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Is my AIS Transponder Transmitting ?

Stylised AIS in Action

With lots of AIS Class B Transponders being installed this season, a question we are often asked is “How do I tell if my AIS Transponder is transmitting ?”

This is a very valid question and if you have spent a few hours installing the transponder then knowing that it is working OK is very important. Using the proAIS2 configuration software (supplied with the unit) allows you to see if the GPS position is OK, monitor the AIS reception of other vessels and also ensure that there are no errors or alarms but if you are new to AIS, there is always that nagging doubt as to whether other vessels are seeing you.

The reality is, that as long as the Green Power LED is constantly lit and no other LEDs are illuminated, then the transponder is working correctly and transmitting your position. It usually takes about a minute for the transponder to power up, get a GPS fix and start transmitting. From that point on wards, it is constantly performing a series of internal tests and the Green LED indicates that all of the tests are OK and the unit is operating as normal. It is almost unheard of for the Green LED to be on and the unit not to be transmitting correctly.

The best test of a Class B transponder is to ask someone else in your marina, who has AIS, to check that you are being received on their system. If your vessel is stationary, then a transponder will only transmit every 3 minutes and this increases to every 30 seconds when your speed over the ground (SOG) is greater than 2 knots, so do allow some time for them to detect you. Also when they first receive your transmission, the only data they will see is your position, speed, course and MMSI number, it can take up to 6 minutes for your “Static Data” (boat name, call sign, vessel type, dimensions, etc.) to be received. This is normal and is the way the AIS system regulates the amount of data being transmitted.

The other increasingly common method of testing an AIS transponder is to look on one of the online “live” AIS websites and the most popular of the free services is

However, it is important for you to be aware of the limitations of these online sites and not assume that you will always be picked up by them. Each of the different online services are only as good as their network of AIS receiving stations, that in many cases are operated by enthusiasts/volunteers. In some areas the coverage is great but there are definitely “holes” in coverage.

One of the good features of the Marine Traffic service, is that you can display the locations of the receiving stations and see if they are currently online or offline. From the Marine Traffic home page, zoom in to your location on the map and then on the left hand side of the screen, click on the “Layers” icon and a series of tick boxes to display different layers of data will appear. Click the “Stations” tikck box and you will see a series of Antenna type icons on the map; green if they are online (receiving data), red if they are offline and orange if they are online but giving poor reception performance.

In the image below, you can see the the UK port of Exmouth and if you were testing your Class B transponder here today, then you would think you were not transmitting. Just one receiving station being offline can create a big hole in coverage and the next receiving station going East is Bridport around 35 miles away. This is a popular section of water and yet you can see a few vessels between Torquay and Lyme Regis.

A Class B transponder transmits at 2 Watts (about a third of the power of a hand held VHF) so even in perfect line of sight conditions, the best range you can expect is about 8NM. It is important to check if the station is within 5-8NM of your location and also if there are any other Class B vessels displayed near you. If you are more than 5-8NM from the receiving station or there is not good clear line of sight between the station and your location, then the chances are that the station is not picking up your transmission or any other class B transmissions from vessels in your marina.

It should be noted that Commercial vessels and some larger pleasure craft have Class A Transponders that transmit at 12.5 Watts and have a much greater transmit range 20-25NM, so these vessels maybe displayed near your location, even if your Class B transmission is not received.

Finally, it can take an hour or so for a new AIS (MMSI number) to be recognised and stored in the database of these online services so if you are confident that you are in range of an online receiving station, leave your transponder on for a few hours to give the online service time to detect and record you in their database.


Another nice feature of Marine Traffic, is that you can drill even further down in to the details of a particular receiving station, by clicking on the icon of the Station you are interested in. This brings up a summary of the receiving Station’s information and if you click the Green “Details and Statistics” button, you can get a wealth of information about the station, its statistics and “live” AIS target reception list.

As you can see, this particular station in Exmouth is regularly offline for quite long periods and even when online, has a rather poor 10NM reception range.

For anyone that lives or has a business close to the sea and who would like to be part of an online AIS network, please contact us about our AISNet product, which can connect to your home/office network and put your local AIS reception data online.

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