Target association (radar S/X, AIS)

So I have been investigating target association lately, and I have found that overlaying AIS targets on the radar is a possibility. This allows you to validate a radar blob with the AIS target I guess.

I have also seen that you can overlay ARPA targets on the ECDIS, and also show AIS targets on the ECDIS. This would duplicate a target on the ECDIS and might clutter the chart.

One question I have in mind is when there is two (or more?) radars on the ship. Many have both S- and X-band radars, and in inland navigation I know many have two S-band radars, one in aft and on in fore. Does each radar have it’s own ARPA? Are these radars completely disconnected? Because then you have three sources of information about targets in your surroundings, one from S-band, one from X-band and one from AIS.

I also know that you can associate radar targets with AIS targets, but can you also associate S-band targets with X-band targets? And if so, can you associate these with AIS so you have all three targets showing as one individual one? That sounds very smooth in my ears, to avoid clutter and having one target to consider from three sensors.

Does anyone have insights into this?

You can have all kinds of fun if the radar menu is not set up correctly with the heading inputs, the AIS version of the target can end up 5 or 10 degrees off the radar heading along with all the navaids and so on. It can be a true or magnetic issue or you might have to get into the service menus for other alignment issues.
Is there a function that moves an AIS target between beacons to match what is presumably the radar return of the same thing? I have not used a radar that does that, in close quarters I have seen a boat cross my bow and visually and on radar be on one side and the lagging AIS still be on the other side.
I have also never overlayed X and S, but if you do you can have things like one big blob on S and two close-together buoys on X, which might make for odd results.

It should be possible with something that associates the AIS target with the radar target with the option of merging the two targets into one.

Deep-sea ships are required to have two radars, one X-Band one S-Band, both have ARPA.

The GPS, ECDIS, AIS, log speeds etc are all networked together. AIS data can be displayed on the AIS itself, on either or both ARPAs and/or ECDIS, output to a laptop via pilot plug, transmitted via WIFI etc.

I am sure it could be,the radar would use ARPA to tag an AIS target when they matched and then move the AIS symbol/ident around in real time. I have never actually seen a radar that does that though, but I certainly haven’t seen every kind of radar.

One of the reasons why this could be happening is that the AIS transmitter on the other vessel is not sending data on a frequent interval. The interval can be anywhere from a few seconds to few minutes.

Next time you have an underway AIS target designated, have a look at their nav status. If the nav status says they are moored the AIS may only be transmitting their data every couple of minutes. Your radar blob and ARPA target tracking info will update with every turn of the scanner, but the AIS data may be several minutes old.

Good thread here. How are you using AIS?


I value AIS and I value ARPA, but I do not like the lack of understanding some officers have on each systems limitations. AIS targets on radars are nice until you realize that the person operating the radar might not actually be plotting ARPA. More times than not they highlight the AIS target and think they are looking at ARPA information. Maybe its just the brand radars I use, but I prefer to use the ARPA with no AIS targets turned on and the ECDIS with AIS and brief uses of the radar overlay if I want to verify/match an ARPA and AIS target. They are both side by side so it is fairly easy and removes the potential for someone thinking they are getting two points of information but are only actually getting one.


anybody who cares to read user guide for o/b radar S & X band will find that both have the so called Master /Slave mode and I have seen this feature since 1992 on all ships.

There are 2 types of AIS units, Class A and Class B. Class A are compulsory on commercial vessels >300GT. These units transmit all of their info every 2 seconds for moving and manoeuvring vessels, and up to 3 mins if at anchor. Class B units transmitting times however can be up to 5 seconds apart if SOG exceeds 23 knots, or at a fairly standard time of up to 30 seconds apart at lesser speeds. So your radar image is what you are seeing in ‘real time’ and the second image will be the AIS image some 2, 5, 30 seconds behind. Something to be very much aware of if in a close quarter’s situation.

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The AIS triangle symbol is near, but not on top of the radar target so that you can select the radar target, not the AIS symbol for the ARPA to plot.

On a weak target or at a distance, the ARPA may track the AIS symbol instead of the actual radar target.

There is a very wide variety in the ability of mates to properly tune, and adjust radars for the sea and weather conditions, and to properly use ARPA. There is often over-reliance on “auto” features and often excessively high settings of sea clutter and rain controls.

Captains may get carried away with “my way” is the only way to do it. Prompting mates to go through the motions of pretending to do it the Captain’s way, but with greatly reduced effectiveness.