ACARS

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Example of an ACARS message

ACARS ( english A ircraft C ommunications A ddressing and R eporting S ystem ) is a by Aeronautical Radio Incorporated developed in the 1970s, digital data radio system for the transmission of messages between commercial aircraft and ground stations . It allows airlines to communicate with their aircraft by exchanging simple messages and thus saves radio messages on the radio frequencies, which are particularly congested in urban areas.

history

Before ACARS was deployed, flight operations reports were processed via radiotelephone , which resulted in a high load on the radiotelephone channels. Routine reports such as departure reports, arrival reports, load and fuel reports, reports about engine status, etc. can be digitally transmitted in a short time since the introduction of ACARS.

ACARS was initially only intended for the transmission of AOC messages ( airline operational control ). But then the use of the message service for more and more applications was discovered as an efficiency-increasing solution, and the number of messages sent via the ACARS system steadily increased. Air traffic control has also been sending flight clearances via ACARS since the early 1990s .

Current situation

Today, the ACARS frequencies are at least as congested as the radio frequencies, especially near larger airports. In addition, the ACARS protocol has some inherent limitations that made the development of additional data links necessary in the 1990s. Nevertheless, ACARS is still indispensable for many airlines, which sometimes operate their complete fleet management by exchanging AOC messages via ACARS.

When Air France flight 447 crashed in June 2009, the ACARS signals provided important initial information, and so the expansion of the system into a kind of "online black box" is being discussed. The previous principle now seems antiquated. At the moment, a flight data recorder has to record up to a hundred parameters several times per second - the current ACARS system would be overwhelmed with that. In addition, the satellite connections for these data streams are too expensive. As a middle ground, it is discussed to send more frequent position reports to air traffic control instead of just every 10-20 minutes to the airline's operations management.

Since 2009 ACARS has been replaced in Europe by VDL2 (In America VDL3). VDL2 is often equated with the new system CPDLC (controller-pilot data link communication), which was introduced in 2013 at the Swiss flight control center Skyguide and has been mandatory for all newly registered aircraft since January 1, 2014.

In March 2014, ACARS news and Doppler analysis of satellite communications were vital in efforts to locate Malaysia Airlines Flight 370 . While the primary terrestrial ACARS system on board MH370 was deliberately switched off, the connection to ACARS via Classic Aero , a data service that uses Inmarsat satellites , was later active again as long as the aircraft was in motion. It periodically replied to a ping every hour for the purpose of time synchronization when maintaining the connection to the satellite ( idle ).

application

The ACARS application includes a message service for automatically and manually created messages and provides special on-board hardware for this.

The management unit reacts to events in the on-board systems and automatically sends messages (originally only the so-called OOOI messages: Out, Off, On, In ). In detail, this stands for

  • Out : The aircraft leaves its parking position for pushback
  • Off : Weight-off wheels , the plane has taken off
  • On : Weight-on-wheels , the plane touched down on the runway
  • In : The aircraft comes to a parking position and turns off the engines.

These messages are received by the ground stations and forwarded by telex or satellite to the respective operation centers of the airlines, which means that the flight can be constantly monitored.

In addition to the fully automatic technical data reports, any other messages can also be transmitted between the cockpit and the operation center via the control unit (consisting of printer, screen and keyboard), such as B. Information about delays, weather reports , personal communications to passengers , etc.

The main provider of the network infrastructure for routing on the ground and the delivery of ACARS messages is SITA in Europe and ARINC in North America .

Example of a typical Delta Airlines ACARS message:

Original message
ACARS mode: 2 Aircraft reg:  .N186DN
Message label: 80 Block id: 0 Msg. no: M03A
Flight id: DL0107
Message content:
3C03 0107/24 EDDF/KJFK
/POS OTR /OVR 1015/NXT MASIT /ETA 1118
/ENS N56000W020000/ALT 310/FOB 0961/SAT 52
/WND 275057/MCH 81/TRB LT CHOP /SKY UNDERCAST/ICE NONE
Breakdown of the communication
Aircraft registration number: N186DN, a Boeing 767-300 from Delta Airlines
Flight number : DL107
Flight from Frankfurt-Main to John F. Kennedy International , New York
Position via waypoint OTR at 1015 UTC (formerly GMT ), next position waypoint MASIT expected at 1118 UTC, then N56 W20
Altitude 31,000 feet , remaining fuel 9.61 tons, air temperature −52 degrees Celsius
Wind from 275 degrees with 57 knots, speed Mach 0.81, slight turbulence , clouds below the aircraft, no ice

The text is transmitted via FSK at 2400  bps .

Data link

Operating frequencies for ACARS:
Frequency in MHz Region / remark
131.550 Primarily worldwide
129.125 Additionally in the USA and Canada
130.025 Secondary in USA and Canada
130.425 Additionally in USA
130.450 Additionally in the USA and Canada
131.125 Additionally in USA
131.450 Additionally in Japan
131.475 Air Canada Canal
131.525 Secondary Europe
131.725 Primarily Europe
136,700 Additionally in USA
136.750 Additionally in USA
136,800 Additionally in USA
136,900 Europe secondary
136.850 SITA North America
136.750 Europe new frequency
131.850 Europe new frequency

ACARS works in the VHF range. The ACARS protocol is character-oriented, limited to a length of 220 characters per transmission, has a data rate of 2.4 kbps, and a stop-n-wait method is used to access the channels . The ICAO defined a data link based on the ACARS data link under the name VDL Mode 1 , but this never gained practical importance.

Free reception

Those interested in radio now have the opportunity to decode the ACARS signals and thus gain an insight into air traffic. Even if you are not connected to the global ground station network yourself, you can receive messages from your own region with any aeronautical radio receiver / scanner and the appropriate decoder . A good external antenna increases the radius of the region that can be monitored.

Since the beginning of 2004 it has also been possible to receive the ACARS signals without your own radio scanner. At the end of 2003 / beginning of 2004 the ACARS Network was founded, which enables users with an Internet connection to experience ACARS in real time worldwide. All that is required is software that is available free of charge for Linux and Windows. ACARS can now also be followed live with a current generation web browser.

There are also network-controlled receivers that can receive ACARS signals.

Some large European airlines (e.g. Deutsche Lufthansa , Condor Flugdienst , Austrian Airlines ) encrypt the ACARS messages so that they can no longer be read in plain text without the appropriate key. Only the content of an ACARS message is encrypted, not the header .

literature

Individual evidence

  1. ↑ Online black box should clear up crashes more quickly , Spiegel Online
  2. ↑ Skyguide introduces CPDLC
  3. ↑ Classic Aero. (No longer available online.) SATCOM1, archived from the original on October 15, 2014 ; accessed on May 26, 2019 (English).
  4. ↑ Gordon Rayner, Nick Collins: [ARCHIVE MH370: Britain finds itself at the center of blame game over crucial delays] ( English ) In: The Telegraph . The Telegraph. March 24, 2014. Archived from the original on November 21, 2014. Retrieved on November 21, 2014.
  5. ↑ Known ACARS frequencies - acarsd ACARS Decoder for Linux and Windows. Retrieved May 26, 2019 .