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Satellites expand AIS coverage

Oct 1, 2014
Polar-orbiting satellites carrying AIS transceivers can provide worldwide 
AIS coverage.

Polar-orbiting satellites carrying AIS transceivers can provide worldwide 
AIS coverage.

Courtesy ESA

In the history of maritime safety, a few major technologies have made a huge impact — radio, EPIRBS and GPS. The latest big impact technology is Automatic Identification System (AIS). Now AIS coverage is about to expand with added satellite capacity, bringing far-flung ocean areas under the AIS umbrella.

AIS is a system based on VHF radio digital selective calling (DSC). AIS units, either standalone or built into a VHF DSC radio and with GPS position input, use VHF channel 70 to broadcast a vessel’s name, position, speed, course and a variety of other data. The system is also true to its name; it’s automatic, requiring nothing from the vessel operator. The method AIS uses to communicate with other vessels is called self-organizing time division multiple access (SOTDMA). There are plenty of technical details, but what it boils down to is that SOTDMA is a sharing agreement that allows multiple vessels to send AIS info to each other using VHF channel 70.

Since AIS is based on VHF frequencies, its range is limited to so-called “line of sight” and can be received only within 40 to 50 miles. The AIS signals from vessels beyond that range — from the thousands of vessels at sea in the North Atlantic, for example — are never received by the AIS unit on your boat. This reduces confusion by only showing those vessels nearby.

There is a safety advantage, however, to an AIS system that can track every vessel no matter where it is on the world’s oceans. Should you get into trouble, rescue organizations, like the U.S. Coast Guard, will have access to your position and track data on your vessel to assist them in getting a rescue vessel or aircraft to you.

Given the physical restrictions imposed by VHF radio signals, the viable solution to tracking vessels at sea is with low earth orbit (LEO) satellites in polar orbits. LEO satellites in polar orbit can see thousands of square miles at a time and pick up signals from AIS-equipped vessels within a huge area on each pass. As the earth spins beneath it, a polar-orbiting satellite will eventually pass over the entire globe. A constellation of AIS satellites can cover the globe with only a few minutes between updates.

The first AIS satellite was launched in 2008 by exactEarth, a Canadian company based in Ontario. That satellite was a proof of concept, and is still operating six years later. ExactEarth has since put more small AIS satellites in orbit and currently has five polar-orbiting spacecraft with onboard AIS capability. ExactEarth plans to expand what it calls its ExactEarth Constellation to nine satellites.

The space communications company Orbcomm, based in New Jersey, launched six OG2 satellites in July 2014 with AIS units on board, providing AIS coverage to its customers. The company plans to launch 11 more AIS-equipped OG2 spacecraft before the end of 2014.

Also in July 2014, exactEarth signed a deal with the European Space Agency to provide AIS data on vessels worldwide. ExactEarth will provide the mission definition, for the ground segment, launch and operation. But what about satellite AIS for voyaging vessels? ExactEarth also announced in May 2014 that it would develop a small vessel tracking service in alliance with SRT, a U.K. AIS manufacturer. This service is dubbed Advanced Class B satellite-enabled AIS (ABSEA). ExactEarth claims that small, inexpensive AIS devices that will use ABSEA system will be built by SRT and can be easily installed on small commercial and leisure vessels.

One interesting wrinkle to putting AIS receivers on satellites is that the receiver can now pick up signals from thousands of ships at a time. With so many vessels in view, the elegant SOTDMA method used by terrestrial AIS units for allowing each broadcasting vessel to get its message out now doesn’t work so well.

Two methods have been developed to deal with this problem. One uses on-board processing (OBP). Satellite-based units using OBP work in much the same way as terrestrial AIS receivers, except they are more sensitive to AIS signals and so can resolve the signals from a large number of transmitters. When more than about 1,000 ships are involved, however, OBP loses its effectiveness. So OBP is generally only useful for low-density areas.

The second system has the intriguing name of spectrum decollision processing (SDP). According to exactEarth, SDP “employs receivers capable of detecting and digitizing the RF spectrum for the AIS channels and then processing the raw spectrum files to control the noise floor and reconstruct collided messages with highly specialized software algorithms.” SDP basically digitizes the radio signals, and then filters through them with software tools until it can detect signals from specific vessels. The SDP method works better for high-density vessel areas.

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