Buyers’ guide to AIS receivers and AIS transponders

¿What is AIS?

AIS is a system for ships identification. The acronym AIS corresponds to Automatic Identification System.

AIS was invented in the early 2000, and the class B standard (which is not compusory) appeared in 2006. There is currently no discussion about the utility that AIS provides, and its usage has been growing both in the sea and land, being nowadays an essential tool to ease traffic control and increase safety at sea, since all ships above 300 tons as well as all passage ships have to carry an AIS transponder. The European Union has recently passed a new directive that even makes AIS compulsory for fishing ships.

AIS can also be considered an standard, as it is included in Recommendation ITU-R M.1371-4. In order to transmitt AIS information, different means can be used, but the “official” information transmitted by a ship has to use either of the two maritime VHF channels reserved for AIS.

First important difference: Transponders and Receivers

Let’s start by the receiver, which is the simplest AIS device. With the AIS receiver we can see the messages sent by other ships. But they will not receive any signal from us. This equipment does not require certification of any type. Cheap receivers can be found in the market, but if we can give a recommendation it would be to choose one AIS receiver that is receiving both channels. If we have a single channel receiver we may be losing a lot of relevant information.

Another difference is whether the receiver is capable of decoding the only AIS or also messages between other ships (everything on the VHF Data Link, VDL).

Due to the fact that receivers do not transmit, there is no such difference between class A and class B.

Deciding wether we should install a receiver or a transponder will depend on:

- legal requirements: we may be subject to regulation that makes it compusory for our ship

- our budget: transponders are significantly more expensive than receivers

- our intention to collaborate in a more secure system by giving our position to other vessels. The value of AIS is higher the more ships use it.

Professional use: class A (SOLAS compliant)

Class A transponders are required by IMO (International Maritime Organisation) and SOLAS to be deployed in ships above 300 tons as well as passenger ships, since 2002. Ships transmitting with a class A transponder also use higher power and more frequency of transmission.

Recreational use: class B

Class B AIS is more recent and allows smaller vessels to participate into the AIS system sharing their position.

The maximum transmission power for AIS class B system is 2w.

Class B transponders have improved significantly in the latest years, both in capabilities and installation. Thanks to VHF splitters, the already installed antenna can be shared and this has a nice side-effect, which is that transmission will have a broader reach since the radio antenna is normally located at the top of the mast.

Data sharing capabilities has also improved a lot and today it is easier to connect an AIS transponder to a PC using the USB port (it used to be with serial port) and also make it compatible with marine plotters thanks to the NMEA-0187 and NMEA-2000 standards.

A quick comparison between class A and B can be found here

Details about the AIS messages can be found here

Setting up an AIS transponder

 Required elements:

- AIS transponder

- GPS dedicated antenna for AIS

- VHF antenna. It can be either exclusive for AIS or shared with the radio, with the help of an splitter. The splitter can be internal to the transpoder or external.

- Connect it to a message decoder and display. It can be a PC, plotter or display that can show the data. If you are looking for a cheap set-up you can use OpenCPN, the open source navigation software, together with a laptop.