Aircraft Communications Addressing and Reporting System – ACARS- is a system of addressing digital data exchange between aircraft and ground center. System ACARS proposed and developed in 1978 by company ARINC. Speed of data transmission in ACARS is not great than 2400 bod, but provide high reliability of data transmission.
Airborne equipment ACARS consist from control panel and interactive display. Pilot enters information or automatically obtains it from other aircraft system and transmits to the ground center. Besides, this system allows monitoring the serviceability of the aircraft systems by malfunction signals sending in automatic regime.
For functional providing ACARS needs an extensive ground infrastructure, which consist from large amount of receiver-transmission centers and wiring points. ACARS provides bilateral communication between airlines and their aircrafts which are in flight.
Main function of ACARS
1. Duplex information message exchange between aircrafts and ground station. Messages can be sending automatically or by pilot.
2. Establishment of voice data link (function SELCAL(selective-calling)). Pilot can send message on voice communication organization. On ground after obtained decision about organization of communication, message is formed and send on board of the aircraft with information for pilots with the frequency for coherent equipment setup and with commands of automatic transition to voice mode.
Fig. 116. ACARS data transfer
3. If airborne equipment of ACARS doesn’t transmit messages, system monitors digital messages, which sends from ground. Each message contains encoded unique aircraft address. With the help of address airborne equipment receives only those messages that addressed for particular aircraft.
But the main function of system is giving information about stage of flight and technical state of aircraft facilities. Besides, pilot can use a big amount of information services which available in digital networks of ground telecommunication. For example, it is available to load data about meteorological condition METAR or TAF for any place on the earth surface.
Main examples of information which depends from different phase of flight contain in Fig.117.
General structural scheme of data exchange with the help of ACARS is shown in fig.118. During route flight airborne equipment of ACARS forms and periodically emits some messages. For example, during flight airborne equipment forms messages which contain information about aircraft position in airspace . These signals in VHF range taken by receiver, and this receiver are a part of the radio receiver center. Received signals decode and final messages from board of aircraft saved in special data base.
Fig. 118. ACARS decoder data flow
Access to the ACARS messages, saved in data base, provided through an air digital ground network data exchange and Internet technology.
Security of information is provided by facilities for restricted access. Depends on privacy configuration of service equipment access to data can be free or with user authentication. In general case any user through a Internet network can obtain access to the data with the help of special clients software. Software ClientNG gives opportunities for direct interaction with the database ACARS (Fig.119) . It allows view all ACARS messages for the any time and creates a local database messages. After data storage in local base, she can be used for users representation or for statistical processing.
In ACARS messages transmits information of different type; formats of messages are different too, because majority of airlines is used own formats for information transmission. But message structure about location of aircraft is standard. This give opportunity to create software for ACARS decoder messages selection which is associated with location of the aircraft and display them for the purposes of monitoring air traffic.
Typical ACARS decoder message about aircraft location contains such information-message type
-geographic latitude and longitude
With the help of special software ACARS decodes with further selection about location. After conversion of coordinates defined aircraft location view on map of the earth surface. The source cartographic information can be cartographic servers (Google or Yandex), which provide complimentary access. For tracking of individual aircraft a piece of map defined automatically with the aim that chosen aircraft always be in the center and does not exceed the specified range.
For convenience and clarity every aircraft designated with special sign handed down with a form that displays registration number and the number of occupied flight level. Information about location on map updates every time, when new message coming, herewith aircraft trajectory of flight is saved on map with point representation of its previous location.
Example of monitoring air traffic with specialized software in the airspace near Vancouver (Canada) is shown in Fig. 120. ACARS messages received through the server with public access (address: Ladysmith.no-ip.info) .