Airline Tracking with APRSair.exe

Some TNC's have modems for extracting the airline company packets on the aircraft VHF band. APRSair.exe will monitor such a TNC on its SECOND comm port and do the following additional functions:

All ACARS packets are added to the L-LIST
The standard ACARS packet header will be placed on the D-LIST
If a LAT/LONG string is recognized, it will be placed on the P-LIST and plotted on the map.

A posit is assumed to be the letters N, S, E and W with the right number of characters in between. They may be the current location of the aircraft or future Waypoints, I dont know. Also I don't compute the checksum and these packets often have errors. Control characters are converted to a "*" so that they do not trash up the screens. If you have any details to help in this effort, please forward them to me.

ACARS REPORTING ON APRS: In the latest APRSair, you can now automatically link these reports over to the APRS network. If you have a normal APRS TNC on COMM 1 and use the alt-S-MODES-REPORT toggle to ON, then each aircraft POSIT will be transmitted out of your TNC onto the APRS channel to the address of ACARS. Other APRS stations will NOT normally capture these packets unless they set CONTORLS-FILTERS-OTHER or unless they set ALT-S- MODES-ALTNET and choose the altnet of ACARS... This separation of the traffic allows both ACARS and APRS to share the same channel...

BE SURE TO KEEP THE RADIO VOLUME LOW and the SQUELCH OPEN. If the volume is too high, then NOISE will look like packets!

Unregistered users of APRSair will only see the latest single data packet. Registered APRS users will see ONE PAGE of such plots. If you want to see more than just the one page, you must be a fully registered APRS user with the DF/DR option...

Since the ACARS TNC is on the second COMM port, normal APRS operations can continue on the first COMM port. You can use the P-LIST to HOOK and UPLINK selected aircraft to the APRS net.. The following is additional information provided by PA0DAL:

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

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).


Europe 131.725 US. 131.550,130.025,129.125 MHz Asia 131.450 Air Canada 131.475


"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 The sixteen pre-key characters are 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

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