Thoughts about AIS

[xymotic]

Originally Posted by Dockhead

Ah, I had thought of using negative and positive numbers for the CPA -- good to know that someone has actually implemented it! That should also be in every interface. I'm just curious -- what does a negative CPA mean? Like a decreasing bearing? Because a decreasing bearing can mean either behind or ahead, depending on what direction the ship is coming from. I'm not sure "ahead" and "behind" is always clear, depending on the relative angle of courses.

The Zeus does show the target triangles in bold when they become "dangerous", according to criteria you can set, and will gray out targets which are stationary (you can even hide them). This is useful, but we need to know which way the bearing is going! I think this is much better for those of us who learned navigation in the stone (non-electronic) age, because it corresponds to how we see potential collisions with a HBC. I think even for youngsters who've never used a HBC, it's useful to think about bearings when you see ships on the horizon.

As to AIS alarms: I had never used them until this last trip. I set them in open water and turn them off on approach to harbors. Like other alarms (especially my favorite, radar guard zone), I think its main purpose is when you're in open water out of sight of land and outside of shipping lanes and not keeping an intense visual watch. I know someone will be shocked by the irresponsibility of it, but I actually took a nap or two on deck last month, when out of sight of land and out of shipping lanes. I set a radar guard zone, set the depth alarm for 10 meters, set the AIS alarm, set alarms for large course changes and wind shifts, and went to sleep for half an hour at a time. Was a Godsend during the single handed part of my trip.

The Zeus doesn't throw a line in front of the target Showing it's trajectory and where it will be in X minutes? With "X" being defined as a preference by the user Every AIS implementation I've ever seen has done that and I'd be quite surprised if it's not even in the spec for AIS itself???? Comparing the length of the projected lines it becomes pretty easy to determine how you are going to pass (unless they are real similar in which case there is risk of collision)

[colemj]

Originally Posted by Dockhead

I'm not sure what the real difference is between MARPA and ARPA -- AFAIK it may be just a difference in specification and not necessarily functionality.

Yes, targets are acquired manually -- you have to point the cursor to a target then go into menu and "acquire targets". I thought ships' ARPA systems worked the same way. Interesting yours acquires automatically. How do you stop it from picking up a hundred meaningless blips and overloading itself?

We have the option of manual, automatic, or a mix (pick so many targets automatically and leave the rest for manual). Frankly, we only use automatic on rare times we set a guard zone at night. The guard zone can be set to automatically acquire ships entering it while leaving the others alone. The hundreds of targets problem is solved by it only being able to track 30 max

Mark

[bcn]

Another aspect for CPA/TCPA:

the stability of the Heading and COG of a small vessel.
Or better the variance of several degrees we are observing (and transmitting).

One parameter is the sensor used and it's filters, the other the real data that are quite "vivid" as well - obviously depending on sea state and winds.

Makes it sometimes difficult to judge the situation shown by the AIS correctly, especially for longer distances. TCPA/CPA are jumping.

It would be interesting to hear from the "other side of the fence" - the big guys - how they deal with this/us taking this data noise into consideration.

[xymotic]

http://bandg.com/Documents/products/...language=en-GB page 39 you just need to set the 'extension line' to be over a certain time and then match that pref on your own vessel

When you are approaching a point with another boat if they have a line well past yours you will cross behind If yours is well past theirs you'll cross ahead and if the lines meet in the middle you and they are bold and blinking at you: you got trouble

(i may have those backwards because I haven't actually been on the water in a couple years and I forget but it becomes second nature in practice very quickly

[colemj]

I just had a look at the Zeus manual on MARPA and it is manual-only, 10 targets max and rather cumbersome. You need to go into the menu section, select "acquire target" and then leave the menu and click the cursor on the target. You need to repeat this for all targets you wish to collect.

On the Furuno, in manual acquisition mode, one simply clicks the cursor on the targets - no menu system to go through for each target. For up to 30 targets.

BTW, this isn't a platform debate - I post this info because you asked for how other systems work in these situations.

Mark

[St. Elsewhere]

Originally Posted by Dockhead

The main problem is that you have no good way of determining whether a target is passing ahead or behind. This information is crucial in determining what kind of maneuver you need to make in order to create a safe crossing with the target.

The most absolutely essential is the dynamic of the bearing, but dynamic of the CPA would also be extremely useful. This could be easily represented simply by showing the BRG number in green (for increasing) or red (for decreasing), in three different shades to show the speed of increase or decrease. Or, chevrons, say, next to the number -- one pointing up means slowly increasing, two pointing up means increasing somewhat faster, three means increasing fast; down chevrons means decreasing. Or both color and chevrons.

Then you instantly see what's going on with each target, rather than having to write them down over a period of time. Having to use paper and pencil with a sophisticated electronic navigation system seems kind of ridiculous.

I am running Nobletec Odyssey and it provides this information for the critical target. It is a graphic that projects the future track of your boat and the target to CPA. It also shows a line connecting the two points at CPA. And it updates continuously.

[chuckr]

we run raymarine with a nauticast ais -- all the feeds go our chartplotter and we track more vessels than we can count and a quick click on each and we can see the cpa and time and decide if we need to keep track of it or not -- it puts all the info on our screen --

during the day we generally run the chartplotter in map mode but at night and specially on overnight passages we run the radar screen and the ais feeds it as well - so we get both the ais feed and radar so we can see the targets that do not have ais

would not want to sail without it in the type of cruising we are doing -- we simply love it

best of all is we see the name of the vessel and have at times called and made agreement on how we can pass safely

[Dockhead]

Originally Posted by colemj

And that was the point I was trying to make - when one is getting a good composite graphic of various data, one only needs to glance at it to have the situational perspective sufficient to augment your eyeballs. I don't care about the actual bearing numbers, if they are increasing or decreasing, etc. I have the AIS display thresholds set to complement my eyeball navigation. The composite graphic tells me instantly and dynamically the position of all ships relative to me and each other, their movements relative to me and each other, which ships are anchored, which are not an immediate threat and which are an immediate threat. All on a chart and all in a short glance! When I look back up, that composite picture is floating in my brain.

I have never found a need to take HBC, paper and pencil and start writing down numbers and plotting movements. Or even much care about that level of detail coming into the forefront of my consciousness.

It may be that this type of question bothers only people who sail in very busy areas. But if you don't care about the bearing numbers, I bet you've never been in this situation.

You are crossing a shipping lane, and you are crossing between three ships which are all approaching to you from starboard. You are under power so they are all stand-on. Your AIS shows that the two last have dangerous CPA -- a couple of cables.

What do you do? The "composite graphic" which "tells me instantly and dynamically the position of all ships relative to me and each other, their movements relative to me and each other" doesn't tell you to alter course to starboard, or to port, and doesn't tell you whether you can pass between the two ships, or only behind both of them, or only ahead of them, or what. You might be able to figure it out from projected COG lines -- maybe. But you will only know for sure if you know which way their bearings are going. If the second ship's bearing is reducing slowly and the third ship's bearing is increasing slowly, then you're poised to pass between them at a dangerous distance (based on what AIS is telling you about CPA), and you need to alter to starboard to pass behind the third ship, and maybe a big alteration if that ship's bearing is increasing. You cannot know this from your "composite graphic" -- you need the bearings and their rate of change.

[Dockhead]

Originally Posted by xymotic

http://bandg.com/Documents/products/...language=en-GB page 39 you just need to set the 'extension line' to be over a certain time and then match that pref on your own vessel

When you are approaching a point with another boat if they have a line well past yours you will cross behind If yours is well past theirs you'll cross ahead and if the lines meet in the middle you and they are bold and blinking at you: you got trouble

(i may have those backwards because I haven't actually been on the water in a couple years and I forget but it becomes second nature in practice very quickly

Yes, I mentioned in the original post that you sometimes can see from the COG extension lines whether you're passing ahead or behind.

But it only works when the extension lines are roughly the right length in relation to the range of your target, and it only works if you have a reasonable CPA. If you are having a real close encounter, they will not tell you which way to turn, because the lines appear to meet.

[Dockhead]

Originally Posted by colemj

. . .

BTW, this isn't a platform debate - I post this info because you asked for how other systems work in these situations.

Mark

Yes, of course, it's useful information, and what I asked for.

The Zeus has pluses and minuses -- in general I like it ok, but I'm not a fanboy and unless Navico pays me , I'm not going to engage in advertising for it . So it doesn't bother me at all to talk about its drawbacks. I agree that the MARPA implementation is pretty weak, and I've always heard that Furuno had an implementation different from others, and it's been interesting to hear the details.

I think MARPA is important and it sounds like the Furuno has a really good implementation. I understand that the Furuno radars are the very best of the pulse radars, so that must be a strong combination. The MARPA on my Zeus, on the other hand, is not significantly different from that of my ancient Raymarine Pathfinder system, other than the fact that it is much more usable since it has much better heading data than what the old Pathfinder got from the creaky old fluxgate I used to have. I expected much better from the new setup and was rather disappointed with it.

It's not a big deal because with AIS, I am finding MARPA to be very secondary. And since it is actually usable, unlike the MARPA on the old Pathfinder with the dodgy heading data, I think it's more or less good enough.

Unlike one of the posters above, I have not encountered a single ship not broadcasting AIS. The only vessels I have encountered without AIS were small pleasure vessels, I think nothing over 40', no kind of vessel at night, and a fishing vessel or two in stealth mode. I was glad to spot the FV's on radar at night and notice them despite no AIS -- congratulated myself for keeping a good watch. But the fishing vessels all switched on their AIS when I got within a few miles, then switched them off after I passed. So in this part of the world (third world may be different), the fishermen also seem to keep good watches.

[DotDun]

Originally Posted by Dockhead

I think MARPA is important and it sounds like the Furuno has a really good implementation.

Furuno doesn't have MARPA, they have ARPA. Big difference, IMHO!

[Dockhead]

Originally Posted by DotDun

Furuno doesn't have MARPA, they have ARPA. Big difference, IMHO!

What exactly is this "big difference"? I have just trawled the interweb again, and still can't find any difference, other than manual versus automatic target acquisition. AFAIK, it's almost exactly the same thing, but I would be glad to be enlightened, if I'm wrong and if there really is some "big difference".

[colemj]

Originally Posted by Dockhead

It may be that this type of question bothers only people who sail in very busy areas. But if you don't care about the bearing numbers, I bet you've never been in this situation.

You are crossing a shipping lane, and you are crossing between three ships which are all approaching to you from starboard. You are under power so they are all stand-on. Your AIS shows that the two last have dangerous CPA -- a couple of cables.

What do you do? The "composite graphic" which "tells me instantly and dynamically the position of all ships relative to me and each other, their movements relative to me and each other" doesn't tell you to alter course to starboard, or to port, and doesn't tell you whether you can pass between the two ships, or only behind both of them, or only ahead of them, or what. You might be able to figure it out from projected COG lines -- maybe. But you will only know for sure if you know which way their bearings are going. If the second ship's bearing is reducing slowly and the third ship's bearing is increasing slowly, then you're poised to pass between them at a dangerous distance (based on what AIS is telling you about CPA), and you need to alter to starboard to pass behind the third ship, and maybe a big alteration if that ship's bearing is increasing. You cannot know this from your "composite graphic" -- you need the bearings and their rate of change.

Like I said earlier - we go through the Panama canal entrance a couple of times each year where there are literally hundreds of boats at anchor, milling around in circles and charging forward at full speed. To top it off, there is a large breakwater that one cannot see over with a narrow entrance that allows 2 ships to pass through very closely. We have to get through all the ships and through this breakwater entrance whose goings on on the other side is blind to us (but not blind to AIS).

In your situation above, a quarter mile (if I understand "cable" correctly - who uses "cable" as a distance measurement?) is no problem to us in a shipping lane and none of the ships would be a danger. If it appeared that a ship was going to be much closer than that, the AIS display with predictor lines would easily tell me the situation without needing to know bearings and rate of change.

Heck, I would probably drop down a knot or two in speed since I am under power and see how the situation changes - that is apparent in 30 seconds or so. Or maybe change course 5 degrees and see. That also will tell me a lot very quickly. Unless I hadn't been paying attention and only noticed those two ships bearing down on us at the last moment, I really don't see the problem in your example.

Now you want something to really screw around with you - try a few Disney cruiseliners off a major port at night. Those guys just randomly circle around waiting for daylight to enter a harbor. One minute they are ringing your alarms and the next they are heading off in a different direction, only to reverse and come at you again. You can't see your AIS display because of all the floodlights, disco balls, laser lightshows, etc blinding you...

Perhaps we are just more used to using AIS integrated into our navigation mode? We certainly go through complicated and busy shipping lanes/ports with little problem with navigation.

Mark

[Pelagic]

Originally Posted by Dockhead

What exactly is this "big difference"? I have just trawled the interweb again, and still can't find any difference, other than manual versus automatic target acquisition. AFAIK, it's almost exactly the same thing, but I would be glad to be enlightened, if I'm wrong and if there really is some "big difference".

Great thread Dockhead!

MARPA stands for MiniARPA and initially means it is not IMO compliant. No minimum screen size, no automatic acquisition and is missing a number of look ahead features useful to the watchkeeper.
One is the "Trial" presentation where you can test an avoidance solution in advance by programing into ARPA a specific change in Course (and/or) Speed and for when test will begin
You then push "Trial" and a big T shows on screen.
Targets are all advanced with new CPA data's for each target to confirm if your test solution works. (Automatically reverts back to live screen in 3 minutes)

Useful in high density traffic with multilateral considerations to a course change.

There are other differences as well. Some MARPAs have a few but ARPA I believe means IMO compliant.

You would need to check the latest ratified IMO rules on ARPA to confirm the differences on new radars

[Dockhead]

Originally Posted by Pelagic

Great thread Dockhead!

MARPA stands for MiniARPA and initially means it is not IMO compliant. No minimum screen size, no automatic acquisition and is missing a number of look ahead features useful to the watchkeeper.
One is the "Trial" presentation where you can test an avoidance solution in advance by programing into ARPA a specific change in Course (and/or) Speed and for when test will begin
You then push "Trial" and a big T shows on screen.
Targets are all advanced with new CPA data's for each target to confirm if your test solution works. (Automatically reverts back to live screen in 3 minutes)

Useful in high density traffic with multilateral considerations to a course change.

There are other differences as well. Some MARPAs have a few but ARPA I believe means IMO compliant.

You would need to check the latest ratified IMO rules on ARPA to confirm the differences on new radars

Thanks, that's useful!

I don't care much about automatic acquisition, but the ability to test a proposed change of course or speed would be terrific!! Wow, I wish my AIS set could do that, too.