Internet AIS
on the chart.

A schematic, not to scale. The gold ring is what the boat's own AIS hears; the dashed ring is what the internet adds.

Why bother

Because offshore, the horizon is the limit

AIS is VHF radio, and VHF is line of sight. Your own receiver sees ships out to the horizon and no further. Offshore, that can be too late to be useful.

But thousands of volunteers and coastal stations are hearing those same ships from the other side, and sharing what they hear over the internet. If you can reach the internet, you can see targets far beyond the range of your own AIS.

Where your own AIS comes from

Two ways onto the Pi

• A cheap SDR dongle and AIS-catcher. A dongle of about the price of a dinner, a VHF antenna and free software give you a working AIS receiver. It listens only, and it costs almost nothing. The dongle →
• Or your main AIS transponder. If the boat already has one, it is already hearing every ship around you. Take its data off the network instead of buying anything. That is what we do.

Either one gives you the local picture. The internet part below is what adds everything beyond the horizon.

Two halves

We give, and we get

1. Sending ours out

A small relay takes the AIS the boat already receives and forwards it to AISHub and MarineTraffic, two networks built almost entirely from volunteers. That is how coverage exists in the first place: nobody is watching your creek from orbit. AISHub is a sharing co-operative, so feeding it is also what entitles you to pull the network's data back. Why that matters →

2. Pulling theirs in: the custom script

This part is custom. We wrote it ourselves, because the built-in alternative needs a paid subscription and we wanted a free route. It does one job:

1. Reads the boat's live position from Signal K.
2. Asks AISHub, about once a minute, for every vessel in a box around that position.
3. Turns the answer into standard AIS sentences, which is the language chart software already understands.
4. Drops our own boat from the list, so we never appear next to ourselves.
5. Serves the result on the boat's network, where the chart software picks it up as a second AIS source.

The chart software, qtVlm, keys every target by its MMSI number, so a ship heard both locally and over the internet is one target and not two. A switch on the desktop turns the feed off or picks how far out to look. In one test it returned about 580 vessels.

Rules we kept

Because delayed data can mislead you

• The local transponder is always primary. It is real time and heard directly. Internet targets are delayed, and where the same ship is in both, the local one wins.
• Internet targets never reach the plotter. The commercial plotter stays on real-time AIS only, so a delayed target can never be mistaken for a live one.
• Not for collision avoidance. Internet AIS is for awareness, to see what is out there and where it is heading. Close quarters are decided by your own radar, your own AIS and your eyes.

The honest limits

• It only shows ships that somebody within reach of the networks is hearing. Far from any receiver there may be nothing to pull.
• The free feed allows one request a minute, so it updates in steps, and restarting the script quickly just returns nothing for a minute.
• It needs the internet. Offshore that means Starlink or satellite data, so it is one more reason to switch the link on deliberately.

All projects Crowdsourcing The DataHub