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Satellite AIS – addressing some misconceptions

Two types of AIS:
The confusion arises from the fact that while there is a single standard for the transmission of AIS data, there are two very different ways in which the VHF signals can be intercepted and processed.

The one which most people are familiar with is the standard, horizontal line-of-sight interception where the signals pass between two VHF aerials located at or near sea level. These are generally ship-to-ship or ship-to-shore. These high intensity, short range signals are then processed by an ECDIS or computer-based software to display information on all vessels within range of the receiver. Shore stations connected to the internet can make that data available in near real time to online vessel tracking services, giving an up-to-date picture of the coastal and harbour traffic within their range. The quality of that data depends on the availability and density of shore-based monitoring stations in any particular area, which can be less than optimum in remote and underdeveloped parts of the world. This is known as terrestrial AIS, and it has been around since the 1990s.

However, around ten years ago various organisations discovered that, much to everyone’s surprise, these short range signals could in fact be picked up from above the Earth’s atmosphere. This was not expected as the maximum horizontal range at sea level is around 50 nautical miles (74 kms), yet these same signals could be received on the ISS, 400 kilometres up. The thinner atmosphere, with no atmosphere at all above, provides far less resistance to the VHF signal. The result is that, since 2005, AIS receivers, which can be very compact and weigh just a few kilogrammes, have been mounted in increasing numbers in low earth communications satellites, creating the satellite AIS (S-AIS) network.

The advantages of S-AIS
The key advantage of S-AIS over terrestrial AIS is that, with sufficient, suitably placed satellites, the entire surface of the earth can now be monitored for AIS transmissions, removing the dependence on terrestrial receiving stations. Each individual satellite can also cover a very large area. The result is that individual vessels or entire fleets can now be monitored wherever they are in the world, be it the middle of the Pacific Ocean or in the most isolated inland sea, as long as each has an AIS transponder fitted and operational.

How does it work?
The technology is evolving all the time, but at present there are two techniques used to detect AIS signals from space. On-board processing (OBP) uses sensitive receivers that are similar to their terrestrial equivalents in how they operate. They do not process the signal themselves, and work best in areas of low-density shipping. However, the service quality starts to degrade when more than 1000 vessels are within receiving range of the satellite as the signals start to collide with each other.

SDP (spectrum de-collision processing) uses receivers that detect and digitise incoming signals, then process the raw files to improve their quality. They provide a faster and higher quality flow of data.

For both systems, the information is transmitted back to the surface, processed if necessary, and from there fed to organisations and companies that distribute the data to the marketplace.

At present, hard-wired terrestrial AIS still delivers the fastest and most accurate data for the areas which it covers and therefore continues to be a vital part of the overall AIS package. However, that may not remain the case for much longer.

Who should have it?
Satellite-AIS data is invaluable for anyone who wishes to locate and / or monitor specific vessels, wherever they and the vessels may be, and for whatever purpose.  This will include:

Vessel owners
Vessel operators
Trade and industry associations
Supplier of maritime services
Statistical and economic authorities
Sports organisers and authorities
Port and harbour authorities
Maritime insurers and advisors
Maritime enthusiasts
Government bodies and agencies
Fleet managers – DPA, HSEQ & CSO
Environmentalists and educational establishments

Where can I get it?
There are a number of online AIS monitoring services that offer both terrestrial and S-AIS data together with a range of additional environmental and vessel information. Start your search at


About BigOceanData
BigOceanData is a leader in maritime tracking and telemetry, delivering global vessel tracking and monitoring services along with fleet management tools via its sophisticated browser-based interface. Key features of the BigOceanData product include its ability to fuse data from AIS signals and a range of onboard satellite reporting systems so as to both improve position accuracy and reduce data costs. The system also integrates a series of data feeds and management tools that show users not only vessel locations and movements, but situational data such as marine charting, terrestrial mapping, weather and sea-state (current and forecast), and piracy and other security alerts.

For more information contact or call +44 (0) 207 998 3048.

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