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Improved AIS standard for yachts

Dec 28, 2018
The self-organizing time-division multiple access (SOTDMA) system used for AIS allows vessels to “claim” a broadcast spot so their data can get out to nearby vessels.

The self-organizing time-division multiple access (SOTDMA) system used for AIS allows vessels to “claim” a broadcast spot so their data can get out to nearby vessels.

When you delve into the details of the automatic identification system (AIS), you quickly see its clever design and effectiveness as a tracking and collision avoidance tool. Recently, the Digital Yacht company released a white paper written by Paul Sumpner, chief technology officer, on the difference between the original configuration of Class B AIS and the newer Class B+ approach (the white paper is available here). In an email, Digital Yacht’s Nick Heyes explained the timing: “The [B+] standard was actually only ratified last year under IEC 62287-2:2017. It then takes us a while as manufacturers to complete, test and deliver to market a product. We’ve just done that with our new AIT2500 and AIT5000, which we launched at METS.”

The reason this is important for voyagers is that the improved Class B+ provides better capabilities to Class B users for visibility in busy traffic environments and for tracking via satellite.

AIS transponders come in two flavors. Class A units are for commercial vessels of 300 gross tons or greater, or which carry 12 passengers or more. Smaller commercial vessels, fishing vessels and pleasure craft use Class B transponders. The main differences between Class A and Class B are the frequency of updates and the broadcast power level. All AIS units operate on two VHF frequencies (161.975 MHz and 162.025 MHz).

For Class A units, the number of vessels that can be handled at once is impressive. As per Sumpner in the white paper, “the system … allows up to 4,500 ships to work within close proximity of one another.” How can this many vessels manage to broadcast their AIS data without interference? From the start, Class A units have used an ingenious approach called self-organizing time-division multiple access (SOTDMA). All vessels in range organize their transmissions by each claiming a 26.6-millisecond time slot. If a slot is occupied, then the AIS unit slides over to another slot. That’s the “self-organizing” part of Class A, and the system minimizes conflicts.

The original scheme used by Class B was different. It employed “carrier sense” slot allocation, which is another way of saying that Class B units, like children at a party of talkative grownups, had to wait until a slot opened and then broadcast their data. As Sumpner writes: “Occasionally a Class A transponder will ‘steal’ a time slot from a Class B transponder and the system is designed that Class A transponders always take priority over Class B, so the Class B transponder will have to delay its transmission and start listening again for another empty slot.” The result of this is that a Class B vessel in a crowded harbor might not transmit for 30 seconds or so. If it is a fast-moving vessel, that delay can represent a significant distance traveled.

The new AIS standard, Class B+, makes use of the same SOTDMA technique like Class A. This means that Class B+ units will have the ability to reserve a broadcast slot, making sure that it can get its data out — even in situations with plenty of traffic.

Another part of the upgrade is that Class B+ units will also take into account the speed of the vessel and increase the reporting time. More from Sumpner: “Another feature that the new Class B+ technology has taken from Class A, is the increased and automatic changing of transmission rates depending upon speed. Unlike Class A, the update rate is unaffected by whether the vessel is maneuvering, but as the vessel’s speed increases, the number of transmissions increases so that other vessels get a clearer and more up-to-date view of where the boat is.”

And the final upgrade to B+ is an increase in the transmission power. Original Class B units transmitted at 2 watts. B+ uses 5 watts, increasing the coverage area for B+ and making its signals more easily picked up by AIS-receiving satellites, thus allowing tracking worldwide on web-based tracking sites like AISLive and Marine Traffic.

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