Automatic Identification System (AIS)

Every now and then boats would bump into each other. As a result, the Automatic Identification System, AIS, was invented. AIS is a method for a boat to tell the other boats around it information such as its position, course and speed. This is done using packets of digital information sent over specific VHF radio frequencies. Because this information is sent over VHF it has the same range limitations as voice VHF, around 20 nautical miles. How this actually works is quite interesting so I'll run you through the process.

To make sure boats don't broadcast their location information at the same time and talk over each other, AIS uses Time Division Multiple Access, TDMA. More specifically, AIS uses a version of TDMA called Self-Organising TDMA.

With Self-Organising TDMA, AIS units listen first before transmitting any information to determine which time slots are currently allocated to other boats. One minute is divided into 2250 time slots, each around 26ms long. Once the time slot to use has been decided, the unit will start transmitting packets of data during only that time slot and continue listening for information about other boats during all other time slots. The frequency with which a boat will transmit its position information depends on how fast it is travelling. This can be as often as every two seconds when travelling quickly or as infrequently as every three minutes when at anchor. As AIS has a range of about 20 nautical miles, clusters of boats within range of each other form a cell based on these agreed time slots.

It is possible for a boat to select a time slot and then travel to an area where another boat has chosen the same time slot. This will cause them to transmit at the same time but this generally isn't a huge problem as the boat closest to you will have the stronger signal and override the fainter signal. The closer boat is obviously the one you need to know about! In addition to this, AIS units will stop using its chosen time slot after a random timeout of between 4 and 8 minutes and then also randomly select a new slot so the overlap will only be temporary. As you may have already deduced, it is actually impossible to have more than 2250 boats within a 20 miles radius and not have collisions!

The AIS unit in Renko is the Raymarine AIS700 which is a dedicated AIS transceiver, but it is also possible to have AIS built into a VHF radio as a combined unit. This often makes sense as they share a lot of technology. The downside is that if the radio has a single antenna then it must be tuned as a compromise between the VHF voice frequencies and the frequencies used by AIS (around 162 Mhz) For comparison, VHF channel 16 is 156.8Mhz.

There are two main types of AIS units, class A and class B. Class A units transmit at higher power than class B and they are generally only used on large commercial vessels.

AIS was originally designed to help ships avoid colliding. With course, speed and rate of turn information, AIS units are able to identify potential future collisions in a way that standard radar can't. Eventually companies realised how useful AIS information is for managing their fleets though and as a result satellites were developed and launched to listen to those same radio-frequency AIS transmissions. There's even an AIS receiver attached to the International Space Station! These satellites can then send this information back to a land-based server for access over the internet. MarineTraffic.com is an example of one such service. For boats near the shore, it is also possible for an enthusiast to set up a radio base station that uploads received position information to the internet.

When you first buy your AIS unit it needs to be configured with your Maritime Mobile Service Identity, or MMSI. AMSA is the organisation responsible for recording and issuing MMSIs in Australia. You need to have your Radio Operators Certificate of Proficiency in order to get an MMSI which means you will need your radio license in order to install AIS on your boat.

For example, the Raymarine AIS700 on Renko is configured by connecting my laptop to it and running the software ProAIS2 which can be downloaded from the Raymarine website.

AIS has quickly become very popular and joined the ranks of radio, radar, GPS and sonar on the list of valuable aids to navigation. Not every boat has AIS installed, so it of course can't be relied on to avoid collisions, but it does give you one extra chance of becoming aware of that boat you might have missed and it gives them one more chance of seeing you. It also allows you to see that blip on your radar or those lights on the horizon and know if the boat is a fishing trawler or super tanker.

When you first see a boat on AIS often it will be labelled with only the MMSI of the boat. This is because the boat's changing position information is transmitted much more frequently than static data such as the vessel name and type. That information is only broadcast every six minutes so it may be some time before you see that MMSI magically change into a vessel name.

Another great use of AIS is simply to get to know the name of a vessel within sight before calling them on the radio. You are always much more likely to get the attention of a boat when calling it by name rather than "sailing vessel off Point Danger."