AIS transceivers automatically broadcast information, suchas their position, speed, and navigational status, at regular intervals via aVHF transmitter built into the transceiver. The information originates from theship's navigational sensors, typically its global navigation satellite system(GNSS) receiver and gyrocompass. Other information, such as the vessel name andVHF call sign, is programmed when installing the equipment and is alsotransmitted regularly. The signals are received by AIS transceivers fitted onother ships or on land based systems, such as VTS systems. The received informationcan be displayed on a screen or chart plotter, showing the other vessels'positions in much the same manner as a radar display. Data is transmitted via atracking system which makes use of a Self-Organized Time Division MultipleAccess (SOTDMA) datalink.
The AIS standard comprises several substandards called"types" that specify individual product types. The specification foreach product type provides a detailed technical specification which ensures theoverall integrity of the global AIS system within which all the product typesmust operate. The major product types described in the AIS system standardsare:
Vessel-mounted AIS transceiver (transmit and receive) whichoperates using SOTDMA. Targeted at large commercial vessels, SOTDMA requires atransceiver to maintain a constantly updated slot map in its memory such thatit has prior knowledge of slots which are available for it to transmit. SOTDMAtransceivers will then pre-announce their transmission, effectively reservingtheir transmit slot. SOTDMA transmissions are therefore prioritised within theAIS system. This is achieved through 2 receivers in continuous operation. ClassA's must have an integrated display, transmit at 12.5 W, interface capabilitywith multiple ship systems, and offer a sophisticated selection of features andfunctions. Default transmit rate is every few seconds. AIS Class A typecompliant devices receive all types of AIS messages.
Vessel-mounted AIS transceiver (transmit and receive) whichoperates using either carrier-sense time-division multiple-access (CSTDMA) orSOTDMA; there are now 2 separate IMO specifications for Class B. Aimed atlighter commercial and leisure markets. CSTDMA transceivers listen to the slotmap immediately prior to transmitting and seek a slot where the 'noise' in theslot is the same or similar to background noise, thereby indicating that theslot is not being used by another AIS device. Class Bs transmit at 2 W and arenot required to have an integrated display: Class Bs can be connected to mostdisplay systems where the received messages will be displayed in lists oroverlaid on charts. Default transmit rate is normally every 30 seconds, butthis can be varied according to vessel speed or instructions from basestations. The Class B type standard requires integrated GPS and certain LEDindicators. Class B equipment receives all types of AIS messages.
What is AIS?
The Automatic Identification System (AIS) is a shiptransponder system that is currently used by most of the commercial shippingindustry.
How it works?
AIS uses the globally allocated Marine Band Channels 87& 88. FM modulation is not used. Each ship equipped with an AIS transpondersends out a packet every few seconds with information about the ship and itsvoyage.
AIS uses the high side of the duplex from two VHF radio"channels" (87B) and (88B)
- Channel A 161.975 MHz (87B)
- Channel B 162.025 MHz (88B)
The simplex channels 87A and 88A use a lower frequency sothey are not affected by this allocation and can still be used as designatedfor the maritime mobile frequency plan.
A stand-alone AIS receiver or the AIS receiver built in to aClass A or Class B transponder can pick up these radio signals and translatethem into a NMEA data sentence that can be understood by a computer with theproper software or by an AIS-enabled chart plotter.
Where to use?
Just about every commercial vessel will have an AIStransponder. Specifically, AIS is mandatory on all ships of 300 gross tonnageand upwards engaged on international voyages, cargo ships of 500 gross tonnageand upwards not engaged on international voyages and passenger shipsirrespective of size.
Other smaller commercial ships, such as fishing vessels,will equip themselves with AIS transponders voluntarily since there are majorsafety benefits to using AIS.
In addition, Class B AIS transponders are used by somerecreational vessels. All AIS Receivers are able to pickup both Class A andClass B transponder signals.
What is the range for AIS receivers?
Since AIS uses the same VHF frequencies as marine VHF, ithas similar radio reception capabilities which is basically line of sight. Thismeans that the higher your VHF antenna is mounted, the greater the receptionarea will be.
- Reception from Class A vessels that are 20 or even 30 milesaway on open water is not uncommon.
- Class B transponders use lower power for transmissions,therefore you can expect Class B vessels to be acquired when they are 5 to 7miles away.
Note: AIS has a major advantage over radar since it cansee ships within radio reception range that are behind large objects, such asother ships or points of land.
What kind of information is broadcast for each ship and howoften is it updated?
A Class A AIS transponder broadcasts the followinginformation every 2 to 10 seconds while underway, and every 3 minutes while atanchor:
- MMSI number - unique vessel identification number
- Navigation status - at anchor, under way using engine or notunder command
- Rate of turn - right or left, 0 to 720 degrees per minute
- Speed over ground - 1/10 knot resolution from 0 to 102knots.
- Position accuracy - differential GPS or other and anindication if RAIM processing is being used
- Longitude - to 1/10000 minute and Latitude - to 1/10000minute
- Course over ground - relative to true north to 1/10th degree
- True Heading - 0 to 359 degrees derived from gyro input
- Time stamp - The universal time to nearest second that thisinformation was generated
In addition, the Class A AIS unit broadcasts the followinginformation every 6 minutes:
- MMSI number - same unique identification used above, linksthe data above to described vessel
- IMO number - unique referenceable identification (related toships construction)
- Radio call sign - international call sign assigned tovessel, often used on voice radio
- Vessel name - name of ship, 20 characters are provided
- Type of ship/cargo - there is a table of possibilities thatare available
- Dimensions of ship - to nearest meter
- Location on ship where reference point for position reportsis located
- Type of position fixing device - various options fromdifferential GPS to undefined
- Draught of ship - 1/10 meter to 25.5 meters (noteair-draught is not provided)
- Destination - 20 characters are provided
- Estimated time of Arrival at destination - month, day, hour,and minute in UTC
Class B AIS transponders also provide static vesselinformation every six minutes however the information only includes:
- Radio call sign
- Vessel name
- Length and beam of vessel
- Type of vessel
Position updates for Class B transponders are broadcast lessoften than Class A transponders. Vessels going less than 2 knots transmitposition updates every 3 minutes while vessels traveling more than 2 knotstransmit position information every 30 seconds.
For this reason it is recommended that you considerpurchasing a true dual channel AIS receiver if you plan to use the AIS receiverin an area where Class B transponders are in use.
Differences Between Single Channel and Dual Channel AISDevices
All the units can receive AIS information from either AISchannel. The single channel device can only receive information from onechannel at a time but will automatically switch to the alternate channel if itdetects interference on the current channel.
The dual channel AIS receivers and all AIS transponders canreceive all AIS broadcast information from both AIS channels simultaneously andconsolidate the information from both channels into a single data stream. Theseare true dual radio receivers. This generally means you will acquire newvessels sooner with these receivers and you will also get the full informationabout a vessel in a shorter period of time.
In practical use, this means it can take up to 12 minutesfor the single channel device to acquire the full static information broadcastfrom an AIS transponder since the receiver only listens on one channel at atime. The dual channel receivers pick up all transponder broadcasts thereforethe maximum time to pick up the full static information broadcast is 6 minutes.The single channel AIS receiver also picks up position updates fromtransponders less often than the dual channel receivers since it only picks upevery other broadcast.
For Class A transponder broadcasts, this is normally not anoticeable issue however with the new Class B transponders - which broadcastwith less power (which means less distance) and position updates broadcast lessoften - there may be a noticeable delay in receiving a position update. Forexample, if a Class B vessel is traveling slower than 2 knots you would onlysee position updates every 6 minutes instead of every 3 minutes with a dualchannel receiver.
The dual channel AIS receiver also includes an integratedGPS module as well as a detachable GPS antenna. With this model you can receiveAIS traffic information and GPS position information as a single consolidateddata stream over the RS232 serial port or NMEA.
If I am only receiving AIS information from one channel at atime, does this mean the single channel AIS receiver will not pick up thetransponder broadcasts from half the ships in my area?
No, the AIS system uses two channels for redundancy. Shipsbroadcast information alternating between the two channels. Therefore, you willeventually pick up information for every ship, even if the AIS receiver can onlyreceive on one channel.
However, with the increased popularity of Class B AISTransponders, it may make sense to purchase a true dual channel AIS receivers.This is because Class B transponders broadcast position updates less often thanClass A transponders. The result with a single channel receiver therefore maybe that Class B vessels will "jump" across the screen as each updateis received.
How often do AIS transponders broadcast position and staticinformation reports?
Transponders send out position updates based on the speedover ground (SOG) of the vessel and updates occur at different intervalsdepending on whether a Class A or Class B transponder is used. Depending onwhether you are using a single channel or dual channel AIS device, reporting intervalswill vary based on the tables below:
Receiving Class A AIS Transponder reports
Ships Dynamic Conditions Dual ChannelReceiver Single Channel Receiver
Ship at anchor or moored 3 min 6 min
SOG 0-14 knots 10sec 20 sec
SOG 0-14 knots andchanging course 3.3 sec 6.6 sec
SOG 14-23 knots 6sec 12sec
SOG 14-23 knots andchanging course 2 sec 4 sec
SOG >23 knots 2sec 4 sec
Ship Static Information 6 min 12 min
Receiving Class B AISTransponder reports
Ships Dynamic Conditions Dual ChannelReceiver Single Channel Receiver
SOG <2 knots 3min 6 min
SOG >2 knots 30sec 1 min
Ship Static Information 6 min 12 min
Can I get more information on how the dual channel systemworks?
The best source for more information is to look at thedocuments related to the AIS standard. On the subject of dual channel support,the standard states:
The normal default mode of operation should be a two-channeloperating mode, where the AIS simultaneously receives on both channels inparallel. In order to accomplish this performance, the AIS transponder shouldcontain two TDMA receivers.
Channel access is performed independently on each of the twoparallel channels. For periodic repeated messages, including the initial linkaccess, the transmissions should alternate between AIS 1 and AIS 2. Thisalternating behavior is on a transmission by transmission basis, withoutrespect to time frames. Transmissions following slot allocation announcements,responses to interrogations, responses to requests, and acknowledgments shouldbe transmitted on the same channel as the initial message.
For addressed messages, transmissions should utilize thechannel in which messages from the addressed station were last received.
For non-periodic messages other than those referenced above,the transmissions of each message, regardless of message type, should alternatebetween AIS 1 and AIS 2. Base stations could alternate their transmissionsbetween AIS 1 and AIS 2 for the following reasons:
- To increase link capacity.
- To balance channel loading between AIS 1 and AIS 2.
- To mitigate the harmful effects of RF interference.
Although only one radio channel is necessary, each stationtransmits and receives over two radio channels to avoid interference problems,and to allow channels to be shifted without communications loss from otherships.
Using an AIS Receiver or Transponder with a Computer orChart Plotter
Many modern computer-based marine navigation softwarepackages can be configured to interpret the incoming AIS data stream andconvert the information into human readable information. These packages alsosupport plotting nearby ships on digital charts and can show your position (viaa separate GPS) in relation to the other vessels in the area that have AIStransponders.
Today most modern chart plotters support the connection ofan AIS receiver or transponder and are able to display AIS targets.
All networks that can handle NMEA serial input at 38400 baudand process VDM serial data will be able to handle AIS data. Many of our AISproducts also support NMEA 2000. Simply add the device to your NMEA 2000network and your other NMEA 2000 devices should be able to use the AIS dataassuming the device supports AIS.
What are the basic steps to install an AIS receiver?
The receiver has three connections.
One connection is for a standard marine VHF antenna. Thesome AIS devices use a BNC connector for the VHF antenna connection. If youneed to connect to typical VHF radio antenna cable that uses a PL-259 / SO-239connector, you'll need an SO-239 to BNC adapter.
The second connection is for data. Some models have a 9-pinserial port which can be connected to a computer serial port using a standardserial cable, which is typically included with the receiver. If your computerdoesn't have a serial port, you can use a USB to serial adapter to make theconnection.
The most of the current products use a USB connection tointerface with a computer and also have data cables to connect to a NMEA-basedchart plotter.
The third connection is for 12 volt DC power.
Once these connections have been made, simply configure yourmarine navigation software or chart plotter to utilize the new serial portconnection as an incoming listener serial data stream. Note that AIS receiversuse 38400 baud by default so be sure you configure your software program orchart plotter appropriately.
For Dual Channel AIS transponders and receivers, you willneed to also connect the GPS antenna and make sure it is placed in a locationthat has a clear path to the sky.
Please see the product installation documentation for fulldetails on installing your AIS device.
What electronic chart programs can handle AIS?
Most of recent software packages support AIS receiver data.
For mobile platforms such as the Apple iPad, a number ofapps are available.
It is possible to use AIS solutions with Windows and Mac OSX operating systems.
What type of VHF antenna do I need for my AIS receiver ortransponder?
The VHF antenna should fulfill at least the followingrequirements:
- Antenna type: Vertical radiator
- Antenna gain: 3 dB or similar
- Impedance: 50 ohm
Can I use an existing marine VHF antenna with an AISreceiver or transponder?
Yes, you can use a splitter to share your existing VHFantenna to your VHF/DSC radio and your AIS receiver. You must use an activesplitter that automatically disconnects the signal to the AIS device if atransmission is detected from the VHF radio.
Splitters that are designed to work with an AIS transpondercan be found on our transponder splitters page.
Caution: Do not use splitters designed for use with AISreceivers with AIS transponders (e.g. the Smart Radio VHF Antenna Splitter) oryou may damage your splitter and / or radio.
Since you have two receiving devices on one antenna, you mayalso experience some small amount of signal degradation with some splitters.While broadcasting on the VHF radio, you will likely see some interruption ofincoming AIS signals. Since AIS broadcasts from each ship are repeated everyfew seconds, this is not normally noticeable in your tracking software.
Caution: If you use a common T-connector to share a VHFantenna with a radio and an AIS device and then broadcast on the VHF radio, youwill damage your AIS device.
Another option is to use a dedicated VHF antenna that ismounted away from your VHF radio antenna. This will mean less interference andwill allow both the AIS device and the VHF radio receiver to get maximum receivingdistances. In order for the antenna to work properly, you must install your AISantenna at least 4 feet from any other radio antenna or vertical metalstructure and mount it as high as possible.