AIS vs Satellite AIS.
Terrestrial receivers see ships near the coast. Satellites see them everywhere else. When each one kicks in, and what the gaps between them mean for your tracking.
Every commercial vessel over 300 gross tonnes broadcasts a short radio signal — its name, position, speed, heading — every 2 to 10 seconds. That signal is called AIS, the Automatic Identification System. It's a maritime standard. It's also the foundation under every modern ship-tracking platform.
What people don't always realise is that "AIS" actually refers to two completely different listening networks. Knowing which one is feeding you data — and where the gaps are — is the difference between a tracking system that works reliably and one that lies to you.
The two networks
Coastal radio receivers.
A worldwide network of land-based VHF receivers. Tens of thousands of them, run by port authorities, coastguards, hobbyists (yes, really — many AIS feeds aggregate hobbyist receivers), and commercial AIS providers.
- Range: ~40 nautical miles from each station
- Refresh: Position reports as they come in — every 2–10 seconds
- Cost: Cheap or free
- Coverage: Excellent near ports and busy lanes
Listening from orbit.
Constellations of small satellites (Spire, ORBCOMM, exactEarth/L3Harris) that pick up AIS broadcasts from low earth orbit. Each satellite has a footprint of ~2,000 nm; together they cover the whole planet.
- Range: Global, including mid-ocean
- Refresh: Per-vessel passes ~every 30 min to 4 hours
- Cost: Expensive — subscriptions run $10k+ per month for serious coverage
- Coverage: Patchy near land (signal collisions), strong in open ocean
The handover
A serious tracking platform fuses both feeds. The handover looks like this:
- Vessel within ~40 nm of a coastal receiver → terrestrial AIS. Position refreshes every couple minutes; tracking dashboard "feels live."
- Vessel sails offshore, past terrestrial range → satellite AIS. Refresh slows to once every hour or two. Dashboard shows position as the satellite passes overhead.
- Vessel approaches destination port → terrestrial picks up again. Refresh accelerates.
The gap to watch for: mid-ocean, with no recent satellite pass. If your dashboard claims your vessel is at position X but the timestamp is 3 hours old and a 24-knot ship is moving, the actual position is up to 70 nautical miles away. Most well-designed UIs surface this — they show "last reported" timestamps, not pretend the position is current.
The aisstream.io feed
For most operators, building a direct relationship with the satellite AIS providers is overkill. aisstream.io is a free, WebSocket-based aggregation service that bundles terrestrial + satellite feeds and exposes them as a clean stream. You connect, subscribe to a list of MMSIs, and receive position updates as they arrive.
aisstream is the primary AIS source under Broadpath. It's free for non-commercial use, has a paid tier for production, and serves real-time globally. The data quality is good enough that the difference between aisstream and a $20k/month proprietary feed only shows up at the edges (very remote oceans, ships running with AIS off).
aisstream.io primary, datalastic pluggable.
Our default AIS source is aisstream.io. The provider layer is pluggable — when we need redundancy, regional optimisation, or specific data types aisstream doesn't carry (like AIS Type 5 voyage data with destination strings), the datalastic adapter slots in.
What gets lost
AIS has known limitations:
- Spoofing. AIS is unauthenticated. A ship can broadcast a false MMSI or false position. Fishing fleets and sanctions-evasion vessels do this regularly. For commercial shipping it's rare.
- Going dark. A ship can simply turn its AIS transponder off. Legal in some jurisdictions for security; illegal under SOLAS in most situations. When a tracked vessel goes dark, you'll see the position timestamp stop updating.
- Vessels under 300 GT. Not required to carry Class A AIS. Many smaller commercial vessels carry Class B AIS, which is lower-priority and transmits less often.
Why this matters for container tracking
Container ships are the easy case: all over 300 GT, all heavily monitored, all incentivised to keep their AIS on (no AIS = no port entry). For a tracked container, the AIS feed is reliable.
What you want to confirm in a tracking platform:
- The "last seen" timestamp is shown, not hidden
- The map can distinguish a freshly-updated position from a stale one
- Historical positions are preserved (so you can play back the voyage later — see historical vessel tracking)
- The platform doesn't fabricate intermediate positions to fake "live" updates
The last point matters more than you'd think. A few platforms interpolate vessel positions between known AIS reports to make the map look smooth. That's fine for visualisation. It's not fine if you're treating those interpolated points as ground truth.
Broadpath shows AIS positions with their actual timestamps, doesn't interpolate, and falls back to satellite AIS gracefully when terrestrial coverage drops. See it on the live map.