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AIS - a practical explanation for those unfamiliar.
AIS (Automatic Identification System) can be thought of as a VHF radar system using 2 channels in the VHF marine band frequencies (161.975 MHz & 162.025 MHz). Designed as a collision avoidance system, it was made mandatory throughout the world in 2002 to be installed in all vessels of 300 gross tonnes and over and all passenger ships, More recently, within the European Union, this has been extended to include fishing vessels over a certain length which at the moment stands at vessels over 15 metres.
There are three classes of AIS; Class A, the type mandatory for commercial vessels, Class B for leisure and small craft and just available, Class B+, an enhanced version for leisure boats that sits around half way between Class A and Class B. All new Onwa AIS plotters are now equipped with Class B+ SOTDMA AIS modules. Class A is more sophisticated than Class B sending more detailed data out at a faster rate than Class B and will have transmit priority at all times. However, the introduction of Class B+ changes this as SOTDMA technology allows leisure users the same access to time slots as for Class A.
Class A is also much more powerful - transmitting at 12.5 watts whereas Class B is only 2 watts. B+, however, is a nice compromise for leisure sailors having a 5 watt transmission and the same faster SOTDMA technology of Class A.
For leisure sailors and small boat users AIS doesn't, and never will, replace true radar but it has certain advantages over radar. Better in one respect than conventional radar AIS can 'see' (to some extent) around corners as its wavelength is longer. Other advantages are considerably lower cost than radar and lower power consumption. Radar consumes a lot of power - at least X band does, whereas AIS current drain is quite small. My Onwa KP-1299A, with its large 12 inch screen consumes around 800 m/a whereas my radar is over 5 amps.
AIS - what does it look like?
You have probably seen AIS displayed as a radar ring or rings with your own vessel at the centre and nearby AIS transmitting vessels displayed as small triangles somewhere within the rings. This is an image of vessels that we can see from our AVES Marine base in Kent. The 'blob' of vessels just above the *red ring are transmissions coming from Tilbury Docks in London, about 8 miles away from us. Note that the outer ring in this example is set at 20nm so we are seeing vessels from as far away as 20nm.
*The red ring is a boundary set into the plotter by the user to trigger alarms, in this image the boundary is set at 6.5 miles, in practice this might me more or even less depending upon the traffic around your boat. In crowded waters, for example, you might set the ring a lot closer to reduce false alarms and in an open ocean situation very much further. We set out plotter alarm at 6.5 miles long distance in order to trigger an alarm from vessels in the River Thames and you can see this in our 2nd radar image below.
The red boxes on the right without any information in them are empty until a collision alarm is triggered, they then fill with details of the vessel calculated by the plotter to be on a collision course with your vessel.
Whilst the 'radar ring' is very useful to see what vessels are around your boat, it only gives you a representation of the vessels in your vicinity and has quite limited use. Look at the bright mass of light between the inner green and red rings, this is, in fact, several vessels but it isn't possible to know from this display anything about them - far better is to see them in real time overlaid on a chart. The benefit of seeing a large vessel displayed on your chart as a moving target in relation to your own boat has to be seen to realise just how valuable that is. At the moment my sailing yacht is moored on the River Midway in Kent. This is a river used by commercial and leisure vessels and it is reassuring to know from the AIS overlay on the chart on my Onwa plotter that whilst I am tacking across the river there isn't a huge freighter just around that blind band in the river.
Image showing three of the vessels shown in the the previous radar ring image but now overlaid on our chart and shows the vessels underway just outside Tilbury Docks. Note that the chart has been zoomed to a quite small area (as it would be for normal navigation) so only shows some of the vessels that can be seen on the radar ring.
This is so much clearer than the information we were previously presented with by just looking at the radar ring. Vessels will show a short line in front of them to indicate if they are underway and, as time passes they will leave a line behind them on the display showing their progress over the water. In the above image there are two vessels proceeding down the southern side of the river and a vessel is just leaving its mooring on the North bank and is beginning to also move down stream. You can see how much more informative this information is than just another target on the AIS Radar Ring. Were you navigating in this area the chart information with the three vessels shown would alert you instantly to their presence whereas the Radar Ring would be a meaningless jumble of targets.
Note...In real life your own vessel's position would also be shown at the centre of the chart enabling you to see where the 'big ships' are relative to your own position. However, as AVES Marine's base is inland from Tilbury, it would be confusing to show our own boat position some miles inland and also the zoomed out chart to show AVES Marine and Tilbury would reduce the detail to an impractical level .
Same image as previous but having placed the curser over the vessel leaving its mooring we can see its MMSI number, vessel name and position. More information on the vessel can be obtained from the AIS radar page/menu/AIS Detail List. Select the vessel name and press Enter.
Same vessel in the AIS ship list, we can see it is a Cypriot ship, that is 6.6 miles away on a course of 156 degrees. We don't have a name at this point but this would show were we to monitor the vessel for a slightly longer period. Whilst essential data is transmitted by Class A vessels every few seconds other information, such as name, is transmitted every six minutes so we have likely missed that part of the AIS transmission.
Advantages of an AIS Receiver
If you have ever had to cross a shipping lane in poor visibility you will know it can be a worrying event and suddenly seeing a merchant vessel emerging from the mist and bearing down upon you isn't something anyone will want to experience. However, with your AIS receiver you will have seen that vessel when it was several miles away and this, I think, really makes the case for AIS. Not only does the AIS show you AIS equipped vessels in your vicinity it also tell you quite a lot about them; their COG, SOG, name, MMSI number, type of vessel. Comforting is the knowledge that your Onwa AIS will alert you well before a real danger exists by the plotters collision alarm going off . If the vessel is approaching on a collision course and your avoiding action is uncertain this is the time to call the vessel by name on the VHS and alert the vessel to your position and course and your intentions. This is quite important as otherwise you might make a course change to avoid being run down at the same time as the commercial vessel does the same and you both stay on the collision course!
You may think from the previous comments that there is an implication that you need to spend the whole of your voyage with your eyes 'glued' to the plotter screen when, as a good skipper, you should actually be looking around you at all time. Of course, keeping a vigilant look out is what you must do and are required to do under maritime law. However, provided you have set up the collision warning alarms, which I will describe a little later in this article, you can leave it to the plotter to alert you to any vessel approaching that may enter the area around your boat that you consider (as defined by your own set-ups) is getting too close.
How far away can you 'see' other vessels?
How far away you can 'see' other vessels on an AIS receiver depends upon the height of the transmitting vessels antenna and the height of your own (VHF transmissions are line of sight). Large commercial vessels will have their own AIS aerial high up on their superstructure so line of site, even if your own aerial is mounted on a stern rail, will be quite good. Also, keep in mind that Class A vessels transmit at 12.5 watt whereas Class B and B+ transmit at only 2 and 5 watts respectively, so, whereas commercial vessels may be seen from many miles away, Class B will be seen only when fairly close up . For Class A reception it can be around 16 miles + and even more in unusual atmospheric conditions. On one occasion, whilst we were based in Gravesend, we received AIS transmissions from vessels in Denmark - around 500 miles! This was freak atmospheric conditions and only lasted around 2 hours. Class B leisure vessels will transmit at only 2 Watts power from an aerial likely much lower down so expect reception from other leisure boats to be considerably reduced over commercial vessels.
From our AVES Marine base in Kent we were able to see Class A vessels 23 miles away although it must be said that we are at quite a high point in Kent.
Transmitting distances
Transmission distances will be much lower than receive using Class B equipment and will be very dependent upon the aerial system components (AIS aerial, cable and connections) and height of the aerial, expect around 5 miles. Whilst this may seem a rather short distance, do keep in mind that a warning of a collision occurrence developing at 5 miles distance should be sufficient to avoid such a situation worsening. Some manufacturers of AIS equipment have adopted the new Class B+ AIS for leisure use (all current Onwa AIS transceivers are B+) which gives a small advantage in terms of transmission as a result of its greater 5 Watts power. Although a little over twice the power of Class B, this doesn't quite equate to twice the distance for transmission - even assuming a perfect aerial system. Class B+ expect 8 to 12 miles transmi. However, are other advantages that B+ has over ClassB. They use different technology, Class Bb using CSTDMA and Class B+ using SOTDMA. SOTDMA ia anm enhanced technology resulting in having no wait time for an AIS slot to transmit together with faster refresh rates. Also, transmission rates increase with speed, which is essential for fast moving power boats.
The table below shows the differences between the two systems.
Comparison SOTDMA and CSTDMA
Comparison SOTDMA and CSTDMA | SOTDMA | CSTDMA |
Transmit Power | 5 Watts | 2 Watts |
Transmit refresh rate | 5 seconds | 30 seconds |
Guaranteed Time Slot Allocation | Yes | No |
Transmit Rate | 5 seconds | 30 seconds |
Vessel at anchor/moored | 3 mins | 3 mins |
SOG 0-2 knots | 3 mins | 3 mins |
SOG 2-14 knots | 30 seconds | 30 seconds |
SOG 2-14 knots and changing course | 30 seconds | 30 seconds |
SOG 14-23 knots | 15 seconds | 30 seconds |
SOG 14-23 knots and changing course | 15 seconds | 30 seconds |
SOG > 23 knots | 5 seconds | 30 seconds |
Other vessel information | 6 minutes | 6 minutes |
You can see that for a fast moving vessel, B+ is pretty much essential as waiting 30 seconds for your transmission to show a course change is just too long.
The additional information that the AIS receiver can bring to you:
Once you see a commercial vessel that is getting close to you it is a very easy process to find out the course and speed of the vessel. With Onwa plotters - by placing the curser over the target vessel its position and MMSI are displayed. Again, on Onwa plotters, an additional button press will bring up a window that shows; Name, MMSI number, Ship name, Call sign, position, SOG, COG, ship type and if it is underway. Note, however that although essential data such as position, COG,SOG and MMSI are refreshed at (for Class A) every 2 to 10 seconds when underway, other less essential data such as vessel name are transmitted less frequently so vessel name may not be immediately displayed.
Antenna considerations
AIS uses two channels at the upper end of the VHF marine band - 161.975 MHz & 162.025 MHz. A standard VHF antenna will receive AIS and also transmit it. However, a standard VHF antenna will not be ideal as it will be tuned around 156 MHz, the centre of the marine band, and not at the upper end where it should be for AIS. In practice, for a receiver, this isn't going to make a huge difference but for an AIS transmitter it is an entirely different matter. This is an important thing to consider as transmitting efficiency is much more dependent on your hardware than receiving only. Whilst any old VHF band antenna mounted anywhere may pick up transmissions from close up vessels, when it comes to transmitting a whole new set of problems arise - antenna design, height, impedance matching from transmitter output to antenna input, connector quality and probably the most often neglected factor - cable type. You may not be aware that a kink/tight bend in the cable, over tight cable ties - anything that compresses and distorts the dielectric insulator (the inner insulator around the centre conductor) may change the impedance and causes signal losses. Even without the added possibility of cable damage, signal loss over the cable is an inevitable significant factor; 50 ohm RG58 co-axial cable - very often used on small boats as it is cheap, thin and easy to install, has significant losses over long cable lengths; this cable is often packed with an AIS cable by manufacturers. RG58 usually works OK for reception, however, using RG58 for a transmitter is different as cable attenuation will definitely reduce your transmission efficiency - maybe to the point where it doesn't really work. If you are running more than 10 metres of cable for your new AIS install you should consider a lower loss cable although you will be looking at a thicker and more expensive cable. Possibly one of the best cables would be RG213 (the cable Radio Hams use) as it has very significantly lower signal loss over long distances, however, because of its thickness - around 10mm (RG58 is 5mm) it is unwieldy and very hard to install; its also a lot more expensive but the difficulty installing is more the reason for it not being ideal on a boat. A compromise would be RG8X which has lower loss than RG58 yet is considerably thinner than RG213 at 6.5 to 7 mm and cheaper also. In a nutshell, to receive AIS is possible on a standard marine VHF antenna mounted anywhere within reason on your boat but for AIS transmission you do need to consider the height of the antenna and also the possible losses in the connecting cable. However, as mentioned earlier you need to see and be seen by vessels close by not those many miles away. Seeing other vessels from 20 miles away is equally unnecessary although it does have a novelty value that a lot of sailors quite like.