What is AIS and how it works
Theory and practice

What is AIS and how it works

All details about a system that can save, or at least make the life of a yachtsman much easier.
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The number of vehicles is constantly increasing, not only on the road. Recently the world's civil fleet has been replenished by about three thousand vessels with more than 100 tons of displacement each year. As the number of smaller plankton vessels increases, one can only guess, but when one observes a forest of masts in another Turkish marina, one can easily think that yachts there every day are multiplied by simple division - like infusion.

Of course, with such a rapid growth of the world fleet of ships will cross paths more and more often.

Mathematical statistics transparently hint that not all these meetings will end in a safe distance, and practical experience shows that radars alone will not solve this problem.

Mainly to reduce the risk of ship collisions in 2000, AIS - Automatic Identification System, i.e. Automatic Identification System was developed. Its functionality turned out to be so successful that only two years later the International Maritime Organization demanded that AIS terminals be obligatory installed on all cargo ships with over 500 register tons of displacement, on «trucks» over 300 tons on international voyages and on all ships for passenger transportation regardless of tonnage.

Unlike radars, which can detect the appearance of large floating objects near the ship and roughly estimate their current direction and speed, AIS provides much more detailed and accurate information about the navigational situation.

To better understand the capabilities of the new system, let's first understand how it works.

The AIS ship module is a digital VHF transceiver connected to ship navigation systems. Depending on the ship's speed, it automatically transmits the following operational information every 2-10 seconds (every 3 minutes at a berth): MMSI identification number, navigation status («at anchor», «in motion under the engine», etc.), current coordinates, true course and speed, angular rotation speed and precise time mark.

In addition to dynamic data, static data is transmitted every 6 minutes: IMO ship identification number, its type, name, radio call, dimensions, type of positioning system (GPS, GLONASS, LORAN) and even position of its antenna relative to the bow of the ship. Route information is transmitted with the same frequency: destination with estimated arrival time, draft, cargo category and number of people on board. In addition, in the event of a threat to the safety of the ship, it is possible to send text messages from the ship that are entered manually.

The received information can be deduced on the terminal in the form of the table with the information on ships being nearby, and also in the form of their symbols superimposed on navigating charts (for example, in a chart plotter) - clear business, in this case it is much easier to estimate a mutual arrangement and dynamics of movement.

In short, according to AIS reports, the captain can absolutely accurately assess the current navigation situation. By the way, radio exchange in the system is carried out in the range of 162 MHz, that is at a much lower frequency, compared to radar radiation. Longer radio waves are able to bend around obstacles such as large ships and small islands, so the AIS range is pleasantly impressive. Under favourable conditions, it can reach over 40 miles, but keep in mind that the antenna installation height is crucial here, as with other on-board transmitters.

For yachtsmen, at least those whose boats are not listed in the charts of Forbes magazine, the subtlety of the system is that only the terminals of simplified version marked «as Class B»are allowed on ships of less than 300 tons displacement.

They have a significantly reduced transmitter power (2W vs. 12.5W), which limits the range of receiving their messages to about five miles. Another disadvantage is a simplified algorithm for data transmission, which allows you to send information only if there is free space on the air during radio exchange of older brothers, equipped with class A terminals. The trick here is that at any given time on any of the two AIS channels it is possible to transmit a single block of digital data, and class A devices are able to negotiate the order of their issue in advance.

However, you have to agree: despite this discrimination, being in a nightly troubled sea, it is very pleasant to know that on a passing supertanker nearby, a watchman is sure to know about the presence of your 45-foot yacht.

There is another way to use AIS, and it is to install a receiver that does not allow you to send any data at all, but is able to track the movements of all ships equipped with complete terminals. By and large, even a separate device is not required for this, as manufacturers such as Icom and Standard Horizon began to equip with this function top models of mounted VHF radio stations.

Convenient, compact, not expensive, but there is one big «but» - on a small screen with low resolution, even a text table is difficult to place, and even to build even the most primitive likeness of the map ...

That is why AIS receivers were developed, which do not show graphic information at all, but can convert data into standard NMEA protocol packages, understood by the vast majority of chart plotters. In addition, some of them can connect to computers via USB, or even transfer data to mobile gadgets running Android or iOS via Wi-Fi. Such devices are produced, for example, Weather Dock.

By the way, when installing AIS equipment even in an additional antenna there is no need for 100% due to the fact that it works in the same frequency band as an on-board radio. However, keep in mind that the splitters used to connect to the antenna of two different devices, as a rule, slightly reduce the signal level, and in case of problems with a single antenna, you will lose two security systems at once.

It would be naive to believe that such an advanced information exchange system was designed solely to assist the helmsman in operational manoeuvring. AIS is also tasked with global ship movement control for the benefit of a variety of shipping companies, traffic control centres and government services that may require information on the location of certain ships or cargo. For this reason, AIS equipment can be based not only on ships, but also on shore stations, many of which are connected to a global network.

Well, in order to use the system more efficiently for search and rescue of sailors in emergency situations, there are emergency buoys capable of transmitting AIS information with higher priority. There are also so-called virtual buoys - this is the only type of device in the system whose actual location may not coincide with the coordinates in their messages. As a rule, they are transmitters installed on the shore, warning passing ships about dangers like badly visible rocks or beaconless capes, which stand out far out at sea.

It should be said that AIS receivers are placed even on satellites. It is only on the Earth's surface that its signal radius is limited by visibility to the horizon, and in space it can be received from hundreds of kilometers without any problems. Today, there are more than a dozen spacecrafts around the planet that are engaged in monitoring maritime traffic.

It is particularly gratifying that global ship movement data can be accessed without having to be the owner of a shipping company or a secret service agent. The information is available on a paid basis (for example, in the full version of Google Earth), but in a somewhat truncated form it can also be seen for free, for example, on the resource www.marinetraffic.com, whose interactive maps and user-friendly interface are mapped to many other sites of maritime subjects.

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