ACARS
A
Users Guide
for the
Aviation Enthusiast
by Lionel K Anderson MSc
ACARS
A
Users Guide
for the Aviation Enthusiast
First Edition 2010 by Las Atalayas Publishing
Author and Editor Lionel K Anderson MSc
Copyright © Lionel K Anderson 2010
The moral rights of the author have been asserted. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical or otherwise without the written permission of the Publisher.
ISBN 978-1-4457-8847-0
Design and Typesetting by kenandglen.com
Contents
1 Introduction 1
2 Receiving ACARS Radio Signals 3
3 ACARS Message Format 4
4 Decoding the ACARS Messages 6
5 More on Decoding 17
Appendix A - ACARS Frequencies 23
Appendix B - ACARS Mode Codes 24
Appendix C - ACARS Message Types 25
Appendix D - Abbreviations used in ACARS Messages 27
Appendix E - IATA ICAO Airport Codes 30
Appendix F - ICAO IATA Airport Codes 64
Appendix G - ICAO IATA Airline Codes 98
Appendix H - ICAO IATA Aircraft Codes 115
Appendix I - ARINC Specifications 131
About the Author
Lionel K Anderson developed an early interest in aviation and aircraft during his schooldays living on the Isle of Wight. This encouraged him to build model aeroplanes and experiment with radio control designing and building simple system using thermionic valves and electro-mechanical escapements.
Those were the days when great things were happening in aviation and he recalls seeing the Fairey Delta 2, flown by Peter Twiss, overhead in 1956 when a new World Air Speed Record 1132mph was made, a significant achievement considering the old record had only been set the previous year by an American F100 Super Sabre, and was so much slower.
Leaving school he joined the British Army as an Apprentice Radar Mechanic and pursued a career in the maintenance of aircraft tracking and surveillance radars.
Later as a professional electronics engineer he was involved in the design and development of radar and microwave engineering systems.
Now retired he continues his many interests and activities in electronics and computing publishing a number of books on various allied subjects.
Glossary
ACARS Aircraft Communications Addressing and Reporting System
ACMS Aircraft Condition Monitoring System
AMS ACARS Message Security, as specified in ARINC 823
AOA ACARS Over AVLC. With the introduction of VDL Mode 2, the ACARS protocols were modified to take advantage of the higher data rate made possible by Mode 2. AOA is an interim step in replacing the ACARS protocols with ATN protocols.
ATN Aeronautical Telecommunications Network. As air traffic increases, ACARS will no longer have the capacity or flexibility to handle the large amount of datalink communications. ATN is planned to replace ACARS in the future and will provide services such as authentication, security, and a true internetworking architecture. Europe is leading the US in the implementation of ATN.
AVLC Aviation VHF Link Control. A particular protocol used for aeronautical datalink communications.
CDU Control Display Unit
CMF Communications Management Function. The software that runs in a CMU, and sometimes as a software partition in an integrated avionics computer.
CMU Communications Management Unit. Successor to the MU, the CMU performs similar datalink routing functions, but has additional capacity to support more functions. CMU standards are defined in ARINC Characteristic 758.
FDAMS Flight Data Acquisition and Management System
FMS Flight Management System. FMS standards are defined in ARINC Characteristic 702 and 702A.
HFDL High Frequency Data Link is an ACARS communications media used to exchange data such as Airline Operational Control (AOC) messages, Controller Pilot Data Link Communication (CPDLC) messages and Automatic Dependent Surveillance (ADS) messages between aircraft end-systems and corresponding ground-based HFDL ground stations.
HF High Frequency. A portion of the RF spectrum.
LRU Line Replaceable Unit. An avionics “black box” that can be replaced on the flight line, without downing the aircraft for maintenance.
MCDU Multifunction Control Display Unit. A text-only device that displays messages to the aircrew and accepts crew input on an integrated keyboard. MCDU standards are defined in ARINC Characteristic 739. MCDUs have seven input ports and can be used with seven different systems, such as CMU or FMS. Each system connected to an MCDU generates its own display pages and accepts keyboard input, when it is selected as the system controlling the MCDU.
MIDU Multi-Input Interactive Display Unit (often used as a third cockpit CDU).
MU Management Unit. Often referred to as the ACARS MU, this is an avionics LRU that routes datalink messages to and from the ground.
OOOI Shorthand for the basic flight phases—Out of the gate, Off the ground, On the ground, In the gate.
POA Plain Old ACARS. Refers to the set of ACARS communications protocols in effect before the introduction of VDL Mode 2. The term is derived from POTS (Plain old telephone service) that refers to the wired analog telephone network.
SATCOM Satellite Communications. Airborne SATCOM equipment includes a satellite data unit, high power amplifier, and an antenna with a steerable beam. A typical SATCOM installation can support a datalink channel as well as several voice channels.
VDL VHF Data Link
VHF Very High Frequency. A portion of the RF spectrum.
Disclaimer
The use of radio receivers and scanners to monitor radio frequencies is not permitted in every country of the world. Similarly some countries and authorities prohibit the use of ACARS data. Thus readers are advised to check with their licensing authorities prior to doing so as the author cannot be held responsible for any legal consequences.
Chapter 1
Introduction
Aviation enthusiasts have for many years listened to air traffic voice messages by monitoring VHF air band transmissions using radio scanners or dedicated air band radio receivers. By monitoring the broadcast messages the enthusiast could ascertain only a limited amount of information in respect of an aircrafts positions and its route.
In the mid to late 1990s most communications between aircraft and the ground used voice communications. The expansion in air traffic generated the need for a faster and more efficient system for handling communications.
Aeronautical Radio Incorporated (ARINC) then developed the Aircraft Communications Addressing and Reporting System (ACARS), a digital data-link system for transmission of short and relatively simple messages between aircraft and ground stations and designed to utilise existing ground station and aircraft radio equipment, and enhance air-ground-air communications.
SITA, a multinational information technology company, later augmented their worldwide data network by adding ground radio stations to provide ACARS service.
ACARS carry a lot of traffic between aircraft and ground stations with the type of information in the ACARS transmissions varying widely. The ACARS messages can range from simple arrival/departure reports to lengthy aircraft computer downlinks of navigation, engine, and performance data as well as weather observations, flight plans, navigation positions, aircraft and engine performance data, arrival/departure/delay reports, equipment malfunction reports, crew reports and connecting gate lists.
As ACARS messages are transmitted in the VHF air band, it is now possible for the aviation enthusiast to receive the messages on their existing radio equipment and by connecting the receiver to a PC or laptop computer to decode the messages for display in a plain text format.
ACARS messages are either transmitted from an aircraft to a company ground station, called a downlink message, or transmitted from ground stations up to aircraft, called uplink messages.
Before a message is transmitted, the ACARS computer first listens to see if the frequency is clear before transmitting its message. This technique is sometimes called Self Organizing Time Division Multiplexing (SOTDM) or ‘Listen before Transmit.’ SOTDM applies equally to both aircraft and ground stations.
This book provides information about the format of ACARS messages and how they may be decoded.
Although this book concentrates on the VHF aspects of ACARS as it is written with the aviation enthusiast in mind, it should be mentioned that ACARS is an integrated system where communications between an aircraft and the airline company may satellite communications, as well as using a number of both VHF and HF transmitter receivers—all of which are linked together by the company’s communications network.
Chapter 2
Receiving ACARS Radio Signals
The ACARS radio transmission is an amplitude-modulated (AM) signal to make it consistent with the historical use of AM voice mode on the aircraft bands in the early days of radio. The transmission is in the VHF band around 130 MHz and uses a 2400 bps NRZI-coded coherent audio frequency MSK (Minimum Shift Keying—a particular form of FSK) on AM to make use of standard aircraft AM communications equipment. A list of the frequencies commonly used for ACARS can be found in Appendix A.
The demodulated audio signal fits perfectly in the audio spectrum of an AM receiver (300-3000 Hz); thus, almost any make and model of air band receiver or scanner covering the VHF 130Mhz band is suitable for receiving ACARS transmissions.
In most cases the earphone output of your receiver can be used to connect the receiver to a standalone ACARS Decoder, to a PC, or to a Laptop Computer running one of the many suites of ACARS Decoder software.
It is essential that the air-band receiver or scanner should have a Squelch control such that the Squelch is turned off. It will also be necessary to adjust the volume control of the receiver so that the decoder is not overloaded. As a brief sync tone is sent before each transmission, many scanners’ squelch will not open up fast enough, thus another reason to keep the squelch turned off. Suit some receivers and scanner are listed below.
N.B Squelch is normally used to remove the background noise and static when receiving voice transmissions. But as the ACARS signal is in effect a short burst of noise typically under one second in length, it necessary to be able to hear it.
|
Make |
Model |
|
Signal |
R532/R53 |
|
Uniden |
USC230, UBC-30XLT, USC230, UBC3500XLT |
|
Yaesu |
FT-817 |
|
Yupiteru |
VT-125/150 , VT-225, MVT-3100, MVT-7100, MVT-7200 |
Chapter 3
ACARS Message Frame Format
Each message frame consists of at least 50, and up to a maximum of 272 characters or bytes. Each character uses a 7-bit ACSII code with an additional eighth parity bit. This results in a total message transmission duration of between 0.17 and 0.91 seconds.
The message frame format is rigidly defined to include synchronization, address, acknowledgment, mode and error checking characters, in addition to the actual message text. Imbedded message-label characters indicate the type of message. The message format is shown in Table 2 below.
|
Parameter |
Value |
No of Characters |
|
Pre-key |
Tx warm-up/Rx AGC adjustment |
16 characters |
|
Bit Sync |
Establish bit synchronisation |
2 characters „+„ , „*„ |
|
Character Sync |
Establish character synchronisation |
2 characters SYN, SYN (16h) |
|
Start of Heading |
Start of Heading |
1 character SOH (01h) |
|
Mode |
Ground system interface configuration |
1 character |
|
Address |
Aircraft registration number |
7 characters |
|
Ack/Nak |
Acknowledge/Non-Acknowledge flag |
1 character |
|
Label |
Type of message |
2 characters |
|
Block Identifier |
Message block number |
1 character |
|
STX |
STX (02h) - if no text ETX (03h) |
1 character |
|
Flight No |
Airline flight number |
6 characters |
|
Text |
printable characters only |
maximum 220 characters |
|
Suffix |
ETX or ETB (17h) |
1 character |
|
BCS |
Block Check Sequence |
16 bits |
|
BCS Suffix |
DLE (7fh) |
1 character |
The sixteen pre-key characters are all binary 1 values, thus resulting in the 0.05 second 2400 Hz beep that can be heard at the start of every message.
The Block Check Sequence field contains the value of an error-detection polynomial that can be used to determine if the entire message was received free of errors. Standard 7-bit ASCII is used, bit 8 is an odd parity bit used for the text field of the ACARS message, and the content may be free text or a mixture of formatted and free text.
ACARS communications are divided into Category A and Category B. An aircraft uses a Category A transmission to broadcast its messages to all listening ground stations. This is denoted by an ASCII 2 (Hex32) in the Mode field of the downlink message. When using Category B, an aircraft transmits its message to a single ground station. This is denoted by an ASCII character in the range @ to ] (Hex40 - Hex5D) in the Mode field of the downlink message.
All ground stations support Category A, but may uplink ‘ to } (Hex27 - Hex7D) in the Mode field. The ground station may use either 2 or the range ‘ to } (Hex27 - Hex7D) in the Mode field.
At the receiving end, a Block Check calculation is made and compared to the calculation appended to the packet by the transmitting station. If the downlink messages contain errors, no response will be given and the transmitting station will retransmit the packet a number of times until a positive acknowledgement is received and the message can be deleted from storage, or the aircrew be alerted to its nontransmission.
If an uplink message is found in error, the airborne equipment will generate a negative acknowledgement (NAK) which triggers an uplink retransmission. Retransmission is also triggered by timeout.
Positive acknowledgement from the aircraft consists of the transmission of the Uplink Block Identifier of the correctly received block. Positive acknowledgement from the ground station consists of a similar transmission of the Downlink Block Identifier. Acknowledgements are placed in the Technical Acknowledgement field. The general-response-message label is _DLE (Hex5F Hex7F). Messages with this label contain no information except acknowledgements and are used for link maintenance.
A typical raw undecoded message might look like this:
<SYN><SYN><SOH>2..KC693<NAK>Q05<STX>S68!2\42H;C<FS<ACK><BEL>#########+*#<NAK>*T#^U*#*<DC2>*#*T#U+L#MjU#UM#)5R#+E#T*J#5U#+jU#<SYN>*U#U#######################################vw<SOH>2.<SO>II0?(0_C<ETX>@<DLE>
Chapter 4
Decoding ACARS Messages
The raw message shown at the end of the last chapter could be decoded and translated into plane English as follows, “KC693 operating as MC4208 was first contacted at 19:18 on 21/06/2010 using Ground Station 2 with broadcast message number S68A was conducting Q0 type link test.”
Fortunately, users don’t have to work through the process of translating such raw messages as there are a readily available number of decoders that will do the job for them.
First, though, it is necessary to look at the structure of decoded messages and discuss some of the standard-message formats.
ACARS Downlink Messages
Consider this decoded message:
(#5)15-05-2010 15:33:36 M=06 ADDR= F-ZKXF TA=Q ML=Q0 B=9 MSN=D09D FID=AF9876
which can be displayed more simply in a table like that below.
|
Decoded |
Interpretation |
|
(#5) |
Decoder generated message number |
|
15-05-2010 18:43:32 |
Decoder generated timestamp (optional) |
|
M=06 |
Mode Category A = A, |
|
ADDR=F-KZXF |
Aircraft address |
|
TA=Q |
Technical acknowledgement |
|
ML=Q0 |
Message Label (message type) |
|
B=9 |
Uplink/Downlink Block Identifier |
|
MSN=D09D |
Message Sequence Number |
|
FID=AF9876 |
Flight Identifier |
In this case, record #5 decoded at 15:33:36 contains a message from a French aircraft with registration F-ZKXF using logical channel 06 to transmit and acknowledge uplink block Q and a link test (Q0) with block identifier 9 and message sequence number 0635 (here the time in minutes and seconds after the hour is used—other formats are also in use). The flight is Air France AF9876.
A method commonly used by many software decoders for displaying the decoded details shown below.
|
ACARS mode: 06 Aircraft reg: F-ZKXF |
|
Message label: Q0 Block id: 9 Msg. no: D09D |
|
Flight id: AF9876 |
Some more examples of the more important or frequently seen ACARS messages:
M=06 ADDR= F-KZXF TA=NAK ML=_? B=3 MSN=2810 FID=AF9876
|
Decoded |
Interpretation |
|
(#7) |
Decoder generated message number |
|
16-05-2010 09:33:12 |
Decoder generated timestamp (optional) |
|
M=06 |
Mode Category A = A, |
|
ADDR=F-KZXF |
Aircraft address |
|
TA=NAK |
Technical acknowledgement |
|
ML=_? |
Message Label (message type) |
|
B=3 |
Uplink/Downlink Block Identifier |
|
MSN=2810 |
Message Sequence Number |
|
FID=AF9876 |
Flight Identifier |