Have questions about AIS? We have answers!

This FAQ page will hopefully answer your questions. If it does not, contact us here.

AIS General Information

Differences Between Single Channel and Dual Channel AIS Devices

Using an AIS Receiver or Transponder with a Computer or Chart Plotter

Purchasing Products from Milltech Marine

AIS General Information

What is AIS?

AIS or the Universal Shipborne Automatic Identification System (AIS) is a ship transponder system that is currently used by most of the commercial shipping industry.

AIS uses two marine VHF channels. Each ship equipped with an AIS transponder sends out a packet every few seconds with information about the ship and its voyage.

A stand-alone AIS receiver or the AIS receiver built in to a Class A or Class B transponder can pick up these radio signals and translate them into a NMEA data sentence that can be understood by a computer with the proper software or by an AIS-enabled chart plotter.

Where can I find out more specific information about AIS?

See the US Coast Guard (USCG) AIS FAQ.

Which VHF channels or frequencies are used with AIS?

AIS transponders and receivers use two VHF radio frequencies: 161.975 MHz (AIS1, or channel 87B) and 162.025 MHz (AIS2, or channel 88B). The USCG has asked the Federal Communications Commission to authorize any US vessel to operate AIS on these two channels under its existing ship station license.

What type of ships show up on an AIS display?

Just about every commercial vessel will have an AIS transponder. Specifically, AIS is mandatory on all ships of 300 gross tonnage and upwards engaged on international voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size.

See the US Coast Guard web page on AIS for more information. Other smaller commercial ships, such as fishing vessels, will equip themselves with AIS transponders voluntarily since there are major safety benefits to using AIS.

In addition, Class B AIS transponders are used by some recreational vessels. The AIS Receivers we sell are able to pickup both Class A and Class B transponder signals.

What is the range for AIS receivers?

Since AIS uses the same VHF frequencies as marine VHF, it has similar radio reception capabilities which is basically line of sight. This means that the higher your VHF antenna is mounted, the greater the reception area will be.

Reception from Class A vessels that are 20 or even 30 miles away 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 7 miles away.

Note that AIS has a major advantage over radar since it can see ships within radio reception range that are behind large objects, such as other ships or points of land.

What kind of information is broadcast for each ship and how often is it updated?

A Class A AIS transponder broadcasts the following information every 2 to 10 seconds while underway, and every 3 minutes while at anchor:
  • MMSI number - unique referenceable identification
  • Navigation status - at anchor, under way using engine or not under command
  • Rate of turn - right or left, 0 to 720 degrees per minute
  • Speed over ground - 1/10 knot resolution from 0 to 102 knots.
  • Position accuracy - differential GPS or other and an indication if RAIM processing is being used
  • Longitude - to 1/10000 minute and Latitude - to 1/10000 minute
  • 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 this information was generated
In addition, the Class A AIS unit broadcasts the following information every 6 minutes:
  • MMSI number - same unique identification used above, links the data above to described vessel
  • IMO number - unique referenceable identification (related to ships construction)
  • Radio call sign - international call sign assigned to vessel, often used on voice radio
  • Vessel name - name of ship, 20 characters are provided
  • Type of ship/cargo - there is a table of possibilities that are available
  • Dimensions of ship - to nearest meter
  • Location on ship where reference point for position reports is located
  • Type of position fixing device - various options from differential GPS to undefined
  • Draught of ship - 1/10 meter to 25.5 meters (note air-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 vessel information every six minutes however the information only includes:
  • MMSI
  • Radio call sign
  • Vessel name
  • Length and beam of vessel
  • Type of vessel
Position updates for Class B transponders are broadcast less often than Class A transponders. Vessels going less than 2 knots transmit position updates every 3 minutes while vessels traveling more than 2 knots transmit position information every 30 seconds.
For this reason it is recommended that you consider purchasing a true dual channel AIS receiver if you plan to use the AIS receiver in an area where Class B transponders are in use.

Differences Between Single Channel and Dual Channel AIS Devices

Milltech Marine sells a dual channel scanning receiver (SR161) and full time dual channel receivers (SR162/SR162G/Comar/AMEC receivers). What are the differences between these models?

All the units can receive AIS information from either AIS channel. The SR161 can only receive information from one channel at a time but will automatically switch to the alternate channel if it detects interference on the current channel.

The SR162/SR162G, all Comar and AMEC AIS receivers and all AIS transponders can receive all AIS broadcast information from both AIS channels simultaneously and consolidate the information from both channels into a single data stream. These are true dual radio receivers. This generally means you will acquire new vessels sooner with these receivers and you will also get the full information about a vessel in a shorter period of time.

In practical use, this means it can take up to 12 minutes for the SR161 to acquire the full static information broadcast from an AIS transponder since the SR161 only listens on one channel at a time. The dual channel receivers pick up all transponder broadcasts therefore the maximum time to pick up the full static information broadcast is 6 minutes. The SR161 also picks up position updates from transponders less often than the dual channel receivers since it only picks up every other broadcast.

For Class A transponder broadcasts, this is normally not a noticeable issue however with the new Class B transponders - which broadcast with less power (which means less distance) and position updates broadcast less often - there may be a noticeable delay in receiving a position update. For example, if a Class B vessel is traveling slower than 2 knots you would only see position updates every 6 minutes instead of every 3 minutes with a dual channel receiver.

The SR162G also includes an integrated 12 channel GPS module as well as a detachable GPS antenna with a 16 foot cable. With this model you can receive AIS traffic information and GPS position information as a single consolidated data stream over the RS232 serial port. The Comar SLR200G also includes an integrated GPS module.

If I am only receiving AIS information from one channel at a time, does this mean the SR161 will not pick up the transponder broadcasts from half the ships in my area?

No the AIS system uses two channels for redundancy. Ships broadcast information alternating between the two channels. Therefore, you will eventually pick up information for every ship, even if the AIS receiver can only receive on one channel.

However, with the increased popularity of Class B AIS Transponders, it may make sense to purchase a true dual channel AIS receivers such as the SR162 or any of the Comar AIS receivers. This is because Class B transponders broadcast position updates less often than Class A transponders. The result with a single channel receiver therefore may be that Class B vessels will "jump" across the screen as each update is received.

How often do AIS transponders broadcast position and static information reports?

Transponders send out position updates based on the speed over ground (SOG) of the vessel and updates occur at different intervals depending on whether a Class A or Class B transponder is used. Depending on whether you are using a single channel or dual channel AIS device, reporting intervals will vary based on the tables below:
Receiving Class A AIS Transponder reports
Ships Dynamic Conditions Dual Channel Receiver Single Channel Receiver
Ship at anchor or moored 3 min 6 min
SOG 0-14 knots 10 sec 20 sec
SOG 0-14 knots and changing course 3.3 sec 6.6 sec
SOG 14-23 knots 6 sec 12 sec
SOG 14-23 knots and changing course 2 sec 4 sec
SOG >23 knots 2 sec 4 sec
Ship Static Information 6 min 12 min
Receiving Class B AIS Transponder reports
Ships Dynamic Conditions Dual Channel Receiver Single Channel Receiver
SOG <2 knots 3 min 6 min
SOG >2 knots 30 sec 1 min
Ship Static Information 6 min 12 min

Can I get more information on how the dual channel system works?

The best source for more information is to look at the documents 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-channel operating mode, where the AIS simultaneously receives on both channels in parallel. In order to accomplish this performance, the AIS transponder should contain two TDMA receivers.

Channel access is performed independently on each of the two parallel channels. For periodic repeated messages, including the initial link access, the transmissions should alternate between AIS 1 and AIS 2. This alternating behavior is on a transmission by transmission basis, without respect to time frames. Transmissions following slot allocation announcements, responses to interrogations, responses to requests, and acknowledgments should be transmitted on the same channel as the initial message.

For addressed messages, transmissions should utilize the channel 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 alternate between AIS 1 and AIS 2. Base stations could alternate their transmissions between 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.
Also the US Coast Guard site has the following information:
Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships.

Using an AIS Receiver or Transponder with a Computer or Chart Plotter

How can I use the data stream from an AIS receiver or transponder?

Many modern computer-based marine navigation software packages, such as Rose Point Coastal Explorer and Nobeltec Visual Navigation Suite 9.0 or higher, can be configured to interpret the incoming AIS data stream and convert the information into human readable information. These packages also support plotting nearby ships on digital charts and can show your position (via a separate GPS) in relation to the other vessels in the area that have AIS transponders.

Is it possible to connect my AIS receiver or transponder to a chart plotter?

Today most modern chart plotters support the connection of an AIS receiver or transponder and are able to display AIS targets. Some of the manufacturers that support AIS include Raymarine, Garmin, Standard Horizon and Furuno. For further information contact your supplier and ask about AIS support in their product. For more information on how to use the AIS receivers or Class B transponder that we sell with an AIS-capable chart plotter, see our special chart plotter information page.

Note Caution: We do not recommend using Smart Radio AIS receivers with chart plotters since Smart Radio uses RS232 serial signals which are not compatible with many chart plotters which expect true NMEA RS422 electrical signals. While some customers have been able to use Smart Radio AIS receivers with some brands of chart plotters, other customers have not been successful or have experienced failures at time of installation or in some cases months after the install. Smart Radio AIS receivers do work well with computer serial connections and the AISWatchMate.

Therefore we now only support the connection of a chart plotter to the NMEA-compatible AIS receivers found here and all AIS transponders.

Do these AIS receivers and transponders work with Raymarine's E Series and C Series chart plotters?

Yes, the Comar and AMEC AIS receivers and all AIS transponders that we sell work well with Raymarine E series and C series chart plotters with the latest ROM upgrade from Raymarine. Normally, you would connect the NMEA-compatible AIS Receiver or Class B transponder to the high speed NMEA input on the chart plotter. For more information on how to use our AIS Receiver and Class B transponder with Raymarine AIS-capable chart plotters, see our special chart plotter information page and our product documentation. Smart Radio AIS receivers are not supported for use with Raymarine chart plotters.

Will an AIS receiver work with my network solution onboard?

All networks that can handle NMEA serial input at 38400 baud and process VDM serial data will be able to handle AIS data. Many of our AIS products also support NMEA 2000. Simply add the device to your NMEA 2000 network and your other NMEA 2000 devices should be able to use the AIS data assuming the device supports AIS. Ask your network provider for further information.

What are the basic steps to install an AIS receiver?

The receiver has three connections.
  1. One connection is for a standard marine VHF antenna. The some AIS devices use a BNC connector for the VHF antenna connection. If you need to connect to typical VHF radio antenna cable that uses a PL-259 / SO-239 connector, you'll need an SO-239 to BNC adapter. These are available from Milltech Marine's online store. 
  2. The second connection is for data. Some models have a 9-pin serial port which can be connected to a computer serial port using a standard serial cable, which is typically included with the receiver. If your computer doesn't have a serial port, you can use a USB to serial adapter to make the connection. We recommend the Sabrent USB to serial adapter and offer these as an accessories in our online store. The most of the current Comar and AMEC products use a USB connection to interface with a computer and also have data cables to connect to a NMEA-based chart plotter.
  3. The third connection is for 12 volt DC power. The Comar AIS-3R and the AMEC CYPHO-150/150S AIS receivers can be powered via the USB port.
Once these connections have been made, simply configure your marine navigation software or chart plotter to utilize the new serial port connection as an incoming listener serial data stream. Note that AIS receivers use 38400 baud by default so be sure you configure your software program or chart plotter appropriately.

For AIS transponders and the SR162G and Comar SLR200G receivers, you will need to also connect the GPS antenna and make sure it is placed in a location that has a clear path to the sky.
Please see the product installation documentation for full details on installing your AIS device.

I don't have a spare serial port on my computer. How do I hook up a Smart Radio AIS receiver to my computer?

You typically have three options.
  1. The first option is to use a USB to serial adapter. Connect the serial end to the AIS receiver and the USB end to a spare USB port on your computer. Make sure you know which COM port has been assigned to the USB serial port and configure your software appropriately. Note that some cheaper serial to USB adapters can be unreliable or cause system crashes. Test the adapter thoroughly before deploying it on your boat. Milltech Marine has tested the Sabrent USB Serial Adapters and recommend these solutions. The Sabrent can be ordered from our online store. The units support both the PC and the Mac. 
  2. If your computer only has USB ports, then you should consider the Comar AIS-3R or AMEC CYPHO-150 / 150S products which interface directly to a USB port on a computer. 
  3. The other option you have with the Smart Radio and the Comar and AMEC receivers is to use the unit as a NMEA data stream consolidator. What does this mean? You can direct the GPS NMEA data stream into the input pin or NMEA input on the AIS receiver and the unit will then combine the GPS sentences with the AIS sentences and output a combined data stream on the USB, serial or NMEA serial port. You would then only need one serial port or USB port on your computer and this port would have both GPS and AIS data coming through it. Full details are in the installation manual that comes with the AIS receiver. You can also purchase an AIS/GPS Cable from Milltech Marine for use with Smart Radio AIS receivers.

What electronic chart programs can handle AIS?

Today several software packages support AIS. The ones known to work are:
  • Rose Point Coastal Explorer
  • Maptech Chart Navigator Pro
  • Nobeltec Visual Navigation Suite 8.0 or later See sample screen!
  • Nobeltec Admiral
  • Capn 8.0
  • Boatcruiser 2.0
  • Fugawi Marine ENC
  • Global Navigation Software NavPak
  • DigiBOAT Software-On-Board
  • Y-tronic Yacht-AIS
  • Navicon AIS Navigator
  • COAA Ship Plotter
  • Memory-Map Professional
  • MaxSea v12.5.3 with Mobiles Module
For the Mac, the recommended packages include:
  • GPSNavX 
  • MacENC
For mobile platforms such as the Apple iPad, a number of apps are available. See our blog article here for more information.

Software that can handle the NMEA VDM messages and display the data should work with our AIS receivers and transponders. Ask your software provider for further information.

If you are just getting started with computer-based marine navigation, then we recommend the Rose Point Coastal Explorer product. This product works great with all of our AIS devices.

Can I use AIS solutions with all Microsoft Windows operating systems as well as Mac OS X?

Yes, it is possible to use our AIS solutions with Windows XP, Vista, Windows 7, Windows 8.x and Windows 10. The Coastal Explorer product we sell works well on all of these. Vesper Marine products include configuration programs for Mac OS X but most of our other products do not include a Mac OS X version. You run Windows programs in a Windows emulator on Mac. For products that require a USB driver, such as our FTDI-based cables, drivers for all Windows and Mac operating systems are available.

What type of VHF antenna do I need for my AIS receiver or transponder?

The VHF antenna should fulfill at least the following requirements:
  • Antenna type: Vertical radiator
  • Antenna gain: 3 dB or similar
  • Impedance: 50 ohm

Can I use an existing marine VHF antenna with an AIS receiver or transponder?

Yes, you can use a splitter to share your existing VHF antenna to your VHF/DSC radio and your AIS receiver. You must use an active splitter that automatically disconnects the signal to the AIS device if a transmission is detected from the VHF radio.

Milltech Marine sells mulitple active VHF Antenna Splitter solutions for use with AIS receivers and transponders. Splitters that are designed to work with an AIS transponder can be found on our transponder splitters page.
Note Caution: Do not use splitters designed for use with AIS receivers with AIS transponders (e.g. the Smart Radio VHF Antenna Splitter) or you may damage your splitter and / or radio.

Since you have two receiving devices on one antenna, you may also experience some small amount of signal degradation with some splitters. While broadcasting on the VHF radio, you will likely see some interruption of incoming AIS signals. Since AIS broadcasts from each ship are repeated every few seconds, this is not normally noticeable in your tracking software.

Note Caution: If you use a common T-connector to share a VHF antenna with a radio and an AIS device and then broadcast on the VHF radio, you will damage your AIS device.

Another option is to use a dedicated VHF antenna that is mounted away from your VHF radio antenna. This will mean less interference and will allow both the AIS device and the VHF radio receiver to get maximum receiving distances. In order for the antenna to work properly, you must install your AIS antenna at least 4 feet from any other radio antenna or vertical metal structure and mount it as high as possible. Milltech Marine sells a number of excellent VHF antennas specially designed for use with AIS devices. See our products page for more information.

I have hooked up my AIS device. Why don't I see ships immediately?

It normally takes a few seconds for ships to appear since the receiver needs to pick up a transmission from the remote ships transponders. The system allows for ships to rebroadcast their information every few seconds so within a minute you will typically see nearby ships appear on your navigation package.

The ships show up as numbers? I thought I would also see the name of the ship.

Just wait. Ships broadcast voyage information every few seconds but also broadcast full ship information every 6 minutes. So after a few minutes, you should see complete information for every ship that the AIS receiver has picked up.

Purchasing Products from Milltech Marine

Do you ship products outside the USA and how do I get information on shipping options and costs?

Yes, we are happy to ship products to most countries around the world. For information on shipping options and prices for shipping, use the online shopping cart in our online store. For high value shipments, we do not recommend US Postal Service delivery services for international orders as these shipments are not tracked and typically take longer than expected. For international orders, additional fees, duties and taxes may be charged by your local authorities.
0 Items