AIS Marine Tracking with SDR: Build Your Own Ship Receiver

AIS, or Automatic Identification System, is a global ship tracking standard. It is mandatory for vessels over 300 gross tons, all passenger ships, and most commercial vessels.

Ships broadcast their position, identity, course, and speed. The system uses two frequencies: 161.975 and 162.025 MHz. An RTL-SDR dongle can pick up these signals. No software is needed, just tune in.

I use an RTL-SDR V4 for this, and it works fine. Some people use SDRplay, which seems solid too. To decode the signals, I use rtl_ais, dump the data to KML, and visualize it on Google Earth.

Receiving AIS signals requires line-of-sight. The system works well near the coast. Inland, reception is not as good. Antenna placement matters. I have mounted mine on my roof, which has improved reception.

AIS signals are not encrypted. The standard is open and fairly straightforward. Decoding with rtl_ais allows you to feed the data to mapping software. You can then see ship positions and their plotted courses.

Why Build Your Own AIS Receiver

MarineTraffic and VesselFinder use volunteer receivers to gather AIS data. You can boost coverage in thinly covered spots, like coasts and ports, with your own receiver. Real-time data comes without the 5-15 minute lag of free tiers. You can also pick up vessels not listed on commercial platforms. This is a useful edge.

Equipment

To get started, grab an RTL-SDR Blog V4, about $40, and a simple vertical antenna for 162 MHz. The included telescoping dipole will work. A marine VHF antenna delivers much better performance, especially near coastlines or harbors.

For a better antenna, try the Shakespeare 5101, a 3dB gain antenna tuned for 162 MHz AIS. Or build your own 1/4 wave ground plane. You'll need a 43.5 cm vertical element and four ground plane radials at 45°, with an SMA connector.

Placement is everything. Line-of-sight to water is key. Your best bets are a second-floor window facing the harbor, a roof mount, or a mast mount. AIS is VHF and strictly line-of-sight. Hills and buildings block the signal.

Software Options

AIS messages arrive as VDM/VDO, decoded by your SDR as audio. You'll need a decoder to turn that into vessel data.

OpenCPN is a solid choice. It is an open-source chart plotter with a built-in AIS decoder and map display. You can download it from opencpn.org, set your location, and then pipe the SDR audio into OpenCPN via virtual audio cable.

Other options exist. AIS Dispatcher handles AIS decoding on Windows. AIS-catcher is cross-platform and made for RTL-SDR AIS reception. There is also RTL_AIS, which is a command-line decoder.

AIS Message Types

AIS sends 27 message types. For OSINT, focus on position reports, static and voyage data, and navigation aids.

Position reports are key, they contain MMSI, navigation status, speed over ground, position, course over ground, and true heading.

Static and voyage data provides vessel name, IMO number, call sign, vessel type, dimensions, destination, ETA.

Contributing to Marine Traffic Networks

Sharing data gets you a free premium account on most platforms. At MarineTraffic, sign up as a sharing station here. You'll need to point your AIS-catcher or OpenCPN at their servers. NMEA data flows in, and you get an Unlimited plan — $59 a month, free. AISHub and FleetMon also take data. Contribute and get access.

Interpreting AIS Data for OSINT

To get the most out of AIS data, you need to crack the MMSI format. Every vessel has a unique MMSI. The MMSI is a country code, MID, followed by a unique vessel ID. MMSI numbers can be looked up on MarineTraffic, VesselFinder, or the ITU GMDSS Master register.

The flag state does not always provide meaningful information. Vessels often fly flags of convenience, such as Panama, Liberia, Marshall Islands. This can be a deliberate attempt to obscure their identity.

Ships trying to evade sanctions or smuggle goods may disable AIS. They might also spoof their position or use duplicate MMSI numbers. A receiver can help identify dark periods. Comparing AIS-reported positions to satellite imagery or radar can help detect a spoof.

Satellite AIS

Standard AIS receivers on land can only pick up signals up to the horizon, about 40-60 km range if you're at sea level. Vessels beyond that disappear from your feed.

Satellite AIS, or S-AIS, solves this. Orbiting satellites collect AIS signals from anywhere on the oceans. Companies like Spire Maritime, exactEarth, ORBCOMM sell S-AIS data to maritime platforms.

For worldwide vessel tracking, paid services like MarineTraffic and VesselFinder are the way to go. Your RTL-SDR gives you real-time AIS data locally. That's it.

Troubleshooting

If you're not pulling in any signals, check your setup. You need to be near water with boats moving. Verify your antenna is hooked up correctly and positioned well. Try both AIS channels.

If signals show up but decoding isn't working, check your audio routing and decoder settings. If decoding drops in and out, try setting the SDR gain between 30 and 50 dB.

Updated April 2026. See also: MarineTraffic review, VesselFinder review, ADS-B Flight Tracking Setup.