ACARS AIRCRAFTARINC COMMUNICATION ADDRESSING AND REPORTING SYSTEM ACARS

ACARS AIRCRAFT(ARINC) COMMUNICATION, ADDRESSING AND REPORTING SYSTEM

ACARS • WAS DEVELOPED IN 1970 S AS AIRCRAFT BEGAN TO BE EQUIPPED WITH DIGITAL AVIONICS • WAS DEVELOPED TO PROVIDE AUTOMATICALLY, THE ARRIVAL AND DEPARTURE TIMES AND AIR GROUND EXCHANGE OF OPERATIONAL DATA BETWEEN AIRCRAFT AND AIRLINES OPERATIONS CENTERS

AIRLINE DATALINK EVOLUTION • DATA LINK WAS FIRST INTRODUCED TO PROVIDE AIRCRAFT MOVEMENT REPORT IN PLACE OF AIR GROUND VOICE REPORT • ONE COMMON USE NOW IS TO PROVIDE AIRCRAFT SYSTEM STATUS REPORT TO THE AIRLINE ON REAL TIME BASIS • PROVISION OF FLIGHT CREW INITIATED INFORMATION REQUEST AND GROUND INITIATED REQUESTS

ACARS USE • ALLOWS AIRCREW TO SEND ARRIVAL/DEPARTURE TIMES, FUEL STATUS, FLIGHT DELAY etc. • GROUND MONITORING CAPABILITY OF AIRCRAFT ENGINE AND OTHER PARAMETERS, EFFICIENT EXCHANGE OF INFORMATION CONCERNING CONNECTING FLIGHTS AND RELIABLE SELECTIVE CALLING SYSTEM

ACARS AVIONICS • TO COMMUNICATE VIA AN ACARS NETWORK, AN AIRCRAFT MUST BE EQUIPPED WITH AVIONICS THAT PROCESSES THE ACARS PROTOCOL. THE ORIGINAL IMPLEMENTATION OF THE ACARS PROTOCOL IN AVIONICS WAS IN ACARS MANAGEMENT UNIT (MU). THE MAJOR SUPPLIERS OF ACARS MU ARE TELEDYNE, ROCKWELL COLLINS AND HONEYWELL

ACARS IMPLEMENTATION • ON BOEING AIRCRAFT ACARS MU IS PERFORMED BY THE AIRCRAFT INFORMATION MANAGEMENT SYSTEMS (AIMS) DEVELOPED BY HONEYWELL • ON AIRBUS AIRCRAFT, AEROSPATIALE HAS DEVELOPED AN AIR TRAFFIC SERVICE UNIT (ATSU) WHICH HANDLES ACARS MU FUNCTIONS AND ALSO HOSTS APPLICATIONS

DATA LINK ACCESS FOR AIRCRAFT SYSTEM • ACARS AVIONICS DEVELOPED SINCE 1990 PROVIDE OTHER AIRCRAFT COMPUTER DATA SUCH AS PERFORMANCE MONITORING AND FLIGHT MANAGEMENT SYSTEMS ACCESS TO AIR GROUND DATA LINK • AIR GROUND PROTOCOL AND MODULATION SCHEME IS SPECIFIED IN AEEC SPECIFICATIONS 618 “AUR/GROUND CHARACTER ORIENTED PROTOCOL SPECIFICATION” • CHANNELS USED IN THE AERONAUTICAL MOBILE EN-ROUTE SERVICE OR AM(R)S 118 -137 MHz • EACH CHANNEL HAS 25 KHz BANDWIDTH

DATA LINK ACCESS FOR AIRCRAFT SYSTEM • MU CONTAINS A MODEM THAT USES MINIMUM SHIFT KEYING (MSK) AMPLITUDE MODULATION PROVIDING A DATA RATE OF 2400 BITS/SECOND • VHF ACARS STATIONS ON GROUND PROVIDE OVERLAPPING RANGE USING THE SAME FREQUENCY • AS ON 31 ST DEC, 1999 THERE WERE 517 REMOTE GROUND STATIONS WERE OPERATING • ASSIGNED FREQUENCIES ARE: – 131. 725 MHz FOR EUROPE, SOUTH AMERICA, CARIBBEAN, MIDDLE EAST, AFRICA AND ST. PIERRE ET MIQUELON(FRENCH CANADA) – 131. 550 MHz FOR ASIA, NEW ZEALAND THE PACIFIC – 136. 850 MHz FOR United States of America AND CANADA – TO PARTLY MEET THE REQUIREMENT OF DENSE TRAFFIC IN EUROPE, 131. 525 MHz PROVIDED FOR ENROUTE AND 136. 900 MHz PROVIDED FOR USE ON GROUND

DATA LINK ACCESS FOR AIRCRAFT SYSTEM • TO SWITCH AIRCRAFT FROM ONE FREQUENCY TO THE OTHER WHILE ENROUTE, VHF MULTI FREQUENCY MANAGEMENT SYSTEM (VMMS) IS USED WHICH AUTOTUNES THE AIRBORNE SYSTEM • VMMS RECEIVES COPIES OF AIRCRAFT FLIGHT MOVEMENT MESSAGES TO IDENTIFY WHEN THE AIRCRAFT IS AIRBORNE AND SHOULD USE THE ENROUTE FREQUENCY AND WHEN IT IS ON GROUND SHOULD USE AIRPORT FREQUENCY • AEEC 620 PROTOCOL SUPPORTS EXCHANGE OF MESSAGES CONTAINING APPLICATION DATA ENCODED AS CHARACTER. USES A 2 CHARACTER ACARS LABEL ON AIR GROUND CONNECTION AND 3 CHARACTER STANDARD MESSAGE IDENTIFIER (SMI) ON THE INTERFACE TO GROUND SYSTEM

DATA LINK ACCESS FOR AIRCRAFT SYSTEM • AIRCRAFT, SUITABLY EQUIPPED CAN ACCESS THE DATA LINK VIA INMARSAT AERONAUTICAL MOBILE SATELLITE SERVICE (AMSS) • FOUR GEOSTATIONARY SATELLITES PARKED OVER ATLANTIC OCEAN (WEST), ATLANTIC OCEAN (EAST), INDIAN OCEAN AND PACIFIC OCEAN PROVIDE COVERAGE BETWEEN 80 DEG NORTH AND 80 DEG SOUTH • AIRBORNE/GROUND SYSTEM MAY INDICATE A PREFERENCE FOR THE USE OF VHF OR AMSS MEDIA

HOW ACARS WORKS • DATA CONCERNING DEP/ARR TIMES, FUEL STATUS, FLIGHT RELATED PARAMETERS etc. AUTOMATICALLY COLLECTED OR ENTERED INTO SYSTEM, CONVERTED INTO IT 5 (7 UNIT WITH 8 TH FOR PARITY) AND FORMED INTO BLOCKS OF 220 CHARACTERS MAX. • MESSAGE TRANSMITTED TO GROUND WHICH IS SENT TO ARINC/SITA CENTRAL SWITCH.

HOW ACARS WORKS • WORKS IN TWO MODES: – 1. DEMAND MODE: EITHER THE GROUND OR AIRBORNE SYSTEM TO INITIATE COMMUNICATION. AIRBORNE SYSTEM WILL INITIATE WHENEVER A PREDEFINED EVENT HAPPENS OR WHEN PILOT COMMAND IS ENTERED. GROUND STATION DETERMINES IF MESSAGE IS ERROR FREE, ROUTES IT TO DESTINATION AND ISSUES ACK. IF ERROR IS FOUND, ACK MESSAGE IS NOT ISSUED AND AIRCRAFT MAKES UPTO 6 ATTEMPTS AND THEN ALERT THE CREW. IN UPLINK MESSAGE HOWEVER NEG. ACK (NAK) MESSAGE IS ISSUED

HOW ACARS WORKS • - 2. POLL MODE: ALLOWS ONLY GROUND PROCESSOR TO INITIATE. GROUND SYSTEM CONTINUOUSLY POLL MESSAGES AND ANALYSES AIRCRAFT REPLIES

MESSAGE FORMAT BLOCK IDENTIFIER(1) (UPLINK ONLY) LABEL(2) ACKNOWLEDGEMENT(1) END OF PREAMBLE(1) ADDRESS(7) (START OF TEXT) MODE(1) START OF HEADING(1) TEXT (220) CHARACTER SYNC(2) BIT AMBIGUITY(2) PERKEY(16) PREAMBLE SUFFIX (1) END OF TEXT OR END OF BLOCK BCS SUFFIX BLOCK CHECK SEQUENCE (2) 16 BITS

MESSAGE FORMAT • Preamble: Identifies the message, synchronization of message • Prekey: 16 char for MU to establish bit and character sync • Bit Ambiguity: two char (+, *) to establish bit recognition correct • Char Synchronization: Two consecutive char are sent to establish char synchronization • Start of Heading: Char sent to indicate start of message heading also block check sequence • Mode: To be used in future (RTCA) • Address: Identifies destination. MU matches the address. Down link identifies aircraft originating • Acknowledgement: MU clears from memory last down link message only after receiving ACK message. Upon receiving NAK ground will retransmit • Label: Identifies message type like ‘QT’ for departure report, ‘ 5 D’ ATIS request etc. • Up link Block Identifier: Changes char every time a new message or general message is sent • End of Preamble: Identified by characters ‘ETX’ for message containing no TEXT and ‘STX’ for message containing TEXT. • Text: Free talk included + others • Suffix: End of Text(ETX)/End of Block(ETB) • Block Check Seq: Gen. ACK or NAK

ATN/VDL HISTORY • 1988 FANS Committee proposes concept of ATN Internet • 1990 ICAO decides to specify ATN subnet provision over a VHF Link • 1991 Aeronautical Mobile Communications Panel initiates VDL Standards • 1997/98 ICAO adopts VDL and ATN Standards

ATN INTERNET • ATN Internet provides transparent data connectivity across diverse terrestrial and air-ground network • Difference compared to generic Internet is the ATN addresses which contain airline or country codes and creates a routing domain for each airline/civil aviation authority

ATN INTERCONNECTION • ATN user system communicates via ATN routers which handle the ATN Internet Protocols • ATN Systems are interconnected by subnetworks which provide the foundation for ATN internet • ATN subnetwork can be provided using the X. 25 protocol

ATN SUBNETWORK • ATN ground systems connect over existing X. 25 networks such as SITA etc. • ATN avionics can connect to ATN ground systems using the X. 25 based services of AMSS, VDL, Mode ‘S’ and HFDL • There is no limit to message length over VDL X. 25 connections • Aircraft can establish an explicit VDL a/g link using a unique address for each ground station

VDL SERVICE PERFORMANCE • One VDL channel provides a 20 fold capacity increase over one ACARS channel • VDL link protocol reduces transmission overhead • Applications: – ACARS based – AOC bit oriented – ATS bit oriented