AIS Packets (Automatic Identification System)

AIS signals are GFSK modulated signals at 161.975 or 162.025 MHz. Packets are sent with a preamble and the HDLC packet protocol is used. Below are pictures showing frequency vs time plots.

Download flow graph here: https://www.funwithelectronics.com/wordpress/other_data/demod_gfsk_for_blog.grc

When connecting the sync_and_strobe (gr-capture_tools) block to this signal and configuring it correctly, we get this:

Zoomed in on the first part:

 

AIS packets are “encoded” in multiple steps:

  1. The fields of various sizes are packed together as a bit sequence.
  2. Every 8 bit bundle is then reversed
  3. Bit-stuffing is performed: Every 111111 is converted to 1111101
  4. The stream is inverted, 1101 becomes 0010
  5. Differential encoding is performed. So every 1 becomes a transition, every 0 causes no change
  6. Then the header and end word are added at the start and the end.

Leave a Reply

Your email address will not be published. Required fields are marked *