transmission of the Downlink Block Identifier.

Acknowledgements are placed in the Technical Acknowledgement field.

The general response message label is "_DEL" (0x5f 0x7f). Messages with this label contain no information except acknowledgements and are used for link maintenance.

The traffic exchanged can be requests for voice communication, weather reports, access to airline computer systems, reading of aircraft automatic sensors, flight plans, messages to be routed to destinations in the international airline data network - in fact much traffic previously carried by voice, has been transferred to ACARS.

The text field of the ACARS packet is used for messages with a fixed format, free text, or a mixture of formatted and free text. Standard 7-bit ASCII is used; bit 8 is an odd parity bit and LSB (bit 1) is transmitted first.

ACARS Downlink Message Example

(#8) 06-11-1996 18:43:32 M=06 ADDR= HB-INR TA=Q ML=Q0 B=6 MSN=0635 FID=SR6767

(Bold typeface indicates decoder generated characters)

In this case record #8 decoded at 18:43:32 contains a message from a Swiss aircraft with registration HB-INR using logical channel 06 to transmit and acknowledgement of uplink block Q and a link test (Q0) with block identifier 6 and message sequence number 0635 (here the time in minutes and seconds after the hour is used - other formats are also in use). The flight is Swissair SR6767.

A few examples of the more important or frequently seen ACARS messages:

M=06 ADDR= HB-IND TA=NAK ML=_ B=3 MSN=2810 FID=OS005

Using logical channel 06 an unsolicited (TA=NAK) general response _ without information is transmitted as block 3 from aircraft HB-IND on flight OS005 with sequence number 2810. General responses are mainly used for block acknowledgement purposes.

M=06 ADDR=  TA=NAK ML=SQ B= 00XSZRH

This is a "squitter" - an ID and uplink test message transmitted at regular intervals from ground stations. This one is a squitter (SQ) version 0 (00) from a SITA (XS) ground station in Zurich, Switzerland (ZRH). The  denotes the ASCII NUL character (0x00) used for broadcast. A block identifier is not used.

M=06 ADDR= OY-MDS TA=5 ML=:; B=131125

This is a data transceiver auto tune message (:;) from ground station 06 commanding the ACARS transceiver of aircraft OY-MDS to change its frequency to 131.125 MHz. At the same time acknowledgement is given for the aircraft's downlink block 5.