Collision Avoidance Using AIS

Most navigation programs (electronic charting systems, ECS) will read and display AIS targets provided to them by a live AIS receiver on the boat or via an internet connection. The former source is much preferred because the internet sources are notably delayed; internet AIS can be very useful for training, but underway it is mostly for a general awareness of who might be in the neighborhood, not where they are at the moment and it cannot be relied upon for collision avoidance.

Most of the ECS AIS displays also show the age of the individual reports in seconds that can be compared with the standards shown below, which is adapted from our post Introduction to AIS.  This makes it clear to the user that they are being updated as required.

A typical display of AIS traffic is shown below. In this ECS (qtVlm) there are several options for learning about a target and also our interactions with it, which is the subject at hand that we get to shortly.

The dark blue reports are tooltips. Only one at a time would show and then disappear when the cursor moves. The CPA info in white background can  be turned on for any vessel. Ours is the white one in center screen. The Details can be turned on for one or multiple targets.  It is not the subject at hand, but the voyage data sent out may not be right. Here we see a vessel reported as "moored" that is moving at 12.7 kts, and another marked as engaged in fishing, which in fact will not be fishing till it gets back to Dutch Harbor. We can't blame the ECS for marking it with a fishing daymark, because that is what it was told.  Again our reminder: To see details, click the pic, then right click and open in new tab, and then zoom.

In the above picture we do not have the collision avoidance functions turned on. We are seeing the COG predictor lines set for 6 min. The end of the green dashed lines is where the vessels will be 6 minutes from now.  It is usually best to have this reckoning time be the same for our vessel as it is for the target vessels. This much is already very valuable information, but in fact the ECS can do much more than that.

Short of using those computed collision avoidance displays, that is what we have to work with, the COG predictor lines of our vessel and the targets, and the goal at hand here is see what can we learn from just these predictor lines.  

For any discussion of collision avoidance we must return to the Navigation Rules that tell us about collision risk in Rule 7, and in particular Part (d) (i)

Rule 7 - Risk of Collision

(a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt such risk shall be deemed to exist.

(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.

(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.

(d) In determining if risk of collision exists the following considerations shall be among those taken into account:

(i) Such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change.

(ii) Such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range.

First, modern court cases have established that "all available means" definitely includes ECDIS (for commercial vessels) or ECS for those without ECDIS, and that AIS is now on par with the radar requirements.  In short, these are powerful tools and it is essentially negligent not to use them if available.

For present considerations, the key point is (d) (i), which tells us how to detect risk of collision using compass bearings. Note the word "shall"; this is not a suggestion, nor an option, it is required.  In basic navigation we often teach a shortcut using relative angles on the bow.

The above is from our textbook Inland and Coastal Navigation. This method assumes the heading of our boat does not change during the observation. Using actual compass bearings removes that uncertainty.

We can apply this principle to the display of AIS projectors, because we have in effect two views of the target taken 6 minutes apart, or 12 minutes if we used that. If the bearing does not change we are on a collision course. If the bearing moves toward the bow, the target is crossing in front of us; if it moves aft, we are crossing in front of it, as illustrated below.

Below is a compilation to show that the concept of bearing moving toward or away from the bow can be applied to targets seen in any direction.

So even without sophisticated closest point of approach (CPA) functions, we can determine if a target is crossing in front of or behind us... that is, as soon as we are able to measure or estimate the bearing to the target from preset positions and also from the tips of the projection lines which would be the bearings we see after watching the target for 6 min or 12 min.  Six minutes is a popular reckoning choice, because at 6 min the length of the predictor line (nmi) is one tenth of the boat speed  (kts).  At 12 min prediction, we divide the length by 2 to get the speed.

Most ECS display the range and bearing to each target, so we do not have to measure the first one, just read it.  Also all ECS  have a quick measurement tool or ruler tool that we can use to measure the bearing from our projector tip to the target projector tip, and that is then the second bearing. Below is a sample from qtVlm. We are left then to reason through the direction we are looking and decide if this is toward or away from the bow.... or if we are on a collision course. 

Keep in mind that a collision course is indeed a rare thing. Just a degree or so (that we cannot really measure in this method) will bypass a collision, and a notable degree change is a fairly big separation.

Here the red lines and dashed magenta line were just added for perspective. We read first bearing from the tooltip and then use ruler tool to learn that the next bearing would be notably forward so this target is passing in front of us.

If we then turned on the closest point of approach (CPA) data we would see this immediately. Below is that view about when they cross.

We see that indeed this target passes well in front of us, and we will have CPA in 8 min, and the CPA will  be 1.1 nmi. Notice where our 6 min position is compared to 2 min later at CPA.  With this type of data we can get these forecasts from a long distance off, but that brings us back to Rule 7 (c), which is the good advice for radar observations that also apply to AIS computations. We do not want to rely on such data from a long ways off, where just slight course changes or current sets can affect the later CPA. 

Note that the AIS gives us both COG and true heading for those vessels with a heading sensor, so we can often get an idea about the currents—and maybe understand why their tracks are drifting off to one side of the traffic lanes or other obvious fairway routes.  We saw this very nicely in our reenactment of the grounding of the Ever Given in the Suez Canal.

I will add more examples later on, with and without the CPA info to illustrate this point.

To keep this in perspective, our logical first choice is to learn how to use the CPA configurations in our ECS and these then typically solve the problem of collision risk evaluation. But if you are new to an ECS on a new boat, you may not know or understand what is being shown, so this basic view of the targets based on COG predictors alone could be helpful. Also sometimes the CPA configuration for AIS can have multiple settings to account for various conditions, which means there is the chance that they are not set up in the optimum manner for present conditions.

Also, needless to say, learning whether a target is crossing in front of you or behind you is just the beginning of our obligation in the Navigation Rules.  From Rule 7 we move on to Rule 8, Action to Avoid Collision.