Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[Juho]

Originally Posted by Paul Elliott

As an option, NavMonPc does move AIS targets along their ded-reckoned path. I call it "extrapolated" position, and it merely uses position, SOG, and COG to advance the icon. I ignore rate-of-turn.

Obviously the longer the interval between AIS updates, the less reliable the DR position might be. However, nearby ships generally send reports often enough that this doesn't become a big issue. I do display the time since update on the target info window so you can decide how much to trust the displayed position.

Cool stuff

[transmitterdan]

Originally Posted by Juho

Cool stuff

Yes, it is cool. My opinion is that some (most?) AIS displays imply a precision that is not really there. OpenCPN displays a line to the CPA. But the reality is that the CPA must be within some shape other than a dot on the chart. Would be nice to hear other opinions on how to estimate the uncertainty in the CPA "zone". Probably should be the "danger zone" and we could use Top Gun soundtrack as the warning when entering the zone.

[evm1024]

Originally Posted by Dockhead

That will work!

That's how you plot relative motion, and the line you are talking about, the "14 degree line", is called the relative MOTION line (motion because speed is also part of it, not just course). It's an analogue technique and works. It's easier to interpret if you do it on a plotting sheet or maneuvering board.

You can also do this with math, but it needs to be done right -- you will have to solve 6 right triangles. There have been some calculations on here by home-made methods which do not give the correct result.

Note that you can use a single line with multiple targets only if they are moving at the same course and speed.

Thanks, I forgot the name of it (relative motion line). Been a while since I let my license expire...

[Paul Elliott]

Not to plug NavMonPc, but there is yet another option for the AIS screen that displays all target vectors as relative. Normally, our own boat's course/speed vector is displayed along with target course/speed vectors. In the Relative mode, our own vector is subtracted from all the target vectors, resulting in relative vectors being displayed.

I usually run in absolute vector mode, but it can be instructive to switch the display over to relative mode.

[carstenb]

Actually I've done something akin to what Rod is proposing several times, although it has been in the straits between Denmark and Sweden where the big ships are sailing in defined channel and I have to cross. I sail outside the channel on the opposite course of the freighter and turn in behind him to cross.

So there are several points that need to be addressed:

1- He knows that I will stay out of the shipping channel and therefore is not going to take evasive action, but he will continue just as a stand on vessel should
2- I've learned when I make the turn in behind him not to turn too quickly because there is one hell of a turbulence back there
3- When we have done this we are generally lying a cables length or so from the channel.
4- He flies by - in the time it takes us to sail the cable or so - he's long gone.
5- if we were trying to be a half cable behind him - we would be lying right on the edge of the channel and turning right about when his bows passed us
6 - no way I would do number 5 above - no matter what. simply isn't safe.

Rod argument is that "a miss is as good as a mile" in the Colregs sense - but that is a falsum. There would be brown shorts all the way around on the ship and most probably the master of the ship would report Rod adn his boat to the authorities. In Europe - Rod quite likely would get either a huge fine or jail time

[TJ D]

Originally Posted by carstenb

Actually I've done something akin to what Rod is proposing several times, although it has been in the straits between Denmark and Sweden where the big ships are sailing in defined channel and I have to cross. I sail outside the channel on the opposite course of the freighter and turn in behind him to cross.

So there are several points that need to be addressed:

1- He knows that I will stay out of the shipping channel and therefore is not going to take evasive action, but he will continue just as a stand on vessel should
2- I've learned when I make the turn in behind him not to turn too quickly because there is one hell of a turbulence back there
3- When we have done this we are generally lying a cables length or so from the channel.
4- He flies by - in the time it takes us to sail the cable or so - he's long gone.
5- if we were trying to be a half cable behind him - we would be lying right on the edge of the channel and turning right about when his bows passed us
6 - no way I would do number 5 above - no matter what. simply isn't safe.

Rod argument is that "a miss is as good as a mile" in the Colregs sense - but that is a falsum. There would be brown shorts all the way around on the ship and most probably the master of the ship would report Rod adn his boat to the authorities. In Europe - Rod quite likely would get either a huge fine or jail time

To the last point, I would report Rod's maneuver as reckless if he did it to me, particularly if I happened to be participating in a TSS or was in any way restricted. In open water-he'd never get that close. I'd view his action as insufficient and would make my own move. Rule 2 and all that.

[TJ D]

Originally Posted by Paul Elliott

Not to plug NavMonPc, but there is yet another option for the AIS screen that displays all target vectors as relative. Normally, our own boat's course/speed vector is displayed along with target course/speed vectors. In the Relative mode, our own vector is subtracted from all the target vectors, resulting in relative vectors being displayed.

I usually run in absolute vector mode, but it can be instructive to switch the display over to relative mode.

Paul, can you enlighten me a little on the NavMonPC? I'm running an ancient toshiba in the nav station with CM93, and wouldn't mind beefing up the functionality a bit. It doesn't look like it'll work with Windows 10 from the site. Any feedback?

[JPA Cate]

TJ D,

I'm running Open CPN with CM 93 on my windows 10 tablet, so it does work.

Ann

[Juho]

Originally Posted by transmitterdan

Yes, it is cool. My opinion is that some (most?) AIS displays imply a precision that is not really there. OpenCPN displays a line to the CPA. But the reality is that the CPA must be within some shape other than a dot on the chart. Would be nice to hear other opinions on how to estimate the uncertainty in the CPA "zone". Probably should be the "danger zone" and we could use Top Gun soundtrack as the warning when entering the zone.

Variation in the results comes probably mainly from variation in the headings of the two vessels. The heading of a small sailboat can vary quite a lot. Location information is probably more stable. Certainly some variation there too, but quite regular, impacting accuracy when the other vessel is very near, but less harmful when it is far away.

If this is the case, the latest headings can not be trusted too much, and should not be considered accurate information. You can improve this e.g. by taking the average of the two last headings. This of course introduces some delay in the measurements. Also the longer history of AIS data can be used to estimate how much we should assume the course to be stable (i.e. can be derived quite accurately from few last measurements, including the location data), or if we should assume that the other vessel might be turning right now. (The turning radius information might not be any more accurate, I don't really know.) We can assume that our own vessel will keep its course, or at least we know the parameters of our AIS system, like its delay after we change course.

When drawing the anticipated CPA on the display, one could display an area rather than a single spot. The size of the area would depend on the estimated uncertainty in the measurements. No problem to display the last and few other latest measurement points there too (you can see on what side of the area they are). The displayed area should take into account also estimated movements since the last received AIS messages.

[Juho]

Originally Posted by carstenb

Actually I've done something akin to what Rod is proposing several times, although it has been in the straits between Denmark and Sweden where the big ships are sailing in defined channel and I have to cross. I sail outside the channel on the opposite course of the freighter and turn in behind him to cross.

So there are several points that need to be addressed:

1- He knows that I will stay out of the shipping channel and therefore is not going to take evasive action, but he will continue just as a stand on vessel should
2- I've learned when I make the turn in behind him not to turn too quickly because there is one hell of a turbulence back there
3- When we have done this we are generally lying a cables length or so from the channel.
4- He flies by - in the time it takes us to sail the cable or so - he's long gone.
5- if we were trying to be a half cable behind him - we would be lying right on the edge of the channel and turning right about when his bows passed us
6 - no way I would do number 5 above - no matter what. simply isn't safe.

Rod argument is that "a miss is as good as a mile" in the Colregs sense - but that is a falsum. There would be brown shorts all the way around on the ship and most probably the master of the ship would report Rod adn his boat to the authorities. In Europe - Rod quite likely would get either a huge fine or jail time

I have met with a ship on a (roughly) 90° course with CPA shorter than 180'. I don't know if this really counts since the speed of the ship was maybe around 10 knots. At the CPA time my speed went down close to 0 knots.

We met in a long narrow strait, coming from opposite directions, and I escaped to a small bay like formation, making a 270° turn, waiting for the ship to pass by. I would have stayed further away, but there was no space to do that. The place was quite safe in the sense that the "bay" was shallow and all the ship could do was to stay in the middle of the strait.

[poiu]

Originally Posted by transmitterdan

Yes, it is cool. My opinion is that some (most?) AIS displays imply a precision that is not really there. OpenCPN displays a line to the CPA. But the reality is that the CPA must be within some shape other than a dot on the chart. Would be nice to hear other opinions on how to estimate the uncertainty in the CPA "zone". Probably should be the "danger zone" and we could use Top Gun soundtrack as the warning when entering the zone.

There can be a big error in the normal presentation of the data. If the ship is a big oil tanker or bulk carrier it will have its GPS located above the bridge near the stern probably and if they have set up the AIS system to just report that position then the bow of the boat will arrive nearly a quarter of a mile before the stern. Even if they put the GPS in the middle of the ship then you will have more than a cable of error just from this.

[conachair]

Originally Posted by poiu

There can be a big error in the normal presentation of the data. If the ship is a big oil tanker or bulk carrier it will have its GPS located above the bridge near the stern probably and if they have set up the AIS system to just report that position then the bow of the boat will arrive nearly a quarter of a mile before the stern. Even if they put the GPS in the middle of the ship then you will have more than a cable of error just from this.

The Opencpn is , as usual, excellent. Using AIS and thinking it's OK to get so close that the transponder position makes a big difference will end in tears sooner or later..

[Dockhead]

Originally Posted by Juho

. . . If our intention is to cross the track of the other vessel from behind the vessel, it is obvious that the polite and correct way to do it is to change the direction of our vessel so that it points behind the other vessel, not in front of it. At least that is what I regularly do when I meet with other vessels, and we must decide and indicate to each others how we plan to cross. Turning my bow so that it points to the other side of the other vessel would carry a false message of my intentions.

If we plan to cross the track of the fast vessel as close to its aft as possible, one may have to take a 90° course against its route (in front of its bow). Or actually the most efficient angle (to cross the track as close to the transom as possible) is even slightly more towards the direction where the other vessel is going.

And since out plan is to cross right next to the transom of the other vessel, our course is about the same that we would use when trying to cross in front of its bow. Such a course would be really confusing to the other vessel. In such a situation it would make sense to the other vessel to react, and maybe turn or speed up or slow down. That would be quite bad behaviour on out side.

Maybe one should take also this kind of obvious colregs clarity requirements into account when counting what kind of manoeuvres are possible and what kind of actions we must avoid. Keeping one's nose on the correct side with respect to the other vessel could be one such requirement. At least we should not keep the other vessel uncertain of which side of it we plan to cross (VHF might help if we really want or have to make this kind of moves).

Yes, yes, yes.

Now we're getting to the heart of the matter.

Look at the 1 mile plot of 5 knots yacht vs. 20 knot ship. Can you see from either vessel, who's passing ahead and who's passing behind? You can't, not by any means, because the planned crossing takes place well within the uncertainty cone. The yacht appears to the ship fine off the port bow, and the ship appears to the yacht less than two points ahead of the beam, and the "nose" can't be on the "right side". Depending on slight variations of course and speed, the yacht may end up ahead or under the bows of the ship, rather than passing close behind.

This is not just a matter of clarity of intentions -- the statement in the Rules against small repeated alterations is intended to require decisive maneuvering from a safe distance so that the other ship has a chance to evaluate and has time to make its own move if it is not satisfied.

That's why you can't approach a ship, sailing into close quarters on a collision course, intending to correct later -- if you are passing behind, it needs to be clear and visible that you are passing behind, from a safe distance away. This is one of the elements of a safe CPA.

[Dockhead]

Originally Posted by evm1024

Thanks, I forgot the name of it (relative motion line). Been a while since I let my license expire...

The name is not important; the concept is. Obviously you didn't forget anything important.

[Dockhead]

Originally Posted by conachair

. . . Using AIS and thinking it's OK to get so close that the transponder position makes a big difference will end in tears sooner or later..[/IMG]

Indeed. Using any data source and overestimating it accuracy will end in tears, or worse, when doing collision avoidance. A key element of a safe CPA is giving enough space to account for what we don't or can't know.

I suspect that that data which is most susceptible to misuse and misinterpretation, is bare eyes observations. I bet someone has studied what you can and cannot see and discern with the naked eye, but I haven't been able to find any sources on it.