How are you using AIS?


Besides staring at the electronics, I also require the Mates to look out of the windows.


Yes, all of that. Basically, it lets you get an idea on what they’re doing and how they’re going to maneuver. You don’t get that with relative vectors.


Both trail modes, relative and true, are a true history of a target’s motion, relative or true. 100% reliable information, because it’s memorialized in the past. It can’t change. It is what it was.

Vectors, relative or true, are only mathematically generated predictions of the vessels’ future motion. True trails give the navigator an instantaneous true representation of past movement. With vectors, as soon as a vessel (yours or a target) leaves a steady state (course/speed), the ARPA must recalculate and so the plot is suspect until both your own ship and the vessel(s) being plotted are again in a steady state of motion. This can often be several minutes of unreliable vector representation on the screen This is why I prefer determining initial risk of collision using an EBL, then acquiring priority targets with ARPA, and monitoring their past history of movement using true trails. As soon as I see a target I’ve prioritized leave a steady state, I put an EBL on it.

ARPA is prone to error because it’s a predictive method. Trails and EBLs are past history combined with real-time monitoring that does not rely upon a computer. Then, visibility premitting, the eye out the window trumps just about everything to determine relative motion.


I preferred relative trails as well, especially in busy places like the English Channel. It really helped in prioritizing traffic. Nowadays, with a dedicated patch of real estate I like the true trails to see if the target is stationary or moving. I use AIS for a vessels ID, geographic position, base course and speed. I plot the target via ARPA but if I’m only getting the AIS signal I’ll plot that until I get a signature. If I can visually see the target and it fits the customary traffic pattern or I have contacted the vessel in question I may simply leave the plotted target as an AIS plot. I also use the AIS feed to my PDA (inavx or transas isailor) with an App that recognizes AIS signal sent via wifi or Bluetooth.


Don’t forget the vulnerability of all this lovely gear to cyber attack or simple software glitches.
After a recent system failure on one of our solid state radars (nothing as quaint as a magnetron here) the Windows based programme would not start because it had lost the licence key; they had to email it to us before we could get it going again.
In fog.


Ugh. Transas ECS/ECDIS licensing and updating is a nightmare too.


A standard standing order of mine: compare and cross-check, as frequently as possible, with any and all tools at one’s disposal. So, for collision-avoidance purposes, I encourage my mate to use the AIS solely for the purposes of target identification and comparing against the radar and the :eyes:. If all three agree you’ve got a winner. AIS is by far the lowest ranking of the three, and the first to be disregarded if there is any disagreement.

I generally will not ever use AIS, or allow it to be used, as a standalone means for collision avoidance. Radar, radar, radar + :eyes: all the time. But, never say never. If the radars or ARPAs are all blown out in rain, or fail outright, I’d much rather have the AIS option than not.

It’s like anything else: it’s okay if used carefully and wisely and with restraint, and a full understanding of its limitations. Never trust it too much or let it become a crutch to compensate for weak radar skills or laziness.


Years ago I sent email to the office to ask the lawyers and they just used the IMO spec thats says AIS might be a collision avoidance tool in the future ( so maybe never)
You certainly need that clear in your company and standing orders


Things have to be set up for maximum effectiveness with minimum cognitive workload.

On the Furuno radar model I’m using the AIS fuction can be on and the AIS display off. So the display can be turned off and on without going into a menu.

I like having the 10 cm (S-Band) radar 100% APRA but being able to quickly switch the AIS on/off is helpful, for example it’s a quick way to get the name of the vessel

AIS updates every 2 to 3 1/3 seconds for a turning vessel and the Furuno radar also has a feature that targets with a set minimum CPA will turn red (from green) on the screen. This would be useful back-up, neither APRA or visual are 100% error free. The 3 cm could be set up with 100% AIS or a mix of AIS/ARPA in accordance with the mate’s preference.

The ECIDS can display either AIS or APRA, I’m thinking the ECDIS should be used to display APRA only to insure the watch officer is not relying too heavily on AIS or turning into an ECDIS zombie.


IMO says AIS is for situational awareness not collison avoidance, be hard to say it better than that. I can’t think of any serious ops question I’d ask the CYA office lawyers.


[SARCASM] Those guys have never been on a ship and have no idea what goes on out there [/SARCASM]


Never heard that
Its an american thing forced through the IMO to automatically identify vessels entering the usa
And the situation in current is wrong which is why it was never flagged as a collision avoidance tool

Ps nor has the judge been to sea when your sitting in court!


I’d love to have a copy of that.

How so? COG takes into account wind and current of both vessels. I can’t think of anything more accurate than GPS stabilized ARPA.


Wind it down to a vessel making way say 4kts in 4kts of current and a vessel not making way so drifting at 4kts
Tell me what your ais says versus your arpa?
Who is the stand on vessel?


Resolution A.1106(29)

Adopted on 2 December 2015 (Agenda item 10)

40 The potential of AIS as an assistance for anti-collision device is recognized and AIS may be recommended as such a device in due time.
41 Nevertheless, AIS information may merely be used to assist in collision avoidance decision-making. When using the AIS in the ship-to-ship mode for anti-collision purposes, the following cautionary points should be borne in mind:
.1 AIS is an additional source of navigational information. It does not replace, but supports, navigational systems such as radar target-tracking and VTS; and
.2 the use of AIS does not negate the responsibility of the OOW to comply at all times with the Collision Regulations, particularly rule 7 when determining whether risk of collisions exists.
42 The user should not rely on AIS as the sole information system, but should make use of all safety-relevant information available.


Sounds like a fudge as the previous version of that document was due to the issue i mentioned


Need more info: 4 knots water speed? Stemming? Or 4 knots ground speed? “Who’s the stand-on vessel?” Who knows? it is a crossing, overtaking, or meeting?


For a ship drifting in a 4 kt current ground stabilized APRA will show a true vector of 4 kts.

If it’s water stabilized the other vessel will not have a true vector. Relative vectors of course will be those vectors added to own ship’s vector.

AIS is going to show the other vessel’s true vector over the ground, 4 kts in this case and the other vessels heading.

For vessels in sight of each other determing stand on / give way would require knowing the aspect, not the vector.

For vessel in sight of each other AIS heading information, assuming it’s correct of course, would be more useful in verifying the aspect in this scenario if for some reason aspect could not be determined visually.


AIS is very useful in areas such as the East China Sea. The Chinese fishermen position an AIS unit on every fishing buoy. Helps other ships to navigate safely.
I always use Rel vectors and true trails. There is a toggle switch VECTOR on the control panel to change between Rel/ true vectors , so it is really not difficult.
Most navigators today cannot use the anti sea clutter and anti rain clutter properly because they do not know the electronic design of how these two work. The anti-sea clutter suppression strength is inversely proportional to the range; whereas the anti-rain clutter is applied uniformly over the entire range. Also these controls require to be frequently adjusted…every 30 min in my view.


Ground stabilized ARPA should match AIS perfectly.
Water stabilized ARPA would incorrectly say that the drifting vessel is stationary.
Either way would give the same relative motion and the same CPA and TCPA.

Neither ARPA mode can tell you that, you need to see the other vessel. If you can’t see the other vessel there is no stand on vessel.