Google it mate, there are over 100,000 entries relating to ACARS scanning. Why you'd want to eavesdrop ACARS messages I don't know. Mostly sent by VHF when in range and SATCOM otherwise, never by HF as far as I know.
Anyone can scan the ACRS messages, which isn't a good thing IMHO. My last airline had an incident where a PDA was found in a seatback, evidently left from a previous flight. The crew ACARSed ops to say they found it. Some idiot spotter was scanning ACARS, saw the message and though it would be a joke to send a bomb phone hoax to the company saying a bomb was on the flight hidden in a PDA. Flight diverted mid Atlantic.
Idiot is still probably in prison. He got a long stretch!
ACARS is not encrypted. There are standards for many ACARS messages, they're available to anyone willing to pay ARINC (the ACARS inventor) to get them. Routing information (source/type/date if you will) is always standardized for automatic processing, data may or may not be standardized depending on its purpose.
Greetings ACARS is encrypted using williamsburg protocol and encapsulated by the service provider to match the protocol BATAP or something else. there are prety good internet ACARS sites able to display the ACARS messages transmitted through VHF, these messages have been decrypted.
I've never flown an ACARS equipped aircraft but as far as I know the data "package" sent by the ACARS contains the actual FMC position info. If its true that the investigators have an ACARS msg reporting certain system abnormalities they must have the attached POS info. They should have a pretty exact last pos to start SAR. But the inital search covered a wide area. Any explanations?? (Perhaps the POS info was garbled? Or the airplane flew on for long minutes after the first sytem failures.?)
The airplanes installed with ACARS may have several functions: 1. The airplane installed with ACARS & activated FANS capability, this airplane is able to communicate ATC via ACARS (CPDLC function). 2. My company modifes ACARS software, let flight crews caculate V1/Vr/V2 via ACARS, and request & receive loadsheet, flight plan, send flight log etc..
an ACARS message does NOT include position. It include message ID, aircraft ID, checksum, send sequence, reply sequence, message type and subtype, and message data. the message data format is unique for every message type and subtype. every message is 220 characters or less in length. longer mesages are split up into "Multipart messages" and are limited to 3000 chars total.