Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[El Pinguino]

Originally Posted by Lodesman

.....
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.

I don't think it implies that at all, although the other ship's bearing relative to your ship's head plays a role which is spelt out in the rules.... a ship fine to port is a different animal to one on your starboard quarter. .... and true bearings and true headings don't come into that side of things...

However, lets say you are steering 090*, other ship is bearing 135*..... bearing changes to 130*... is 'closing'... is simple....

[Dockhead]

Originally Posted by DotDun

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).

Yeah, I guess fishing boats are to us, as we are to ships We try to steer a wide berth around them in the Channel, especially on the French side. In the North Sea, the F/V's seem to be larger and more professionally run. They actually seem to maintain some kind of watch, if you can imagine that, and switch on their AIS when we approach them at night, which ids much appreciated.

I might really be changing to Furuno next electronics upgrade.

I doubt that the calculated data can be anywhere near as accurate as what we get on AIS -- which is not calculated. But ultimate accuracy is not important if you follow reasonable procedure and maintain reasonable CPAs. Not being dependent on the target's broadcasting would be priceless. I would very much like to have properly functioning ARPA/ MARPA.

[Lodesman]

Originally Posted by El Pinguino

... is 'closing'... is simple....

"is drawing left" or "is decreasing"... is simple and unambiguous. But it's all semantics, use what you want.

[transmitterdan]

I doubt ARPA is “less accurate” than AIS. Precision does not equal accuracy.

AIS depends on accuracy of 2 GPS units plus the competence of the person entering the vessel specific data. I see quite a lot of “anchored/moored” vessels doing 15 knots.

[Dockhead]

Originally Posted by transmitterdan

I doubt ARPA is “less accurate” than AIS. Precision does not equal accuracy.

AIS depends on accuracy of 2 GPS units plus the competence of the person entering the vessel specific data. I see quite a lot of “anchored/moored” vessels doing 15 knots.

Well, course, speed, CPA and TCPA as provided by AIS don't require any human data entry, so don't depend on the "competence of the person entering" etc. GNSS is accurate to a few meters.

IMO standards for accuracy of big ship ARPAs are 0.3 miles for CPA, 5 degrees for course, and half a knot for speed. Our radars with their tiny antennae will fall far short of that I suppose. I guess AIS will be at least two orders of magnitude more accurate than even a very good boat radar ARPA or MARPA.

But again - that much accuracy is not critical - because the thing being measured is itself not so stable. That's we pass with reasonable margins of error and don't cut it so close in open water.

[transmitterdan]

Pedantic mode on:

CPA requires correct entry by two different humans of the GPS antenna location for both vessels. Not saying they get it wrong often but merely that a person has to do it.

[Paul Elliott]

Originally Posted by transmitterdan

Pedantic mode on:

CPA requires correct entry by two different humans of the GPS antenna location for both vessels. Not saying they get it wrong often but merely that a person has to do it.

But does *any* plotter or program calculate AIS CPA based on vessel dimensions and GPS antenna location? And don't forget that for such a calculation to work, the ship heading needs to be accurately indicated, and I've seen more than a few where the HDG is "not available".

For that matter, how does ARPA deal with ship dimensions?

Regardless, I think if we would be crazy to rely on either AIS or ARPA to let us cut it so closely that the ship dimensions were a significant factor. If we need to get that close I would be using my eyeballs on the ship, and on the radar screen if appropriate. And yes, I realize that we are being pedantic, and no one is suggesting that we aim for such a close crossing solely using AIS or ARPA for guidance.

[transmitterdan]

Paul,

I agree with you about making a close approach “on instruments”. But a lot of people think because it is “electronic” then it is also “accurate”.

I think AIS is supposed to take vessel size and antenna location into account when computing CPA. For a 500-1,000 foot ship it can make a significant difference.

[Lodesman]

Originally Posted by transmitterdan

I think AIS is supposed to take vessel size and antenna location into account when computing CPA. For a 500-1,000 foot ship it can make a significant difference.

Interesting point. If a 1000-ft ship has its AIS antenna mounted on the bow, and that information is programmed into it, does it:

a)subtract that 1000 ft when reporting the cpa a vessel going astern?; and

b)transmit that data to other ships, and if so do they add 1000 ft to the GPS location transmitted?

Or do I on my little boat, being told by AIS that I'm passing 1000 ft astern of the ship, slam into her transom?

[evm1024]

I was down on my boat yesterday and took some time to check on the accuracy of both my Class B AIS rig and my Raymarine radar MARPA.

Looking at the position of the boat on the chart plotter showed me to be on the dock where I actually was. Chart, AIS, reality all lined up.

Taking a bearing to a buoy mid channel and plotting that from the GPS indicated position put the boat and the buoy on the same line of position. Crosscheck GPA. Spooky accurate!

I "think" I recall that AIS transmissions transmit the GPS position which is quite accurate, the placement of the GPS antenna as entered by humans and the size of the ship as entered by humans. Transmission rated for each data type vary based on the AIS spec (class A/b, speed, ROT etc). The point being that if I remember correctly, the use of (or not) all this data to calculate CPA was left for the receiving end.

Switching on the RADAR and autopilot (which gives my 10 Hz heading) I sought to verify that the heading was accurate.

So to start with to get heading I needed to mount the (fluxgate) compass in an area that was as free from stray magnetic fluxes as possible, that there was no electromagnetic flux when I turn on various devices and was in a portion of the boat with minimal motion. All human setup variables.

Using a hand bearing compass I was able to see that the ship head as indicated by the AP was off by about 5 degrees on this heading. I eventually plotted my position and head using a sextant held sideways to get the most precise angles I could to a few distant objects. (Wow, paper charts and a sextant....).

I say on this heading because there is a deviation table in the AP that corrects for deviation. Who knows how accurate that table is. It has been a long time since I did an AP deviation setup. And, I never swung the boat to check on the accuracy of the electronic compass. More human input to do the cal for deviation and alignment.

With that all up and calibrated to my satisfaction I fired up MARPA and selected the buoy for my first target. MARPS locked right up on the buoy and started reading out its "course and speed". As a few sweeps painted the buoy it was obvious that the echo from the buoy did not align with the charted position. Yikes, Looks like the Radome was not aligned with the boat. Or rather more likely it was aligned correctly when the system was last calibrated but things changed (human error, naaa).

So, taking a little time to adjust the RADAR brought the buoys echo to overlay its position on the chart. A quick check shows other buoys echos are aligned with their charted position.

Of course the echo dances around a bit and MARPA data for the stationary buoy shows that it is going this way then that way at up to 0.5 kt or so. It almost hit 1 kt a time or 2.

I think that my going through the setup for the various bits that MARPA depends on reminded me of the uncertainties involved in RADAR positions and MARPA data.

Reviewing the uncertainties in GPS antenna placement and stitching that to ship size, heading and the like reminded me of the uncertainties in GPS "data".

For me this reinforces the need for the prudent mariner to keep a significant CPA from any other vessel encountered. 1 nm does not sound like too much. 1000' sounds way too close (in a fog - ouch!).

There are many uncertainties in fixing the distance between out boats and other vessels.

[Dockhead]

Originally Posted by transmitterdan

Pedantic mode on:

CPA requires correct entry by two different humans of the GPS antenna location for both vessels. Not saying they get it wrong often but merely that a person has to do it.

Keep the pedantry coming! I don't want to be the only one

Yes, this is true, and it's a good point, but the maximum theoretically possible error is within a ship length, and an error of such a magnitude would be rare. This is quite a bit less than the 3 cables error of ARPA.

I suppose it is another advantage of radar, that it works from (usually) that part of the ship which is closest to you.

[Dockhead]

Originally Posted by evm1024

I was down on my boat yesterday and took some time to check on the accuracy of both my Class B AIS rig and my Raymarine radar MARPA.

Looking at the position of the boat on the chart plotter showed me to be on the dock where I actually was. Chart, AIS, reality all lined up.

Taking a bearing to a buoy mid channel and plotting that from the GPS indicated position put the boat and the buoy on the same line of position. Crosscheck GPA. Spooky accurate!

I "think" I recall that AIS transmissions transmit the GPS position which is quite accurate, the placement of the GPS antenna as entered by humans and the size of the ship as entered by humans. Transmission rated for each data type vary based on the AIS spec (class A/b, speed, ROT etc). The point being that if I remember correctly, the use of (or not) all this data to calculate CPA was left for the receiving end.

Switching on the RADAR and autopilot (which gives my 10 Hz heading) I sought to verify that the heading was accurate.

So to start with to get heading I needed to mount the (fluxgate) compass in an area that was as free from stray magnetic fluxes as possible, that there was no electromagnetic flux when I turn on various devices and was in a portion of the boat with minimal motion. All human setup variables.

Using a hand bearing compass I was able to see that the ship head as indicated by the AP was off by about 5 degrees on this heading. I eventually plotted my position and head using a sextant held sideways to get the most precise angles I could to a few distant objects. (Wow, paper charts and a sextant....).

I say on this heading because there is a deviation table in the AP that corrects for deviation. Who knows how accurate that table is. It has been a long time since I did an AP deviation setup. And, I never swung the boat to check on the accuracy of the electronic compass. More human input to do the cal for deviation and alignment.

With that all up and calibrated to my satisfaction I fired up MARPA and selected the buoy for my first target. MARPS locked right up on the buoy and started reading out its "course and speed". As a few sweeps painted the buoy it was obvious that the echo from the buoy did not align with the charted position. Yikes, Looks like the Radome was not aligned with the boat. Or rather more likely it was aligned correctly when the system was last calibrated but things changed (human error, naaa).

So, taking a little time to adjust the RADAR brought the buoys echo to overlay its position on the chart. A quick check shows other buoys echos are aligned with their charted position.

Of course the echo dances around a bit and MARPA data for the stationary buoy shows that it is going this way then that way at up to 0.5 kt or so. It almost hit 1 kt a time or 2.

I think that my going through the setup for the various bits that MARPA depends on reminded me of the uncertainties involved in RADAR positions and MARPA data.

Reviewing the uncertainties in GPS antenna placement and stitching that to ship size, heading and the like reminded me of the uncertainties in GPS "data".

For me this reinforces the need for the prudent mariner to keep a significant CPA from any other vessel encountered. 1 nm does not sound like too much. 1000' sounds way too close (in a fog - ouch!).

There are many uncertainties in fixing the distance between out boats and other vessels.

Indeed! It also shows the importance of cross-checking and reality-checking data with data from other sources, including your eyeballs. When you see radar targets and AIS carats diverging, that gives you a good idea about the limits of the data.

i don't use MARPA much, simply because it works so poorly on my radar, but I always run the radar if there is any kind of traffic, and I switch on radar overlay on the chart, and double check the AIS carats with the radar targets.

And at the risk of being boring, I'll repeat one more time -- good collision avoidance procedure does not require super-accurate data about the other vessel. Having it sure doesn't hurt anything, but it's not essential.

[Capt. Mike Maurice]

The harsh truth is that the Colregs(or Inland) Rules apply with equal force to all vessels. There is no distinction between commercial and recreational.
This is not my interpretation, but that of the U.S. Supreme Court.
P.D. Carmilla & M.P. Drzal, "Foremost Insurance Co. v. Richardson: If this is Water, It must be Admiralty", 59 Wash. Law Rev. I (1983)

[Dockhead]

Originally Posted by Capt. Mike Maurice

The harsh truth is that the Colregs(or Inland) Rules apply with equal force to all vessels. There is no distinction between commercial and recreational.
This is not my interpretation, but that of the U.S. Supreme Court.
P.D. Carmilla & M.P. Drzal, "Foremost Insurance Co. v. Richardson: If this is Water, It must be Admiralty", 59 Wash. Law Rev. I (1983)

This is an important principle, not recognized by many recreational sailors. I think most of us on here would agree whole-heartedly.

Thanks for posting the cite.

[transmitterdan]

Originally Posted by Lodesman

Interesting point. If a 1000-ft ship has its AIS antenna mounted on the bow, and that information is programmed into it, does it:

a)subtract that 1000 ft when reporting the cpa a vessel going astern?; and

b)transmit that data to other ships, and if so do they add 1000 ft to the GPS location transmitted?

Or do I on my little boat, being told by AIS that I'm passing 1000 ft astern of the ship, slam into her transom?

The way it is supposed to work is the AIS receiver create a virtual outline of the vessels and offsets that by the antenna location. Then it should compute the nearest point of intersection between those two outlines. But I do not know if all AIS units do it correctly or not.