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gnuais

Decodes AIS from the discriminator output of a VHF receiver

What it does

Takes audio from a VHF receiver tuned to an AIS channel -- ideally from the discriminator, before the audio filtering -- demodulates the GMSK, decodes the AIS messages, and stores the vessel data in a local database. It can also forward what it hears to an AIS aggregator.

Why you would want it

It is the route that does not need an SDR. An old marine VHF or a scanner with a discriminator tap is enough, which is worth knowing if you already have one -- and a purpose-built receiver on a fixed antenna often outperforms a dongle on the same feedline.

Before it will work

A receiver tuned to 161.975 or 162.025 MHz, with audio into a sound card. Discriminator audio rather than speaker audio: the receiver's audio filter removes the components the demodulator needs, which is why a tap is the usual advice.

How it installs

  • apt: gnuais

Known problems

One receiver hears one channel. Vessels alternate between the two, so a single-receiver setup sees roughly half the traffic and does not know it -- rtl-ais covers both from one dongle for exactly this reason. Speaker audio instead of discriminator audio produces a very low decode rate that reads as bad reception.

Keeping it current

  • probe: apt policy
  • strategy: apt_upgrade

Where to get help with the software itself

SourceForge project page at the URL above.

Source: catalog/packages/gnuais.yaml