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AIS receivers can be classified by two key characteristics, the number of receivers and the receiver sensitivity. There are many other factors that influence receiver performance but these two are probaby the most important.
Number of receivers
This is either one receiver or two. AIS with one receiver will be amongst the cheaper models on the market and will suit certain budgets. Those with two receivers are usually more expensive and ensure that both frequencies are monitored all the time so that AIS transmissions are not missed.
Receiver sensitivity
Receiver sensitivity is quoted as a figure eg -112 dBm. The more negative the number the more sensitive the reciever. There is a practical issue to be considered here. Whilst the most sensitive receiver will pick up weaker transmissions this may not always be desirable. Because VHF reception is generally limited to line of sight reception there may be no need to have the most sensitive receiver available. More sensitive recievers cost more and they can present too much information and be subject to reception issues. In general sensitivity around -112 dBm should be adequate. The majority of receivers available today meet this specification.
AIS frequencies
The AIS receiver uses frequencies in the upper part of the maritime band. Normally channel 87B, AIS 1 (161.975 MHz) and channel 88B, AIS 2 (162.025 MHz).
VHF Antenna
It is important that the VHF antenna used for AIS should be well installed and since this is a line of sight system, the higher the better. Using an automatic antenna switch means it is possible to use the same antenna for both AIS and VHF use. However when transmitting on either the VHF Radio or the AIS transponder, AIS reception is not be possible.
AIS Transponder
The AIS transponder needs to transmit on a VHF frequency to broadcast the required information. As well as transmitting different data sets the transmitter powers of Class A and B transponders are different.
Class A transponders must have an output power of at least 12 watts and Class B transponders 2 watts.
There are subtle differences between the transmission technology used in Class A and Class B transponders. Class A transmission is organised with allocated time slots (known as SOTDMA). In Class B the transponder listens for when the network is free and then transmits (known as CSTDMA). Class B transponders must also listen for reservations from base stations and comply with these. For Class B this means that the probability of a transmission collision exists if two transponders are listening and here a free slot and then transmit simultaneously. Fortunately the risk of this is minimal and the big advantage of AIS is that because it is limited to Line of Sight a high degree of time slot reuse is achieved.
An AIS transponder needs very precise timing to function corectly, and because they are supplied with a GPS signal this timing is readily available.
The AIS system has many other functions to allow for the effective management of the network. These include the functions that can discard the most distant transmissions when slot capacity is in short supply.
Glossary
AIS - Automatic Identification System
CSTDMA - Carrier Sense Time Division Multiple Access
DGPS - Differential Global Positioning System
ETA - Estimated Time of Arrival
GPS - Global Positioning System
GALILEO - European equivalent to GPS
GNSS - Global Navigation Satellite System
IMO - International Maritime Organization
NM - Nautical Mile = 1852 mtrs
RX - Receive/Receiver
SOLAS - Safety Of Life At Sea
SOTDMA/STDMA - Self Organized Time Division Multiple Access.
VHF - Very High Frequency
VTS - Vessel Traffic Services (Like ATC but for ships)
AIS Technical
Autopilot
Magnetic Compass
NMEA
Satellite Compass
Weather Instrument
True Heading CTRX Carbon+ Class B Transponder
True Heading Carbon Pro Class A Transponder
True Heading Carbon+ AIS Receiver