Many have heard of the AIS system. But how many people know its principle of operation and what class should be installed on your yacht? Let's figure it out now!
What is AIS, or Automatic Identification System? Many people familiar with the marine theme from land think that this is a system that is made so that we can see the position of all ships anywhere on MarineTraffic. In fact, this is one of those cases where the original use of the technology has perhaps surpassed in importance what AIS technology was originally created for.
History of AIS
AIS was invented in the 90s with the aim of improving navigation safety and collision avoidance. Since 2002, the IMO (International Maritime Organization) SOLAS (Safety of Life at Sea) requires the installation of AIS on all vessels over 300 tonnes displacement that go into international waters.
The idea was that before AIS, the only way to warn the crew of a collision threat at night or in bad weather was radar, but the radar only shows that there is some piece of iron floating over there.
The advantage of AIS is the ability to obtain information not only about the coordinates of another vessel, but also many other details - what kind of vessel it is, its speed, whether it is going straight or turning, and much more.
How AIS works
But how does it all work? All AIS transponders receive and transmit data on the same frequency in the VHF band, approximately 160 MHz.
In this range, radio waves travel roughly in a straight line, like regular FM radio, so the range is determined by the curvature of the earth and the height of both antennas above sea level.
For conventional ships, this is approximately 10-20 nautical miles (19-37 kilometers). This does not mean that radio waves do not fly further - they still fly, but it means that they can no longer be received at sea level.
Problems in use
Then the interesting part begins. The frequency is one, but there are many ships around. What happens if several transponders start transmitting data at the same moment? Nothing good, they will mix, and it will be impossible to make out anything.
In order to reduce the likelihood of this, AIS uses a rather sophisticated protocol - a type of TDMA (time-division multiple access), when different stations try to transmit their data at different intervals.
Given that there are usually not too many ships within a 10-20 mile radius, this mostly works. In the worst case, when two transponders transmit simultaneously, most likely, one will be closer than the other, respectively, the signal from it will be stronger, and the more distant and weaker signal will be suppressed. Very good for collision avoidance, closer is more important.
An elegant solution to a problem
But then people figured out that you can simply receive all AIS signals, and know everything about the movement of ships completely free of charge. One problem is the radius of 10-20 miles from the antenna.
The situation was improved by the fact that on the coast it was possible to put the antenna higher, and thus increase the radius, but everything that happens far from the coast remained a mystery. But soon, since about 2005, people realized that radio waves fly not only to the sides, but also upward, they began to try to catch them from satellites - and they succeeded!
Sites like MarineTraffic immediately appeared that show you all ships anywhere in the world. It would seem that now everyone can see everything - but not everything turned out to be so simple.
Most satellites that receive AIS signals fly at altitudes of the order of 200-400 kilometers, and receive signals in a radius of somewhere on the order of 200 kilometers. It would seem that the more - the better? It quickly became clear that in areas of busy shipping, and especially near large ports, satellites work poorly, and there is no replacement for a ground receiver.
Class A and Class B
As mentioned earlier, on ships with a displacement of more than 300 tons, making regular international voyages, and on ships of any tonnage, carrying out passenger transportation, the installation of an AIS system is mandatory. Moreover, they should only be equipped with full-fledged Class A equipment.
For all boaters whose boats do not fit into these categories, there is a class B. This is a simplified version of the transceiver. The difference between AIS class B is that the power of the system is not 12.5 W, but only 2 W, so the range is about 5 miles, and not up to forty, as in the "adult" version.
Another difference lies in the fact that priority in data transmission is always given to Class A devices, and small vessels use the gaps in the air.
It is quite difficult to visually distinguish the class of the device. Often the class is indicated directly on the transponder body. In any case, marine avionics are evolving just as fast as in other areas, so yesterday's innovations seem to be a rarity today. Class B transponders can be made either as a full-fledged module with a display, or as a unit that can be connected to the chartplotter via the NMEA protocol to display visual information on its screen. There are also Wi-Fi modules that connect to a laptop, tablet or smartphone.
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