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Toimi II, Polyteknikkojen Kuoro, 2002

About AIS

AIS, or Automatic Identification System is not much unlike APRS - it is a professional tracking system for ships. It runs at 9600 bit/s (GMSK) on two marine VHF frequencies, with a nice binary protocol which features automatic synchronization and time multiplexing - even with a very high number of vessels transmitting position reports once every 2 to 10 seconds, packet collisions will not be a problem. The protocol is unencrypted, standardized and well-defined, but unfortunately patented.

aprs.fi welcomes everyone to receive AIS data and share it with other users by submitting it automatically to the service.

What is needed to receive AIS?

To receive AIS transmissions you need a 9600 bit/s data capable VHF receiver which can receive the marine VHF channel 87B (161.975 MHz) or 88B (162.025 MHz). Most new amateur transceivers and scanners have DATA plugs with a discriminator output which is good for decoding 9600 bit/s data. The discriminator output is connected to the audio input (line or mic) of a computer and a DSP software demodulator is used to decode the GMSK data.

Decoding can be done using ShipPlotter on Windows, or by gnuais on Linux.

Step-by-step instructions:

  1. Get an 9600 bit/s capable VHF receiver. You can either modify a scanner to have a discriminator output, or buy an off-the-shelf receiver or transceiver which has an 9600 bit/s data port. Many new mobile amateur rigs like the FT-7800 or IC-208H work just fine.
  2. Install a VHF antenna. A vertical antenna tuned around 162 MHz is good, but you can get started with a 2-meter amateur band vertical. Higher is better. If you live by a coast, a slightly directional antenna pointed to the sea is nice, too. Use a good-quality cable (shorter is better) and remember to properly shield all cable connections against the effects of the weather. When self-vulcanizing tape is used for the moisture protection, an additional UV shielding layer of PVC tape is usually needed.
  3. Check that you can receive transmissions on the AIS channels. They sound like static noise, but they have a slightly different tone than the FM static, and your signal level meter should jump up and down on those channels (but not on the ones around them). You can also tune up your local VTS frequency and listen for voice traffic.
  4. Connect the 9600 bit/s data output pin of your radio to the line or mic input of your computer. With speakers or headphones connected to the soundcard's output, check the computer's audio mixer settings so that you can record the received audio.
  5. Install ShipPlotter or gnuais.
  6. Sign up for an account on aprs.fi and look up your AIS password from the account page. Click on 'change' to see the complete submission URLs for each software.
  7. Configure ShipPlotter or gnuais as described below.
  8. After some data has been successfully uploaded, you can upload the position of the AIS receiver itself. This will allow others to see where your receiver is, and enable receiver performance statistics calculation on aprs.fi. If you wish not to reveal your exact location, please place the receiver symbol just a few kilometers away, perhaps on the sea near the coast or in a forest.
    • Click the position with the right mouse button
    • Select Voeg marker toe
    • Drag the marker to the exact right position (zoom in for better accuracy)
    • Click on the marker to open the info balloon
    • Then click Positie AIS ontvanger
    In a few seconds the AIS receiver symbol should appear at that position. The operation can be repeated to move the receiver. You need to be logged in using the account used for AIS data uploading, and you need to upload some AIS data first before setting the receiver position.
ShipPlotter configuration example

ShipPlotter on Windows

Use the following settings:

  • Server: aprs.fi
  • Port: 80
  • Script: /aisfeed/<your-ais-password-here> - for example, /aisfeed/c273rhauwf7 if your password is "c273rhauwf7"
  • Update (secs): 60
  • Enable compression: yes
  • Share ID: any lower-case letter (a-z), not a number or upper-case letter!

You can put aprs.fi as server 1 or server 2, as you wish. Just remember to set the Share ID to a lower-case letter. This is a convention to announce permission to redistribute your AIS data on the web.

gnuais on Linux

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