Demodulating ACARS

ACARS is a digital protocol used by aircraft to transmit messages. It’s been around since the late 1970’s and is decodable using nothing but a sound card and the right software. But, after helping a friend (a technologically-sophisticated one; like me, he has a ham license) who has previously had no luck decoding the messages, it’s clear there’s some tricks involved.

  • Don’t use squelch. Squelch will chop off the first tiny fraction of information in a packet, causing decoding errors (typically, messages simply won’t decode). There’s no need to use squelch, anyhow. Squelch exists to prevent humans from being annoyed by listening to the background noise when a frequency is not in use. Computers don’t care about being forced to analyze static, and can easily distinguish between static and an ACARS packet.
  • Use a wide bandwidth. A big part of my friend’s problem was that he was using the default AM bandwidth on his communications receiver, which was apparently too narrow. I have myself tried using both the wide and narrow filters on my receiver; only the wide one works. ACARS is apparently a wide-bandwidth mode, and a narrow filter throws away critical information needed to decode a message.
  • If using ACARSD, configuration is critical. ACARSD is the most popular freeware package for decoding ACARS. Alas, it’s not exactly user-friendly. To install it you must first configure the installer and manually tell it to create the directories it needs. To configure it you must use a separate program that (re)writes the necessary .INI file. Moreover, that program doesn’t always default to reasonable values as advertised. It claims ACARSD will use the default sound card if none is specified. I found it necessary to explicitly specify the sound card for the default one to be used on my friend’s computer.

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