AVES MARINE Ltd (T/A avesmarine.com)            Authorised and approved by Onwa Marine Ltd                                UK and EU sales and support 


If you spot an error anywhere or disagree with anything mentioned on this page we will welcome your comments. 

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 nboats that sits around half way between Class A and Class B. 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. 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 Onwa 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 we are seeing vessels 20nm miles away.

*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 which would be excessive in real terms. We set this 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 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. 

 Radar Image 1:      AIS radar screen on an Onwa KP-1299A chart plotter.

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 my AIS receiver that whilst I am tacking across the river there isn't a huge freighter just around that blind band in the river. 

K-chart 2.0. River Thames London; AIS vessels underway.

Image showing the previous radar ring information but now, overlaid on our chart and shows 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. 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 long zoom 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 and position.

The 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. 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). 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 transmissions it will be line of site between vessels - 20 miles and even more in ideal conditions whereas for Class B transmissions around 4 to 7 miles and B+_ 8 to 12 is the maximum assuming, again, ideal conditions.  However, do consider that AIS is a close up collision avoidance system so transmitting and even receiving over long distances is not necessary.

From our AVES Marine base in Kent we can see Class A vessels 23 miles away although it must be said that we are at quite a high point in Kent.  

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 high speed (every two to three seconds) other less essential data such as vessel name draft etc 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 center 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. RG58 usually works OK for reception, however, using RG58 for a transmitter is entirely 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 four times the diameter of RG58, it is unwieldy and very hard to install; its a lot more expensive also. A compromise would be RG8X which has lower loss than RG58 yet is thinner than RG213 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;  although it does have a novelty value! 

Comparison of RG213 (on the left) and RG58 cables

Co-axial cable loss comparison at 200 MHz:

RG58   7.3 dB      Loss over 100 ft = 5.9 dB    which is very, very roughly a loss of 75%

RG8x  5.4 dB             "           "          = 4.7 dB                           "                               65%

RG213  3.3 dB           "            "         =  2.5 dB                          "                               40%

You can see from this that transmission loss over cable is quite a consideration.

Antenna height:

The higher your antenna the better will be your transmission and reception efficiency, particularly the former - it's that line of sight thing. However, keep in mind that you don't need to 'see' vessels 20 miles away, just the ones that are getting up close, so for receive AIS it isn't that important to get your antenna 30 feet up at the top of your mast. For transmission of AIS, however, we have that more stringent set of requirements that were mentioned earlier so higher the better is the rule together with the best cable possible.

Using your existing antenna: 

It isn't possible to simply connect an AIS device to your existing VHF antenna because when your VHF is on transmit it will send up to 25 watts of R.F energy into the AIS receiver which will almost certainly not like the experience! With an AIS transceiver the problem goes both ways so you will possibly end up with frying both your VHF and the AIS.  

You can, however do this quite safely by installing an antenna switcher which is an electronic device that prevents damaging either your VHF or the AIS device by allowing only one transmitter to transmit at any one time, preference will automatically be for the VHF. They are quite expensive to buy - for a transceiver antenna splitter it will likely  set you back at least £200 - but will save the hassle of fitting a second antenna and running all that additional cable. We don't supply antenna switchers as we feel that a dedicated AIS antenna is the way to go for AIS reception and transmission.  

Using a dedicated second antenna for an AIS Receiver: 

To avoid the hassle of mounting at the top of a mast if you are using an AIS receiver only I would go for one mounted on the push-pit or the roof of the cockpit. I say this from the practical experience of having a push-pit mounted dedicated AIS antenna which works fine for me as I see commercial shipping from 16 miles away. AIS in Class B and B= vary in their specification, the 16 miles that I can see with my KP1299A may not be achievable with a less sensitive receiver. Y

A dedicated antenna tuned to AIS frequencies together with a short run of decent cable will be a lot less expensive and a lot less hassle than a second installed at the top of a mast.      

Why an AIS transceiver?

All the advantages of having an AIS receiver plus the knowledge that not only do you see the commercial vessels but that they see you also. There is a caveat to this in that commercial vessels, it is said, are able to filter out Class B transmissions. This is understandable in crowded waters frequented by leisure craft and it is something that should always be kept in mind. However, for the relatively small difference in price between an AIS receiver and a transceiver (for Onwa products this is the case for other makes this may not be so) the transceiver version makes a lot of sense - just don't always assume that a commercial vessel sees you either because the Bridge Watch isn't paying attention or they have a Class B AIS filter set on. 

A frequently asked question that we receive: I want an AIS receiver but am unsure if I want to transmit my position via AIS what would you advise?

Answer: Go for the transceiver, you can operate the device with the transmit turned off so in essence you have just a receiver, however, if at some point you want to transmit your vessel's position you have the option of doing so by turning the transmitter on - I can pretty much guarantee that you will do this as soon as that mist starts to come down! The difference in price is small (for Onwa plotters) so you have the best of both worlds. 

Using a dedicated second antenna for an AIS Transceiver:

To be frank I need to do some test to ascertain just how efficient my push-pit mounted antenna performs as a transmitter. I will do this and publish the results at a later date. 

As I previously mentioned when discussing AIS antenna for reception, AIS is intended as a warning system for close up possible collision situations. That, together with the low transmitting power of Class B means that although you may see commercials from 16 miles away they won't see you until both vessels are very much closer. This isn't a negative aspect of Class B as I doubt whoever is in charge of that commercial vessel 16 miles away is particularly interested in another vessel at that sort of distance. However, at 2 or 3 miles they probably may be - so think close up when you are thinking about AIS transmitting distance.

My feeling is though that if you go for an AIS transceiver you need everything going for you and at only 2 watts that means an antenna as high as possible connected to the AIS device via the best low loss cable. This may be a case for an antenna switcher but, bearing in mind that a standard VHS antenna is not ideally tuned for AIS, this might actually suggest the need for a second antenna at the top of the mast although the hassle of installing it is another matter!. Of course, if your vessel is a power vessel without a 30' mast then this is all academic as you are obliged to transmit from low level. In this case get the best aerial you can and connect it with a low loss cable.


Further notes on an Onwa AIS enabled plotter settings that may be useful 

 Using an Onwa AIS plotter

Collision Alarm Settings

In the AIS menu on Onwa plotters there are three user settings that you need to set up for the plotter to calculate collision alerts and alarms. usage and experience will make clear what the appropriate setting might be for your particular circumstance and environment.


Activation ring:

Set this as a kind of barrier around your boat where any Class A or B equipped vessel entering the ring will be tracked by the plotter and will trigger an alarm should the plotter compute that a collision may at some point occur (set by your CPA and TCPA settings)  


CPA - Closest Point of Approach 

CPA is set to the distance that the user feels is as close as he wants another vessel to get before being warned that the possibility of a collision may exist. The plotter computes from the other vessels position, COG and SOG and warns if a vessel is calculated to come within the distance that has been set in the CPA Limit box in the AIS Menu.


TCPA - Time to Closest Point of Approach

TCPA is set by the user to the time that he wants to be warned that another vessel is computed by the plotter to get to the point where it enters the CPA limit that has been set.


 As an example:

You need to decide how close you want to let other vessels get to you (CPA) before an alarm sounds and also how much time (TCPA) it will be before before the vessel gets to that point. For instance, you might decide that 0.5 nm is plenty close enough for large vessels to pass by you and you want to have 10 minutes notice so that you have time to take avoiding action and, if you are unsure what the other vessel may do, call them on the radio using the information that comes up on your plotter in the red boxes on the AIS radar ring page (True for Onwa plotters; others may be different in the way that they display warnings).


A vessel that may represent a real and present danger will show on the AIS radar screen as red and flashing.

Radar image 2:  An alarm has been triggered

A vessel has entered the area that we have defined as being close enough before we want to know about it. The alarm message, which appears in the usually empty red boxes, tells us that the vessel, KEW, will be at that point in 6.2 minutes when it will then be at our set limit of 6.4 nm away 

Note that the ring setting has been made unrealistically far away in this example as our AVES Marine land based site is quite a way from any actual vessels. In practice this distance would be shorter.


A word of advice: when in crowded waters it may be best to turn alarms off to avoid constant alarms. However, using your Onwa plotter's chart you can still visually track nearby AIS equipped vessels and, as always keep a good look out at all times.


 You Tube video of interest

Demonstration of a KP8299A (8" plotter with AIS transponder) in Hong Kong Harbour. Here Vincent Wong Chairman of Onwa sets up the 8299A on one boat which will travel around the harbour while Vincent tracks it from another that is stationary. It is useful as it demonstrates how the plotter on the stationary boat that Vincent is on can track the 8299A on his other boat by way of speed and course. Note how the vessel with the 8299A is represented on the chart of the receiving boat by a triangle with a short line to the front indicating the heading of the boat and a longer line to the rear showing its passage over the ground. 

Note: the patterns in the video that appear on the screen from time to time are not visible in real time to the plotter user - they are a phenomena produced by the video camera and are known as moiré effect. The plotter being used for this demo is not a UK version and the fascia controls and other aspects are written in Chinese. Plotters produced for UK and E.U market use are all in English. By the way, Onwa plotters are multilingual as far as the menu options are concerned


Live link to  map of AIS transponder equipped vessels in the English Channel.


For Technophobes and mathematicians this is the way that Onwa chart plotters calculate the collision warnings CPA and TCPA:

CPA = closest point of approach for 2 boats P and Q, distance between P and Q at closest point of approach.

TCPA = time to closest point of approach

t = time
t0 = start time, now, the time of current calculation

P(t) = position longitude + latitude at time t, let's say P is our boat P (position, velocity u COG+SOG by Onwa-GPS)
Q(t) = position longitude + latitude at time t of other boat Q, position, velocity v transmitted to us/Onwa-GPS by AIS data !

u = velocity/speed vector of boat P in course COG + speed SOG, given by Onwa-GPS
v = velocity/speed vector of boat Q in course + speed, given by AIS
u and v have always the values of t0 , now, the time of current calculation !

P(t0) and Q(t0) are the positions at start time, now, the time of calculation.

|w(t)| = d(t) = distance between boats P + Q at time t, in nautical miles.
w0 = |P(t0) - Q(t0)| = distance between boats P + Q at time t0, current time of calculation, now.

Positions of boats P + Q at any future time t, calculated by vectors u + v and positions now, at start t0, time of current calculation:

P(t) = P(t0) + t * u      , u and v as vectors, each pairs of values of COG + SOG for boats P + Q.
Q(t) = Q(t0) + t * v      , u and v as vectors, each pairs of values of COG + SOG for boats P + Q.

 d(t) = |P(t) - Q(t)|  = |w(t)|     , distance between boats P + Q at any time t
 w(t) = w0 + t * (u - v)   , u and v as vectors, each pairs of values of COG + SOG for boats P + Q.

 So far we have defined now all values and basic equations.
We get now directly (according to reference) the time of closest point of approach = tcpa :

 tcpa   =    -w0 * (u - v) / |u - v|^2

 If |u - v| = 0  then tcpa = 0 by practical definition.

 If tcpa < 0 then cpa closest point of approach was already in past, distances between boats P + Q are increasing, no problems.

 After Onwa-plotter calculated tcpa , it can calculate cpa = distance between boats P + Q at time tcpa, d(tcpa).

 Distance cpa at time tcpa:
(time tcpa = time(now) + tcpa)

 cpa   =   d(tcpa)   =   P(tcpa) - Q(tcpa)  in nautical miles