AIS part 2 – Class B receivers, the choice is yours

admin | February 11th, 2010 - 13:38

nmea_p2 You’re probably aware by now that the original (Class A) Automatic Identification System was designed for commercial shipping. Class B AIS was designed to be simpler (cheaper) to implement and to be suitable for recreational vessels. It is compatible with Class A systems and available either as “Receive Only” or as a “Transceiver”.

In this post we’ll described a range of options for “Receive Only” systems, typically the initial choice of most yachties.

AIS uses two VHF channels to provide redundancy (in case of a failure, one channel might be locked) and responsiveness (you can transmit twice the information). It is a very cleverly designed system which allows thousands of boats to (almost) simultaneously transmit their position without interfering with another (explanation).

RayMarineAIS250 While single channel AIS receivers are available, dual channel units are clearly superior and the price difference is relatively small. A popular unit is the Comar unit (pictured above). Of course to be able to use it you’ll need two additional items, a VHF antenna and a way of displaying the vessels picked up by the receiver. The usual way is to connect the receiver to the boat’s chart plotter via the unit’s NMEA connection.

To save on installation cost, you might consider a unit with a built-in antenna splitter, which allows an AIS unit to share an existing VHF radio antenna. Both Comar and Raymarine have such units in their range.

One interesting newcomer to the scene is the Digital Yacht ANT200, pictured in part one of this series. It is designed to mount on the rail just like a GPS receiver and includes a stubby VHF aerial. It will also combine the signal from the boat’s GPS receiver with the AIS NMEA connection so that only a single connection to a chart plotter is required.

Dual channel Class B receivers start around A$500.

Manufacturer’s web sites:

Comar
Raymarine
Digital Yacht

In our next episode, we’ll discuss the trend to build AIS receivers into radios and chart plotters.

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