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Everything there is to know about AIS

Everything there is to know about AIS

AIS has been used in commercial shipping for many years to avoid collisions at sea. However, due to its many advantages it is also becoming increasingly popular in recreational shipping. Find out about the advantages of an AIS device, why it is a useful addition to a radar, which AIS transponder you should choose and how to install it on board. You can discover our range of AIS devices and matching accessories here.

What is AIS?

AIS stands for Automatic Identification System. AIS devices consist of a GPS receiver and a data radio unit. The AIS system enables ships all over the world to identify each other and transfer important information to other ships, but also to land stations and traffic centres on the coast. In order to ensure that AIS data can be used worldwide, the International Telecommunication Union has specified standardised message types or telegrams that every AIS system must be able to receive or send, provided that the device type is capable of receiving the respective telegram. The most important telegram types in everyday life are ship data, position, speed and course.

What is AIS for?

The AIS system was originally developed for commercial shipping to avoid collisions at sea. AIS on board ensures better planning and decision making. With it the current traffic situation on the water can be monitored dynamically. Course and speed changes of nearby ships are automatically displayed and it is possible to determine exactly when two ships will have the shortest distance to each other, how big the distance will be and how long it will take until that happens. Necessary manoeuvres can easily be coordinated using a marine radio thanks to the transmission of MMSI (Maritime Mobile Service Identity) numbers. When using an AIS-capable chart plotter or alarm sensor, alerts can be set to indicate, for example, when a ship is approaching within one mile or when a collision could occur at the same speed and bearing at a predetermined time.

Nowadays, AIS is increasingly used on small ships that are not required to carry such equipment, for example in recreational shipping. With a low-priced AIS receiver you can display the ship data, position, course and speed of surrounding ships. Your own position is not displayed. AIS is a practical way of "looking around the corner" on busy areas of large rivers, and avoiding collisions or dangerous manoeuvres. When you buy an AIS receiver, you should check whether it is a 1 or 2 channel receiver. AIS data is transmitted alternately on two channels. A 1-channel AIS receiver takes longer to update than a 2-channel receiver. These days it is almost only possible to buy two-channel receivers. In addition to increased safety on the water, AIS also allows you to better plan the use of locks and mooring areas.

Transmitting AIS Live ship positions to land stations and traffic centres on the coast enables the monitoring and guidance of ship traffic. For coastal states, AIS also allows to monitor illegal fishing, by using data on vessels and cargo.

Furthermore, AIS data can be viewed by the general public via MarineTraffic & Vesseltracker. You can even turn your tablet or smartphone into an AIS chart plotter thanks to the MarineTraffic App. All that is needed is mobile coverage and a data plan. However, MarineTraffic depends on volunteers all over the world who send the data received via their receiving station to marinetraffic.com. This can lead to incomplete information. Depending on how good the mobile data transmission currently is, the retrieved data could be a few seconds old when you receive it, making it not 100% reliable.

How does AIS work?

Transmitting AIS Data

AIS data is transmitted by the vessel using AIS transmitters over two VHF frequencies reserved for this purpose (AIS 1: 161.975 MHz, channel 87 B and AIS 2: 162.025 MHz, channel 88B) in a fixed time frame via HDLC data protocol. A total of 4500 time slots - 2250 slots per channel - are available per minute, to which an AIS device is synchronised via its integrated GNSS receiver.

Receiving AIS Data

AIS data can be received and processed free of charge by AIS receivers. An appropriate display is required for processing and using AIS data. This can be a chart plotter with electronic sea chart, a radar with AIS software or a suitable PC programme. Connection to a chart plotter can be carried out via NMEA0183 or NMEA200, depending on the device. If you opt for PC based processing, an additional NMEA input for GPS data is helpful - some AIS transmitters transmit their own GPS position together with the received AIS data sets.

Find out more about NMEA »

Which data is transmitted via AIS?

Depending on whether an AIS transceiver is installed on a sports boat or commercial ship, the transmitted data can be very different. The following data can generally be transmitted via AIS, but is used to its full potential in commercial shipping. In pleasure boating, MMSI number, callsign, position, course and size are usually all that is required.

Static Data

  • Vessel's name
  • Call sign
  • MMSI number and international call sign
  • IMO number
  • Type of vessel (sports boat, tug, freighter, tanker, passenger ship, SAR)
  • Ship size

Dynamic Data

  • Position of the vessel (LAT, LON)
  • Speed Over Ground (SOG)
  • Course Over Ground (COG)
  • Coordinated Universal Time (UTC)
  • Navigation status (using engine, using sails, at anchor, moored, restricted manoeuvrability, etc.)
  • Heading
  • Rate of Turn (ROT)

Travel-Related Data

  • Current maximum static draught in dm
  • Port of destination (UN/LOCODE)
  • Planned arrival time (ETA)
  • Specification of cargo category (dangerous goods class), if applicable

Different types of AIS systems - Which AIS is right for me?

There is a difference between AIS receivers, with which you can see information about surrounding boats, but cannot transmit your own, and AIS transceivers, also called AIS transponders. AIS transponders receive the AIS data of the surrounding boats and at the same time transmit the data of their own boat. Depending on the area of application of the AIS transceiver, very different data can be important. There are different types of AIS transceivers, the use of which is mainly divided into professional and recreational shipping.

Class A transceiver

International maritime vessels of more than 300 GT (gross tonnage) and certain passenger ships are subject to the SOLAS Convention. These ships must be equipped with Class A transceivers. Class A AIS transceivers have a transmission power of up to 12.5 W, which is greater than class B equipment used on recreational craft. Class A transceivers can therefore send and receive data over longer distances. The transmission of AIS data is also more frequent than with class B transceivers. The exact frequency depends on the ship's speed and manoeuvring status and ranges from every 2 seconds to every 3 minutes.

Class A AIS transceivers have a data interface, a so-called pilot port, which, for example, allows pilots to access traffic conditions and navigation data.

Transmission frequency AIS signal

Vessel speed and manoeuvre status

2 seconds

  • Speed more than 23 kn
  • Speed 14-23 kn + course change

3 1/3 seconds

  • Speed up to 14 kn + course change

6 seconds

  • Speed 14-23 kn

10 seconds

  • Speed up to 14 kn
  • At anchor or moored + faster than 3 knots

3 minutes

  • At anchor or moored + not faster than 3 knots

Class B Transceivers

For sports boats used only for recreational purposes or fishing, there is no equipment requirement for AIS transceivers. However, since an AIS transceiver on board has many advantages, so-called Class B AIS transceivers are available. Class B AIS transceivers are generally less expensive than Class A transceivers because there are less stringent operating requirements. Class B transceivers are scaled down to include the most important data. Often only the vessel's MMSI number, current position, course and size are transmitted. This makes Class B transceivers very easy to operate. The name and dimensions of the ship must be programmed only once, after which the AIS transceiver is ready for use. Class B AIS transceivers have a lower reporting rate than Class A transceivers. They have a maximum transmission power of 2W. While Class A transceivers use a complex system (SOTDMA = Self Organising Time Division Multiple Access) to coordinate with other transceivers in the surrounding area to use the transmission frequencies, Class B transceivers usually just use the free time slots to transmit their data (CSTDMA = Carrier Sense Time Division Multiple Access). However, there are also class B AIS transponders that use SOTDMA technology. The transmission frequency of AIS data for Class B transceivers depends on the speed of the vehicle and the frequency load and ranges from every 5 seconds to every 3 minutes.

Transmission frequency AIS Signal

Vessel speed and manoeuvre status

5 seconds

  • Speed more than 23 kn (low frequency load)

15 seconds

  • Speed more than 23 kn (high frequency load)
  • Speed more than 14-23 kn (low frequency load)

30 seconds

  • Speed more than 14-23 kn (high frequency load)
  • Speed 2-14 kn

3 minutes

  • Speed up to 2 kn

Other types of AIS transceiver

In addition to Class A and B AIS transceivers for commercial and recreational shipping, there are also other types of AIS transceivers. There are, for example, inland AIS transceivers which are on a par with Class A transceivers but still have options for inland navigation and send other data. AIS base stations are used by traffic centres on land to monitor shipping traffic. AtoN (Aids to Navigation) transceivers are installed on buoys and other navigation indicators. You can send information about their type, description and position. So-called virtual AtoNs are used for marking restricted areas or hydraulic engineering structures with an area-wide coverage. All you need to do is install an active AIS transmitter on a buoy, which transmits the position of all the other marker buoys in the area. Aircraft and rescue helicopters operating with SAR also use special mobile AIS transceivers.

What exactly is SOTDMA?

SOTDMA stands for Self Organising Time Division Multiple Access. This system is used in Class A transceivers but also in Class B AIS transmitters. Compared to the simple Class-B CSTDMA transceivers, which usually have a transmission power of 2 watts, the Class-B SOTDMA transceivers normally work with a transmission power of 5 watts. Transmission of the AIS signals is also faster with SOTDMA transmitters and occurs at best every 5 seconds instead of twice per minute with conventional CSTDMA transponders.

Another advantage of AIS transponders with SOTDMA technology is active participation in the time slot process. This is particularly advantageous in busy areas, as both SOTDMA transponders and Class A AIS transmitters are able to allocate time slots, whereas Class B CSTDMA transponders must always wait for a free transmission time.

AIS or Radar?

The advantage of an AIS device over an on-board radar is that the reception of AIS signals is not impeded by land tongues, buildings or islands. It is also easier for inexperienced users to identify targets than on a radar image. On inland waterways, transceivers are set up in curves that are sealed off from radio signals, and also transmit AIS signals over mountains.

Nevertheless, an AIS device cannot replace a radar unit. Because AIS data is not always reliable. On the one hand, not all ships are equipped with an AIS device or have it switched on. Smaller vehicles in particular often do not have an AIS transceiver and some military vehicles also do not transmit AIS signals. On the other hand, ship position data is based on GPS technology. Data is transmitted at regular intervals, which may result in minor deviations from the actual position of the vessel.

Find out more about radar systems »

AIS signal range

The AIS signal range depends on the antenna height. For ship-to-ship connections, the range is approximately 20 nautical miles (37 km). Depending on the antenna height, coastal stations receive signals within a radius of 50-100 km.

Which antenna for AIS?

Any conventional VHF antenna can be used for receiving AIS data. When installing the antenna, make sure that it is at least 3m away from the radio's VHF antenna so that the antennas do not interfere with each other.

If you want to avoid having a second antenna on board, you can use the radio antenna for AIS reception by using an antenna splitter (also called an AIS splitter or duplexer). An AIS splitter ensures clean reception and transmission. While the radio is transmitting, the AIS receiver does not receive and vice versa. Never connect the AIS receiver and the VHF radio to the same antenna at the same time, i.e. without splitters! Doing so can lead to damage to the radio transmitter. For radios transmitting at 25 W, the AIS receiver may also burn out.

Choose your VHF antenna now »

Find out how to install your AIS with external splitter

Many newer AIS transponders have an integrated VHF splitter, which means you don't have to buy a separate AIS antenna splitter.

Find out how to install your AIS with integrated splitter

To avoid having two VHF antennas on board, you can also choose a VHF radio with integrated AIS receiver instead of an antenna splitter.

To send AIS data you also need a GPS antenna. Combined VHF/GPS antennas are also available for sending and receiving AIS data. .

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