Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[jackdale]

Originally Posted by Jim Cate

From the point of view of the yacht, it does not matter how often a B class AIS updates the data. It is how often the ships AIS updates, and the ship will likely have A class AIS. From the ship's pov, the slower update rate, coupled with the relatively erratic speed and course of the yacht might lead to inaccurate CPA calculations, and hence influence their choice between ARPA and AIS for decision making.

Jim

Point taken.

[transmitterdan]

Even on our boat I like to identify a “threat” and put a MARPA flag on it. Then I can see near real time info on their relative bearing, estimated SOG and COG. We have an old Raymarine radar but it does the MARPA thing better than AIS in my view. But it can’t handle lots of vessels and I have to mark the ones I want to target. So we can use OpenCPN and AIS to track all other AIS vessels.

[DotDun]

Originally Posted by jackdale

A class A AIS updates every 2-10 seconds.

Position updates for Class B transponders are broadcast less often than Class A transponders. Vessels going less than 2 knots transmit position updates every 3 minutes while vessels traveling more than 2 knots transmit position information every 30 seconds. (https://www.milltechmarine.com/faq.htm#a6)

That might account for the difference.

Correct!

I wanted to show the difference. With a 14kt vessel speed, a class B updates every ~700ft, whereas ARPA update every 50-100ft. The more updates the more solid CPA/TCPA can be calculated.

[wjhutchings]

Originally Posted by Lodesman

Bearings are not normally described as "closing" or "opening" - that would be how one refers to range. The bearing could be said to increase/decrease, draw right/left, or veer/back. If it's on your port side and the bearing is decreasing, then it will pass astern of you; if on your starboard side, the bearing would need to increase for it to pass astern of you.

I am afraid for real seaman they are described as opening or closing the amateurs may use something else

[StuM]

Originally Posted by wjhutchings

I am afraid for real seaman they are described as opening or closing the amateurs may use something else

You are apparently unfamiliar with Lodesman's background

And Mr Google is obviously not a real seaman, because searches on "opening bearing" and "closing bearing" reveal nothing relevant.

[Dockhead]

Originally Posted by DotDun

Correct!

I wanted to show the difference. With a 14kt vessel speed, a class B updates every ~700ft, whereas ARPA update every 50-100ft. The more updates the more solid CPA/TCPA can be calculated.

The update rate is determined by the transmitting station, not the receiving station.

Class "A" sends a new report every 10 seconds for a vessel on a steady course and speed up to 14 knots, and 6 seconds for 14-23 knots. If the vessel is changing course, the update rate increases to 3.3 or 2 seconds.

That's how often the target data for a Class "A" carrying vessel will be updated on your AIS.

Ais Reporting Intervals

I don't know how you keep your radar set up; mine is normally on 24 RPM. I think 48 is max for recreational radars. A radar paint every second or two is NOT equivalent to an AIS report -- every radar paint gives you just a rough idea about bearing and a pretty good idea about range. You need a pretty decent series of them for the errors to average out. AIS doesn't need to calculate the position, course and speed of the target -- the target is reporting that data directly.

Not to take anything away from the impressive ARPA pictures posted above, but even with the best recreational radar, I don't think it's possible for us to get anywhere near as accurate a picture of position, course, and speed of a vessel with Class "A" AIS, or calculation of CPA and TCPA, as we can with AIS.

I also wouldn't mean to take anything away from the great value of seeing the target on the radar screen. I am a big fan of radar, which does far more than AIS does (if I were forced to pick only one, I would pick radar and give up the AIS).

And all this may be different in dealing with Class "B" Mark 1 targets, as someone mentioned above.

[El Pinguino]

Originally Posted by StuM

You are apparently unfamiliar with Lodesman's background

And Mr Google is obviously not a real seaman, because searches on "opening bearing" and "closing bearing" reveal nothing relevant.

I have spent the last 55 years working with bearings that were either 'opening', 'steady/constant' or 'closing'.... but what would I know......

Backing/veering is - btw - a met term.... I have never never ever ever seen it used to refer to a change in bearing.

You don't appear to know WJHutchings background....

Damn!!!! Like a moth to the flame.... I'm sucked back in ....

[Alan Mighty]

Originally Posted by El Pinguino

I have spent the last 55 years working with bearings that were either 'opening', 'steady/constant' or 'closing'.... but what would I know......

My memory has it that from about the mid-1960s, talk of 'bearing drift' and reporting 'bearing drift left', 'bearing drift right', and so on became fashionable.

And the source was a new cohort of sonar operators, radar operators, and target officers, who had been trained to calculate how to maintain zero bearing drift to a target (by adjusting vessel speed and heading) or how to force bearing drift (if bearing drift is zero and range is decreasing, then collision is inevitable).

[TeddyDiver]

Originally Posted by jackdale

A class A AIS updates every 2-10 seconds.

Position updates for Class B transponders are broadcast less often than Class A transponders. Vessels going less than 2 knots transmit position updates every 3 minutes while vessels traveling more than 2 knots transmit position information every 30 seconds. (https://www.milltechmarine.com/faq.htm#a6)

That might account for the difference.

Untill they don't..

[markpierce]

Originally Posted by Jim Cate

From the point of view of the yacht, it does not matter how often a B class AIS updates the data. It is how often the ships AIS updates, and the ship will likely have A class AIS. From the ship's pov, the slower update rate, coupled with the relatively erratic speed and course of the yacht might lead to inaccurate CPA calculations, and hence influence their choice between ARPA and AIS for decision making.

Jim

That would depend on the level of visibility. Regardless, don't rely on AIS regarding small boats and lesser craft as most don't have AIS.

[Jim Cate]

Originally Posted by markpierce

That would depend on the level of visibility. Regardless, don't rely on AIS regarding small boats and lesser craft as most don't have AIS.

No, my statements are true in any sort of visibility. And while it is true that some small ships and many small craft do not broadcast AIS, this discussion is relative to large ships which are mandated to do so.

Jim

[El Pinguino]

Originally Posted by Alan Mighty

My memory has it that from about the mid-1960s, talk of 'bearing drift' and reporting 'bearing drift left', 'bearing drift right', and so on became fashionable.

And the source was a new cohort of sonar operators, radar operators, and target officers, who had been trained to calculate how to maintain zero bearing drift to a target (by adjusting vessel speed and heading) or how to force bearing drift (if bearing drift is zero and range is decreasing, then collision is inevitable).

'Flares' for blokes were also fashionable in the 60's... https://www.prettylittlething.com.au...red-pants.html

And Grey Funnel Line's idea of a 'target' is just that... a target

[Lodesman]

Originally Posted by El Pinguino

I have spent the last 55 years working with bearings that were either 'opening', 'steady/constant' or 'closing'.... but what would I know......

Backing/veering is - btw - a met term.... I have never never ever ever seen it used to refer to a change in bearing.

You don't appear to know WJHutchings background....

Jeez you're ancient - no wonder you're such a cranky barsteward I'm not getting worked up over WJHutchings' comment - he's just taking the piss. It's fair enough - I was mostly chiding him because 1200 posts into the thread, he enters in giving the same basic advice that's been given about 40 times previously.

I don't use backing or veering for anything other than met; just put it in as an example - it's archaic but that's where it came from. Where do you think they got the terms? You must have "veered" an anchor with a manual capstan back in the old wooden sailing ships when you started out - do you recall the direction they turned?

I know that nautical dialects vary from region to region and again through sub-elements of the marine industry. I don't really care to see WJ's CV, but wonder where he has plied his trade, if he cares to tell us? I don't know, or really need to know where and how you've spent 55 years either - perhaps the terminology is antipodean? My background is Canadian Navy, which is derived from the Royal Navy, who've been around for nearly 500 years, but what do they know? Who knows, my understanding might be purely Canuck, but I don't ever recall seeing opening or closing described as you use it, in any of the Admiralty manuals. There 'opening bearing' and 'closing bearing' have specific meanings in the navigational sense as types of transit used for compass/gyro checks.

Anyway, opening or closing in referring to a bearing, would by context suggest that it is opening or closing from your own ship's head; this implies using relative bearings. This is proscribed by colregs. Rule 7 specifically uses "compass bearing", and the various guides explain why.
But I'm sure you knew that.

[El Pinguino]

Originally Posted by Lodesman

Jeez you're ancient - no wonder you're such a cranky barsteward ........... .

Cranky?, you should see me when I'm off my meds......

Ancient? Well I did have this real old bloke say just the other day that he still wanted to be sailing when he was my age....

Moving right along... some light entertainment... collision avoidance the Vesper way... scroll down to the first vid.... https://www2.vespermarine.com

Go below, put on kettle,....'is that an alarm I hear?' ... run back to helm, yabber into VHF while going 'hard a port' or is that 'full left rudder'? and - at the same time - looking at smart watch thingo...

Ship under his bow is 'Trans Future' ........ bet he got closer than 180 feet to it...... ship on his smart watch is 'Acrux'

Hope he turned the gas off.......

[DotDun]

Originally Posted by Dockhead

The update rate is determined by the transmitting station, not the receiving station.

Class "A" sends a new report every 10 seconds for a vessel on a steady course and speed up to 14 knots, and 6 seconds for 14-23 knots. If the vessel is changing course, the update rate increases to 3.3 or 2 seconds.

That's how often the target data for a Class "A" carrying vessel will be updated on your AIS.

Ais Reporting Intervals

I don't know how you keep your radar set up; mine is normally on 24 RPM. I think 48 is max for recreational radars. A radar paint every second or two is NOT equivalent to an AIS report -- every radar paint gives you just a rough idea about bearing and a pretty good idea about range. You need a pretty decent series of them for the errors to average out. AIS doesn't need to calculate the position, course and speed of the target -- the target is reporting that data directly.

Not to take anything away from the impressive ARPA pictures posted above, but even with the best recreational radar, I don't think it's possible for us to get anywhere near as accurate a picture of position, course, and speed of a vessel with Class "A" AIS, or calculation of CPA and TCPA, as we can with AIS.

I also wouldn't mean to take anything away from the great value of seeing the target on the radar screen. I am a big fan of radar, which does far more than AIS does (if I were forced to pick only one, I would pick radar and give up the AIS).

And all this may be different in dealing with Class "B" Mark 1 targets, as someone mentioned above.

ARPA target acquisition takes time, anywhere from 1 to 3 minutes, but once acquired, and what the screen shots show, is ARPA does deliver faster and as accurate data as Class B AIS delivers. I haven't run the same comparison with a Class A AIS target, but Furuno's ARPA is pretty impressive, I'll bet it would be very close. Maybe maxing it out on 30 ARPA targets simultaneously would tax the system, I've never seen that much traffic running AIS to compare. I have no personal experience with MARPA. I have used ARPA to tiptoe thru 18 shrimpers fishing in about a 2 mile radius. That was fun, they perform drastic heading changes at the most inopportune times.

My radar does run at 24 & 48 rpm. IIRC, it is set to auto adjust speed, but I don't know/remember the criteria it uses to adjust speed (I assume shorter range runs faster).