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What is AIS?
AIS is the acronym for the “AUTOMATIC IDENTIFICATION SYSTEM”.
It is a system where ships automatically broadcast their position, course and speed, together with other information about their name, call sign, dimensions, ship type, destination and ETA, on a pair of radio channels in the Maritime VHF band. The system is part of the Safety of Life at Sea system and is intended to assist in reducing collisions between ships.
With suitable equipment, all the participating ships obtain a real-time radar-like display of the ships operating in their area. Better than radar, the AIS system provides name and call sign details of the target ships on the display which greatly facilitates voice communication between them. With suitable equipment, you can monitor these transmissions at home and keep track of shipping movements within VHF range.
It is a requirement for ships:
1) over 300 gross tonnage and upwards engaged on international voyages
2) cargo ships of over 500 gross tonnage and upwards not engaged on international voyages
3) all passenger ships
Some VTS stations, coastguard stations and other commercial vessels such as pilot boats are also fitted with the system.
What channels does AIS use?
AIS uses two frequencies in the Marine VHF band: 161.975 and 162.025 MHz. (international marine channels 87 and 88). All ships use both frequencies and successive message transmissions are radiated on alternate channels. Professional AIS equipment monitors both channels simultaneously. This doubles the channel capacity and also combats interference. Monitoring a single channel with means that you only hear half of the transmissions. Since the transmissions are often made at intervals of only a few seconds, this does not significantly the effectiveness of a system using only one channel. Messages are transmitted at 9600 bits per second using GMSK modulation. Each message is typically only 30ms (1/30 second) long – scarcely more than a click.
What do I need to receive AIS transmissions?
You need a suitable antenna, a receiver with a discriminator output, capable of tuning to one or other of the AIS channels, and a PC with a sound card. You also need suitable software. Some monitoring/scanner type receivers are fitted with discriminator outputs. If your does not, take a look at www.discriminator.nl where Rene has catalogued a huge array of simple modifications to many models of receiver to provide the discriminator output that you will need to receive AIS transmissions. The AIS transmissions use a somewhat wider bandwidth than normal VHF FM communications. You may find, if your receiver has a narrow IF bandwidth, that you will only decode a proportion of the messages. A few receivers have a choice of IF bandwidths; in such a case, choose the bandwidth suitable for 25 kHz channel spacing.
Is it legal to monitor AIS transmissions?
The regulatory framework regarding radio monitoring varies considerably from country to country. In some countries the regulations are very liberal, in others they are very strict. In many, the regulations are strict but are not enforced strictly except in cases of abuse or criminal intent. In deciding whether such monitoring is legal in your own regime, remember that the transmissions from ships are in the nature of a broadcast (there is no destination address in the messages) and are not in any sense private messages. If your interest stems from a boating connection, then you would have a strong safety case for monitoring this traffic.
I am not near the sea. Can I still monitor the traffic?
AIS transmissions are subject to VHF line-of-sight propagation. It all depends on the terrain between you and the shipping.
How can I tell if there are AIS transmissions in my area?
Tune to one of the two channels and select AM mode (normally you would use FM mode for this traffic). If there are AIS messages around, every few seconds you will hear a characteristic ‘pop’ sound. It sounds quite different from static clicks. You should hear some stronger and some weaker signals and the frequency of these pops will give you some idea of the AIS traffic intensity in your area.
This is what AIS sounds like on a receiver. It sounds different in the AM and FM modes. Right click the black square for play options. Your computer must be Flash enabled to hear the sounds. Untick play in the play options to stop the sound.
AIS in AM mode
AIS in FM mode
AIS Class B
AIS for Beginners
Naval ships and AIS
There is no requirement for naval vessels to transmit AIS, although the majority are capable of doing so. Generally naval ships do not want to advertise their position. They of course receive it as part of their electronic navigation system. Modern naval ships have the capability of transmitting spoof (false) AIS positions of their own (and probably other vessels) to confuse the enemy under combat conditions. Generally UK naval support vessels (RFA) transmit AIS.
Class B is defined is a variation in the Class A specification to allow the development of a smaller, cheaper more power efficient variant specifically for the leisure and small commercial market. It also has potential use for mounting on navigation buoys and other Aids to Navigation due to its lower power consumption and so can give the navigator an accurate position of the buoy at all times (important in the case of bars and spits that may move from one storm to the next). It only requires two receive channels, the DSC capability being performed by time-sharing the AIS channels at the appropriate times. The reporting rates are reduced to every 30s, and the standard position message that it sends (msg 18) has been carefully designed so that it fits into a single transmission slot. The power rating of the transmitter has been reduced to 1W, both to reduce the overall power consumption, and to reduce the potential impact that a large number of Class B vessels could have on the overall AIS system (the 1W rating reduces the coverage area compared with the 12.5W of the Class A). This does mean that only the closer Class B targets will be received, but as these are the ones that are likely to pose the most immediate threat, this seems to be a sensible compromise. In all cases the Class B specification has been carefully constructed so that it does not harm or degrade the functionality of the Class A units in the system.
The Class B is nearly identical to the Class A, except the Class B:
Has a reporting rate less than a Class A (e.g. every 30 sec. when under 14 knots, as opposed to every 10 sec. for Class A)
Does not transmit the vessel’s IMO number or call sign
Does not transmit ETA or destination
Does not transmit navigational status
Is only required to receive, not transmit, text safety messages
Is only required to receive, not transmit, application identifiers (binary messages)
Does not transmit rate of turn information
Does not transmit maximum present static draught