watching ships with AIS

SDRangel set up for AIS receive & decode, showing several vessels' information

i was recently by the coast with my SDR. i wanted to try and pick up some seaworthy transmissions, something i don't usually have the opportunity to do. a quick search told me that AIS was probably what i was looking for.

AIS ("automatic identification system") operates on two channels at 161.975 MHz and 162.025 MHz, and carries information such as vessel ID, name, type, heading, and speed. step one was seeing if i could receive anything to decode.

i set up my dipole antenna oriented horizontally (allegedly it's supposed to be vertical for this, but for whatever reason i got better SNR horizontally) and eyeballed it to the requisite 44 cm per leg. i tuned to 162 MHz in SDR++ and saw this:

an SDR++ waterfall showing two lines of blips at 161.975 MHz and 162.025 MHz respectively.

happy little blips! this is what AIS looks like, so it's time to decode it. there are a bunch of purpose-built applications for this but i already have SDRangel so i went with that

setup goes as follows:

  1. tune to 162 MHz (make sure you see the same pattern of blips as in SDR++) and add two AIS demodulators set to +25 Khz and -25 KHz respectively. you'll know when you've done it right because the "received messages" section of the demodulators should start filling up with entries: SDRangel with two AIS demulator windows, one of which is displaying three sets of ship information
  2. add an AIS window to collate the shipping info from both channels, making it easier to keep track of: SDRangel with an AIS window showing the same three ships as the AIS demodulator in the previous step
  3. add a map and focus your location to see where the transmitting vessels are located: SDRangel with a map focused on the english channel. two vessels are visible in the water

now you can sit back and let it collect data. SDRangel will plot vessel tracks over time. i enjoyed watching this fishing vessel moving back and forth in the channel:

the SDRangel map window zoomed in to show a ship making a horizontal back-and-forth path

you can also right click the vessels in the AIS window and see more info about them online, including photos.

fun!