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In aviation, ACARS (//; an acronym for Aircraft Communications Addressing and Reporting System) is a digital datalink system for transmission of short messages between aircraft and ground stations via airband radio or satellite. The protocol was designed by ARINC and deployed in 1978, using the Telex format. More ACARS radio stations were added subsequently by SITA.
Prior to the introduction of datalink in aviation, all communication between the aircraft and ground personnel was performed by the flight crew using voice communication, using either VHF or HF voice radios. In many cases, the voice-relayed information involved dedicated radio operators and digital messages sent to an airline teletype system or successor systems.
Particularly, the airlines paid the flight and cabin crews according to whether the aircraft was Out of the gate, Off the ground, On the ground, or In the gate. The flight crews reported these times by voice to geographically dispersed radio operators. Anecdotally, the airlines believed the flight crews "fudged" these reported times to the crews benefit, so believed they would save not only the cost of the radio operators, but also would realize a labor savings for the flight and cabin crews through more accurate time reporting.
In an effort to reduce crew workload and improve data integrity, the engineering department at ARINC introduced the ACARS system in July 1978, as essentially an automated time clock system. Teledyne Controls produced the avionics and the launch customer was Piedmont Airlines. The original expansion of the abbreviation was "Arinc Communications Addressing and Reporting System". Later, it was changed to "Aircraft Communications, Addressing and Reporting System". The original avionics standard was ARINC 597, which defined an ACARS Management Unit (MU) consisting of discrete inputs for the doors, parking brake and weight on wheels sensors to automatically determine the OOOI times and generate and send as telex messages. It also contained a Medium Shift Keying (MSK) modem used to transmit the OOOI reports over the existing VHF voice radios. The first day of operations saw about 4,000 transactions, but ACARS did not experience widespread use by the major airlines until the 1980s. The original ARINC development team was headed by Crawford Lane and included Betty Peck, a programmer, and Ralf Emory, an engineer. The terrestrial central site, a pair of Honeywell Level 6 minicomputers, and their software were developed by a subcontractor, Eno Compton of ECOM, Inc.
ACARS as a term refers to the complete air and ground system, consisting of equipment on board, equipment on the ground, and a service provider.
Ground equipment is made up of a network of radio transceivers managed by a central site computer called AFEPS (Arinc Front End Processor System), which handles and routes messages. Generally, ground ACARS units are either government agencies such as the Federal Aviation Administration, an airline operations headquarters, or, for small airlines or general aviation, a third-party subscription service. Usually government agencies are responsible for clearances, while airline operations handle gate assignments, maintenance, and passenger needs.
The ACARS equipment on the aircraft is linked to that on the ground by the datalink service provider. Because the ACARS network is modeled after the point-to-point telex network, all messages come to a central processing location to be routed. ARINC and SITA are the two primary service providers, with smaller operations from others in some areas. Some areas have multiple service providers.
ACARS messages may be of three broad types:
Control messages are used to communicate between the aircraft and its base, with messages either standardized according to ARINC Standard 633, or user-defined in accordance with ARINC Standard 618. The contents of such messages can be OOOI events, flight plans, weather information, equipment health, status of connecting flights, etc.
A major function of ACARS is to automatically detect and report changes to the major flight phases (Out of the gate, Off the ground, On the ground, and Into the gate), referred to in the industry as OOOI. These OOOI events are detected using input from aircraft sensors such as doors, parking brake and strut switch sensors. At the start of each flight phase, an ACARS message is transmitted to the ground describing the flight phase, the time at which it occurred, and other related information such as the amount of fuel on board or the flight origin and destination. These messages are used to track the status of aircraft and crews.
ACARS interfaces with flight management systems, acting as the communication system for flight plans and weather information to be sent from the ground to the FMS. This enables the airline to update the FMS while in flight, and allows the flight crew to evaluate new weather conditions or alternative flight plans.
ACARS is used to send information from the aircraft to ground stations about the conditions of various aircraft systems and sensors in real-time. Maintenance faults and abnormal events are also transmitted to ground stations along with detailed messages, which are used by the airline for monitoring equipment health, and to better plan repair and maintenance activities.
Automated ping messages are used to test an aircraft's connection with the communication station. In the event that the aircraft ACARS unit has been silent for longer than a preset time interval, the ground station can ping the aircraft (directly or via satellite). A ping response indicates a healthy ACARS communication.
ACARS interfaces with interactive display units in the cockpit, which flight crews can use to send and receive technical messages and reports to or from ground stations, such as a request for weather information or clearances or the status of connecting flights. The response from the ground station is received on the aircraft via ACARS as well. Each airline customizes ACARS to this role to suit its needs.
ACARS can send messages over VHF if a VHF ground station network exists in the current area of the aircraft. VHF communication is line-of-sight propagation and the typical range is up to 200 nautical miles at high altitudes. Where VHF is absent, an HF network or satellite communication may be used if available. Satellite coverage may be limited at high latitudes (trans-polar flights).
The sound of an ACARS VHF transmission made on 130.025 MHz, recorded at Petaluma, California on 15 August 2006. The short "beep" at the beginning of the message is composed of 16 bytes at 2400 Hz, which is the "alert" preamble in the protocol. The balance of the message which sounds like radio static is the actual data stream of the message.
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In the wake of the crash of Air France Flight 447 in 2009, there was discussion about making ACARS an "online-black-box" to reduce the effects of the loss of a flight recorder. However no changes were made to the ACARS system.
In March 2014, ACARS messages and Doppler analysis of ACARS satellite communication data played a very significant role in efforts to locate Malaysia Airlines Flight 370. While the primary ACARS system on board MH370 had been switched off, a second ACARS system called Classic Aero was active as long as the plane was powered up, and kept trying to establish a connection to an Inmarsat satellite every hour.
In 2002, ACARS was added to the NOAA Observing System Architecture. Thus commercial aircraft can act as weather data providers for weather agencies to use in their forecast models, sending meteorological observations like winds and temperatures over the ACARS network. NOSA provides real-time weather maps.