The operation of AIS base stations is defined by the equipment and test standard IEC62320-1 Edition 1.
AIS base stations use the FATDMA and RATDMA access schemes. FATDMA slot allocations are manually configured and broadcast to other AIS transceivers using data link management messages. RATDMA transmissions are typically used for non-periodic messages such as broadcast of safety related messages.
Both AIS devices and VHF radios use VHF antennas to transmit and receive data. Most smaller vessels either do not have the capacity, or owners the need, for a second VHF antenna. This means that an antenna splitter needs to be used to ensure there is no loss of data, and the antenna is managed in the correct way.
Antenna splitters offer a cost effective way to ensure the antenna is managed efficiently.
The antenna splitter monitors which device is using the VHF antenna and ensures it has access to transmit or receive. Most antenna splitters on the market ensure there is no signal loss when power fails on the vessel, and gives priority to the VHF radio over the AIS device.
The operation of AIS Aids to Navigation is defined the equipment and test standard IEC62320-2 Edition 1.
AIS Aids to Navigation use either the FATDMA or RATDMA access scheme depending on their hardware configuration.
An AtoN AIS unit operating in RATDMA (Random Access Time Division Multiple Access) mode uses its receiver to listen to both AIS frequencies for about one minute, and makes and stores a map of all the AIS "slots" [or message spaces] on the VHF data link [VDL]. It then looks for two free adjacent slots in which to send its [2-slot long] AtoN message 21 or meteorological and hydrological message 8.
RATDMA is ideal for many applications because the AtoN or weather/tide AIS unit can be placed at any location, and requires no reservation of slots by a base station. It can be used whether base stations exist in the area or not. Its drawback is that in order to transmit to the AIS slot map the unit must turn its receiver on for at least one minute before transmitting, and this is the main power consuming factor with an RATDMA AtoN or weather AIS unit.
An AtoN AIS unit operating in FATDMA (Fixed Access Time Division Multiple Access) mode will transmit in a pair of slots which are reserved by an AIS base station. Ships receive a message from the base station, indicating that certain slots are reserved. The ship AIS transponders store this reserved slot information in their slot maps, and do not transmit in these slots. The FATDMA AtoN is programmed to transmit in two consecutive slots of those reserved by the base station. It is possible to "re-cycle" slots by having a number of AtoN units use the same pair of slots, but use them sequentially. FATDMA allows greatly reduced power drain for an AtoN AIS unit, because no receiving period to build a slot map is needed.
AIS receivers pick up all Class A and Class B broadcasts, however, unlike transceivers they do not transmit any AIS data. They are designed to see, but not be seen, aimed squarely at the leisure market for mariners who want to enjoy the fun aspects of AIS, but without the full AIS benefits.
As with Class B and Class A transceivers, AIS receivers use the VHF antenna to receive data, however, there are no connections to a GPS antenna as the device does not transmit its position, so does not require a connection.
Receivers allow you to see the position and other details from AIS devices within range, but they are not able to see you. AIS receivers can have the same outputs as any other AIS device, namely NMEA 0183, NMEA 2000, USB, wifi and RS232, which allows integration too many display options.
The operation of AIS Search and Rescue transceivers is defined in the equipment and test standard IEC61097-14 Edition 1.
AIS SARTs use only the modified-SOTDMA access scheme specifically defined for burst transmission in low volume emergency beacon applications.
SARTs use Pre Announced Time Division Multiple Access (PATDMA) as they continuously transmit but don’t receive.
Once activated, the SART sends its position and heading eight times in a minute. The device sends the information this often to ensure that the life raft is at the peak of the wave on at least one of the transmissions, maximising range. SARTS also send the SART safety message 'SART ACTIVE' every four minutes.
SARTs do not receive any form of data, but keep transmitting data. They are designed to work in an emergency situation.
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