A Beginners Guide to AIS Transponders and the Global Maritime Distress and Safety System  and Maritime Mobile Service Identities - MMSI


If you are a boat owner, especially one who uses busy waterways and ports knows how difficult it is to identify where other users are, and even more importantly, where they are going! Engine powered craft tend to travel in straight lines, or gentle curves, while wind powered sailing vessels often take courses that are close to 90 degrees from their real intended bearing. Some are very slow, and others can be much fasters. Even in controlled waters, such as ports and estuaries, having a clear indication of where all the traffic is, is a huge step in safety.


Common Maritime Electronic Devices


Short Range VHF Radio


A simple to use, but effective voice communication system consisting of vessel mounted and portable radios. Connected to a larger boat mounted antenna, or a much shorter portable antenna, the ranges vary from many miles to less than a mile. The decider on water is simply antenna height – and the horizon. Radio wave travel in straight lines, so distance is largely a factor determined by the curvature of the earth, or in nautical contexts, the sea! There is a rough but quite reliable formula that allows you to estimate likely range. 3 times the square root of the antenna height in metres. Sounds complicated, but the calculator in your phone will work it out for you if you are stuck.  Let’s use the example of a small workboat where the antenna is 4m above the sea. Multiply this by 3, and you get the square root of 4 is 2, so 2 multiplied by 3 gives you the horizon at 6 miles. If you have a handheld radio and when you hold it, it is 1.6m above the water – a realistic figure, the square root is 400mm, and when multiplied by 3, the horizon is roughly 1.2 miles away. In real life this also assumes the antenna is vertical, and we’ve all seen people use handheld radios where the way they hold it means the antenna is a long way from vertical. At the horizon, the actual radio wave just carries on, so with the two examples above, you can add the ranges – the workboat with higher antenna can talk to the handheld at 7.2 miles – roughly! If you have a large ocean-going yacht with the antenna on the top of the mast, at say 16m, then your range to the horizon could be 12 miles. The coastguard might have their local antenna on a waterside building at perhaps 50m above the water – so would have a reach approaching 21 miles. Even a 100m tower only reaches 30 miles or so. Distance as you can see is very important to the success of VHF radio.


This limitation in range led to the development of the Mayday relay protocol, something all new marine band users get tested on in their examination. A ship can very likely be in range of the Coastguard shore station, and in range of the vessel in distress, further away.


The Global Maritime Distress and Safety System (GMDSS) provides a framework to allow ships, boats and aircraft to interconnect their electronic systems – usually GPS, radio, and radar. All ships over 300 gross tonnage must have a technical installation that meets the requirements of GMDSS. Smaller vessels may install this equipment and take advantages of the additional safety systems if they wish – but the equipment standard is slightly simpler, and of course may not always be powered up and in use.


Beacons and Location Devices

Emergency position-indicating radio beacons (EPIRB) are part of the GMDSS system and a series of satellites (Cospas-Sarsat) can receive broadcasts from ships emergency beacons and provide a coordinated response. These EPIRB devices transmit on 406MHz.



A useful world-wide coverage system providing text-based messages primarily about weather and weather related messages transmitter on the shortwave band, centred around 500KHz. Small displays produce text-based display of information.


Automatic Identification System (AIS)

This is an identification system based on data transmitted on two of the short-range channels - Channel A 161.975 MHz (87B) and Channel B 162.025 MHz (88B)

These two channels are NOT available for voice communications, and are usually missing from most commercial radios. Web sites such as Shipfinder and others collate the AIS data received from sites all over the world and present them to the viewer. This AIS data can be received by a device installed on a boat or other vessel and with the position data from a GPS, can display a pictorial view of where AIS equipped vessels are in the local area. As it uses standard VHF short range radio, the maximum distance is again a factor of antenna height. The important feature is that the devices can offer collision avoidance warnings. They know the current heading of their own vessel, and can see the current heading of all other AIS vessels in the area. If conflicting paths are evident, these devices will sound an alarm and give advance warning of course conflicts, allowing changes to be made earlier than by visual means.

AIS transponders and chartplotters

Common devices now incorporate a GPS unit, a display and a VHF radio in one small housing. In practice, you fit a small GPS antenna to get good coverage from the satellites in orbit and you fit a second VHF antenna to your boat. If a second antenna is problematic, you can buy a small device to allow your comms radio and your AIS tranponder to share one antenna.

Even small craft can now afford to have a dedicated VHF radio, and a chartplotter with GPS. Some of the chartplotters can also be loaded with detailed nautical charts that show shipping lanes, underwater obstacles and depth information.  The equipment is priced according to facilities and electronic class. As an example, a modest price small vessel transponder will broadcast location information every 30 seconds, and will normally also send details of the vessel MMSI number, the length and beam and sometimes name. The commercial units send more frequently and detail course information with final destination, and vessel type. For them, the information is mandatory, for vessels less than the 300 tonne limit, the content and identification detail is informationary.

Maritime Mobile Service Identity (MMSI)

This is a data base of unique registration numbers that can be entered into radios, plotters and other devices to enable positive identification. The MMSI number is a 9 digit sequence. UK Individual vessel MMSI numbers start with 232, 233, 234 or 235, then 6 digits. MMSI numbers beginning with a 0 are group numbers – perhaps a group of trans-atlantic craft sailing the same route, although these seem rare. 00232 numbers indicate the station is a shore station. In the UK, applying for a VHF radio licence will generate an MMSI number. In the case of a vessel, this number is allocated for the lifetime of the vessel and will not change. Portable radios are allocated their MMSI, and T prefixed callsign – again, this is intended to be for the lifespan of the radio.


Most devices that can accept an MMSI number do NOT allow this to be changed by the user. It is a single entry and forget process. Most need returning to a dealer if the number needs to be changed – so selling a radio can be less than simple!

For handheld radios there is no legal requirement for the radio to be MMSI/AIS/DSC equipped, it is a user choice and reflected in the price, however, it is an obvious aid to safety.

Digital Selective Calling (DSC)

This data system uses channel 70 156.525MHz which is not available for voice communications. Using the MMSI unique identification it is possible for one vessel to call another, or a group of other vessels without calling them using voice. In practice, users are presented with a list of their favourite other vessels, and they can select one, and press a call button. The other radio, usually monitoring channel 16, will hear the data on channel 70, and sound an alarm. The display indicates who is calling and gives the option to answer. If this is selected, both radios automatically switch to the channel indicated by the sending station and a conversation can immediately follow. This is better frequency management and prevents wasted time calling other ships on channel 16, then agreeing to move to a working channel. In busy areas, channel 16 can be very busy.

In emergency situations, the system can be setup so that pressing a button for three seconds under a hinged spring loaded red cap initiates a mayday  emergency broadcast. If the radio has integral GPS or is connected to a chartplotter containing a GPS, then the radio sends a DSC emergency message containing position, MMSI and other details that automatically sound alarms on other equipped vessels of all sizes, and of course the coastguard. The large over 300 tonne vessels can also initiate Mayday relays, but this facility is NOT supported by voluntary fitted GMDSS installations.

AIS Beacons

These are small floating GPS equipped devices that broadcast data that can be displayed on a chartplotter fitted with AIS. They require an individual MMSI number, which needs to be obtained from OFCOM and added to your licence. They can be attached to fishing nets or other items at sea and will broadcast their position for later recovery.

Paul Johnson 2020







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