Aircraft Communication Addressing and Reporting System (ACARS)

ACARS, an acronym for Aircraft Communication Addressing and Reporting System, plays a crucial role in modern aviation by providing a digital data link system that operates within the VHF range, specifically between 129 MHz and 137 MHz. This innovative system revolutionizes communication between aircraft and ground stations, enabling seamless real-time data transmission without the necessity of human intervention.

By utilizing ACARS, airlines can efficiently manage their fleet by exchanging vital information regarding aircraft performance. This system allows for the monitoring of various parameters such as engine health, fuel consumption, and maintenance requirements. Through both downlink and uplink capabilities, airlines can receive critical updates on the status of their aircraft and proactively address any maintenance needs, ensuring optimal operational efficiency and safety.

Furthermore, ACARS enhances the overall communication infrastructure within the aviation industry, enabling swift and reliable transmission of data between aircraft and ground personnel. This seamless exchange of information contributes to improved decision-making processes, enhanced safety measures, and streamlined operational procedures for airlines worldwide.

ACARS

Currently, there are four major Service Providers offering Air - Ground Link Message Handling.

1. SITA (Society International de Telecommunication Aerienere) operates AIRCOM in Singapore. The most part of the world is covered by SITA-AIRCOM is operated at 131.725 MHz.

2. ARINC (Traffic designated to ARINC is routed to the appropriate Data Service Processor in Chicago via separate telex connections). In USA is ARINC-ACARS dominant at 131.550 MHz.

3. AIR CANADA operates the own Air Canada at 131.475 MHz.

4. AVICOM Messages (at 131.450 MHz) going to SITA or AVICOM (Japan) are sent to SITA’s Service Processor in Singapore always.

The transmitted information is received via the ground stations to a central computer (Singapore for SITA or Chicago for ARINC) where data is converted into airline messages and sent to airline main base.

If there is too much communication traffic at a certain frequency channel, the ground station initiates an automate frequency change to another channel (Remote Tuning).

At uncovered areas of the world, if there is no link to a ground station the ACARS uses the Satellite Communication.

ACARS uses an aircraft’s unique identifier and the system has some features that are similar to those currently used for electronic mail.

The ACARS system was originally specified in the ARINC 597 standard but has been revised as ARINC 724B.

ACARS messages cater for the transfer of routine information such as:

• Passenger loads

• Departure reports

• Arrival reports

• Fuel data

• Engine performance data.

This information can be requested by the company and retrieved from the aircraft at periodic intervals or on demand.

Note that different channels are used in different parts of the world.

A typical ACARS message consists of:

• mode identifier

• aircraft identifier

• message label

• block identifier

• message number

• flight number

• message content

The aircraft ACARS components include a Management Unit which deals with the reception and transmission of messages via- the VHF radio transceiver, and The Control Unit,

which provides the crew interface and consists of a display screen and printer. The ACARS Ground Network comprises the ARINC ACARS remote transmitting/receiving stations and a network of computers and switching systems.

The ACARS Command, Control and Management Subsystem consists of the ground based airline operations and associated functions including operations control, maintenance and crew scheduling.

There are two types of ACARS messages;

• downlink messages that originate from the aircraft

• uplink messages that originate from ground stations.

The data rate is low and messages comprise plain alphanumeric characters.

SYSTEM ARCHITECTURE:

An established data communication via VHF to a Regional Ground Station (RGS) or via LBand (Frequency) to a satellite (SATCOM).

A Remote Ground Station is basically an intelligent VHF transceiver and message converter. Once a RGS has received a Downlink Message from an aircraft, its content is turned into a conventional Telex Message and sent to the Airline Service provider (ASP).

The ACARS Airline Service Provider (ASP) controls all RGS’s or earth-stations (SATCOM) connected via ground network.

It receives downlink messages from an aircraft via Regional ground Service (RGS) and distributes them to the appropriate designators (Airline Host Computer or Telex printer).

Besides the AC-Registration and Flight No., downlink messages coming from a RGS also contain the IATA code of the receivers RGS.

Therefore the ASP knows the approximate position of an aircraft upon the last received downlink.

Airborne Components and Subsystems

The ACARS System consists of a CMU (Communication Management Unit) a data printer, a Control-Unit/Terminal (MCDU) for entering and displaying data.

Communication Management Unit (CMU)

The CMU provides Input (receive) and output (transmit) interfaces through VHF-3 or SATCOM for the up- and down-linked messages. The CMU collects or distributes and formats data from/to various subsystems for transmission to or from the operators ground based computer system.

Out-.Off-, On- and In-event (0001) times are automatically generated by a set of parameters permanently broadcasted by sensors and various aircraft system to the CMU.

These times are automatically transmitted to the ground system and are used for aircraft movement and messages control The CMU controls the VHF-3 communication system in data mode VHF-3 can either be used for digital data link or for voice communication.

Multifunction Control Display Unit (MCDU)

Both MCDU’s are hooked up to the CMU and provide the main interface to the crew. It consists of an alphanumeric keyboard and display to control and enter data.

The CMU shall maintain a dialog with only one MCDU at a time. If selected, the opposite MCDU will duplicate the active ACARS page.

Airborne printer

The multifunction printer is the output device for data.

Furthermore, there is an Cabin Video Information System (Long Haul) providing the display of actual data from ground to all passengers for example: Connecting flights Information about passenger terminals Weather at destination Sport results Operations.

Data Format

The data is transmitted via VI IF with frequency shift keying it analog format via SATCOM in digital format.

Each alphabetic character is represented with 7 Bit Each packet is divided in 3 groups:

Preamble 34 Characters Address and System Protocol Message 220 Characters of data Trailer 7 Characters Parity and Verification