ACARSSTD.STDT

Harderwijk The Netherlands
Date : 31 January 1996
From : PAODAL
To   : acars @ ww


Dear Reader.

Herewith a short description of the ACARS messages and what seems 
to be the defacto message frame decoding standard adopted by both AEA 
and LOWE, (it may be even follow the ARINC characteristic 597-5). 
PA0DAL's ACASORT sorting program follows this message frame layout.

ACARS-Frequencies 

Europe	131.725
US.     131.550,130.025,129.125 MHz
Asia    131.450

Air Canada 131.475  

      
            INTRODUCTION to ACARS

 "Aircraft Communications Adressing and Reporting System"
 

For Ham radio operators an SWL's, ACARS can be regarded as a
commercial type of packet radio communications. The ACARS sig-
nal uses a 2400 baud message data bit stream to differentially
AM modulate the transmitter carrier using 1200 and 2400 Hz tones.

A 1200 Hz tone idicates a bit change from the previous bit and a 
2400 Hz tone indicates there was no bit change. AM modulation is
used, a practice consitent with the historical use of AM voice
mode on the aircraft bans sinds the early days of radio.
The signal is phase coherent.

Each message frame consists of at least 50, and up to a maximum
of 272 characters or bytes. Each charachter 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 synchroni-
zation, address, acknowledgment, mode and error checking characters,
in addition to the actual message text. Imbedded message label charac-
ters indicate the type of message. The exact message format is shown
below, the higlighted characters represent parts of the message that
AEA ACARS actually displays on the screen and/or saves to disk (most
others are nonprinting characters):

#characters   Purpose          Comments
       16     Pre-key          Xmitter warm-up/Rx AGC adjustment
       2      Bit sync         establish bit synchronisation
       2      character sync   establish character synch
       1      SOH              indicate start of message
 *     1      Mode             ground system interface configuration
 *     7      Address          aircraft resgistration number
       1      Ack/Nak          acknowledge/non-acknowledge marker
 *     2      Label            type of message
 *     1      Block ID         message block number 
       1      STX              indicates start of message text
 *     4      Sequence#        message sequence number**
 *     6      Flight number    airline flight number**
 *     210    Text             message text
       1      ETX              indicates end of text
       16     Block Check Seq  error dedection polynominal value
       1      BCS suffix last character     
 
 *)highlighted text
**)air-ground message only
t
The sixteen pre-key characters ar all binary 1 values, resulting in
the 0.05 second 2400 Hz beep you hear at the start of every message.
The Block Check Sequence field contains the value of an error dedection
polynominal that can be used to determine if the entire message was re-
ceived free of errors. detailed documents nored in the Reference section
of thes manual. Although none of theACARS traffic is encrypted, most 
systems include design provisions for doing so. Etc. Etc.

source: a packet radio message

73s Bert van Dalen 
pa0dal @ pi8utr