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Old 11-06-2014, 02:29   #76
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Re: Thoughts about AIS

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Originally Posted by Dockhead View Post
Yes, my display also displays extended COG lines which show where the vessels will be in a given amount of time. This is somewhat dangerous because it is possible to set the time for other vessels and for your own boat differently, in which case you get a completely false idea of relative positions over time. If the times are set the same, then you see the relative positions at that time -- which is very useful in case that time is somewhere near the time of CPA.

Whether or not the lines cross tells you almost nothing.


Yes, AIS shows you what you could never see before -- when and how ships are changing course and speed. It's amazing how much you can see and know which was totally obscure before.

Hi Dockhead

I'm not sure what you mean by setting the times? The arrows shown on my display take into accounthte relative speeds of the boats- no time setting involved as far as I can tell (form my limited experience using this)
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Old 11-06-2014, 03:07   #77
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Re: Thoughts about AIS

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Hi Dockhead

I'm not sure what you mean by setting the times? The arrows shown on my display take into accounthte relative speeds of the boats- no time setting involved as far as I can tell (form my limited experience using this)
I don't know what your arrows are, but on my Zeus displays -- just like on the ancient Raymarine displays they replaced -- you get an extension line coming out of the icon showing your boat which shows where you're going according to COG. The length of the line corresponds to some time period you set in the menu -- 5 minutes, 10 minutes, 30 minutes I think are the usual choices. So the end point of the line is exactly where you will be in that set amount of time. In the Zeus, you can set up the same kind of lines for AIS targets. So if the time period is set the same, then the end points of your own extension line and the extension line of your AIS target will show you the relative position at the end of the set period of time. This is really useful when there's a big difference in speed, because the line will be longer for the fast ship and you can see what happens over the set period of time. Without that, you really have no idea.
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Old 11-06-2014, 03:23   #78
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Re: Thoughts about AIS

I'm still figuring out the various options available, but I'm pretty sure my system is set according to distance not time. This means that when a boat enters the distance radious, a line shows up showing COG. The length of the line is relative to SOG and distance between the two boats

I can see that this will involve some type of time application. Guess I need to play around with the settings some more.
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Old 11-06-2014, 03:38   #79
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Thoughts about AIS

I would be somewhat concerned at using closing vectors to make passing decisions , in tight heavy traffic crossing situations. Various errors can build up to a point where there is concern that the display is lying to you. There is in effect a " cone of error" associated with the cog vectors. Both vessels have a cone of error , once you are within the intersections of those cones, you cannot reliably determine if the vessel will pass ahead or astern ( leaving aside potential future course change issues ). This is often very clearly illustrated when you use manual radar plotting techniques

This was what happened to the moody ( Wakona?) which was run down in the English Channel.

MARPA is even worse as all data is related to your boat and heading is not stable.

Note that APRA by IMO definition must be capable of automatically acquiring and maintaining 30 targets, MARPA ( mini-APRA) is some unregulated subset. In my experience MARPA on small boats unless you have fast display processing and super accurate heading only available from satellite compasses is virtually useless.

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Old 11-06-2014, 04:26   #80
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Re: Thoughts about AIS

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Originally Posted by goboatingnow View Post
I would be somewhat concerned at using closing vectors to make passing decisions , in tight heavy traffic crossing situations. Various errors can build up to a point where there is concern that the display is lying to you. There is in effect a " cone of error" associated with the cog vectors. Both vessels have a cone of error , once you are within the intersections of those cones, you cannot reliably determine if the vessel will pass ahead or astern ( leaving aside potential future course change issues ). This is often very clearly illustrated when you use manual radar plotting techniques

This was what happened to the moody ( Wakona?) which was run down in the English Channel.

MARPA is even worse as all data is related to your boat and heading is not stable.

Note that APRA by IMO definition must be capable of automatically acquiring and maintaining 30 targets, MARPA ( mini-APRA) is some unregulated subset. In my experience MARPA on small boats unless you have fast display processing and super accurate heading only available from satellite compasses is virtually useless.

Dave

I don't think that accuracy concerns mean you shouldn't use AIS for making passing decisions (?!). It was made for that purpose, I think. I think it rather means that you must choose a CPA which is appropriate considering the likely accumulated errors. It is often vitally important to know whether you're passing a mile ahead or a mile behind, and you usually can't know this without either recording the evolution of the bearing to the target, or using AIS. If it's ok to make passing decisions according to bearings taken from a HBC, then surely no one would think anything wrong using much more accurate data from AIS? uzzled:

We use the projected COG line for all kinds of purposes. I find it to be extremely accurate and extremely useful, even lifesaving, especially when you use it to determine whether you can clear some obstacle or not, especially with a strong tide running or other circumstances where your COG may be very different from where your bow is pointing.

As to the uselessness of MARPA: on my old system with a plain old-fashioned Raymarine fluxgate compass, MARPA was nearly but not entirely useless. I have specially -- keeping MARPA especially in mind -- furnished my new system with the most accurate heading data you can get for less than several thousands dollars -- an Airmar H2183 three-axis gyro-stabilized heading sensor. Although I have no illusions that the performance is comparable to what a big ship's ARPA system -- using a heading sensors costing as much as a house and a big computer and all done from a much more stable platform than what we have -- can do, it is already much more useful. AIS calculations of CPA, TCPA, etc. are much more stable -- maybe orders of magnitude -- but MARPA works reasonably well for targets not broadcasting AIS.
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Old 11-06-2014, 04:46   #81
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Re: Thoughts about AIS

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Originally Posted by Dockhead View Post
I don't think that accuracy concerns mean you shouldn't use AIS for making passing decisions (?!). It was made for that purpose, I think. I think it rather means that you must choose a CPA which is appropriate considering the likely accumulated errors. It is often vitally important to know whether you're passing a mile ahead or a mile behind, and you usually can't know this without either recording the evolution of the bearing to the target, or using AIS. If it's ok to make passing decisions according to bearings taken from a HBC, then surely no one would think anything wrong using much more accurate data from AIS? uzzled:

We use the projected COG line for all kinds of purposes. I find it to be extremely accurate and extremely useful, even lifesaving, especially when you use it to determine whether you can clear some obstacle or not, especially with a strong tide running or other circumstances where your COG may be very different from where your bow is pointing.

As to the uselessness of MARPA: on my old system with a plain old-fashioned Raymarine fluxgate compass, MARPA was nearly but not entirely useless. I have specially -- keeping MARPA especially in mind -- furnished my new system with the most accurate heading data you can get for less than several thousands dollars -- an Airmar H2183 three-axis gyro-stabilized heading sensor. Although I have no illusions that the performance is comparable to what a big ship's ARPA system -- using a heading sensors costing as much as a house and a big computer and all done from a much more stable platform than what we have -- can do, it is already much more useful. AIS calculations of CPA, TCPA, etc. are much more stable -- maybe orders of magnitude -- but MARPA works reasonably well for targets not broadcasting AIS.
With HBCs, rather like paper chart position fixing and plotting, it was clear there was a significant error and people acted accordingly.

The danger with projected COG, is that people start to intrinsically believe everything they see on the screen and starting make passing decisions on increasing narrow CPAs, merely because the vectors say its "seemingly" safe. This is especially true of passing ahead of the oncoming vessel.

As for MARPA, the big issue on a small boat is they is actually no easy way to determine the error, IN earlier displays there were serious limitations in processing and screen updates, resulting, in tracking several vessels, widely varying COG vectors and irrelevant alarms. even with stabilised compasses, these are still fluxgate and subject to all the issues associated with them. Having played with most of the modern small boat systems, I retain a healthy scepticism. especially in bad weather ( which is when you most need it )

I know you understand your systems well, but many do not.

dave
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Old 11-06-2014, 04:49   #82
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Re: Thoughts about AIS

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We use a Sony experia as a mouse and kb to drive the pc when in the cockpit.
That sounds a really useful way to control OpenCpn on a cockpit screen - how is it done?
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Old 11-06-2014, 15:16   #83
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Re: Thoughts about AIS

Just a few thoughts... if you are using COG vectors, it only makes sense to me if the time period the vector represents is the same for both yourself and the target. In any case, if you are relying on these vectors, I encourage understanding the way this works on your display.

Fixed time vectors don't reveal where you'll be in relationship to one another at CPA or crossing. For this you need a vector time that is appropriate for each individual target. openCPN does this nicely, but on a relatively small display I think it can become cluttered and difficult to read. It is also a bit complicated to understand until you get the hang of it.

For what it's worth... we chose to display this info on a per-selected target basis. It displays fixed length vectors for the top N collision risk targets and CPA vectors for a selected target. The downside to this is you must select the target of interest to see the CPA vector. But the upside is it's very easy to understand what you are seeing.
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Old 11-06-2014, 20:04   #84
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Re: Thoughts about AIS

Piou, the app is unified remote. The free version works fine.

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Old 12-06-2014, 05:19   #85
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Re: Thoughts about AIS

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Originally Posted by carstenb View Post
Hi Dockhead

I'm not sure what you mean by setting the times? The arrows shown on my display take into accounthte relative speeds of the boats- no time setting involved as far as I can tell (form my limited experience using this)
On my displays (and on the Ray displays which preceded them), the length of the extension lines corresponds to some period of time you can set. So the end point of the line shows where you will be in the set amount of time, assuming you maintain the same COG and SOG. It's really very useful.

If you set this time horizon the same for your own boat and for AIS targets, then you see the relative positions of you and them in a set period of time.
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Old 12-06-2014, 05:21   #86
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Re: Thoughts about AIS

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Originally Posted by jeffrobbins View Post
Just a few thoughts... if you are using COG vectors, it only makes sense to me if the time period the vector represents is the same for both yourself and the target. In any case, if you are relying on these vectors, I encourage understanding the way this works on your display.

Fixed time vectors don't reveal where you'll be in relationship to one another at CPA or crossing. For this you need a vector time that is appropriate for each individual target. openCPN does this nicely, but on a relatively small display I think it can become cluttered and difficult to read. It is also a bit complicated to understand until you get the hang of it.

For what it's worth... we chose to display this info on a per-selected target basis. It displays fixed length vectors for the top N collision risk targets and CPA vectors for a selected target. The downside to this is you must select the target of interest to see the CPA vector. But the upside is it's very easy to understand what you are seeing.
And that seems like the optimum way to do it. To me, at least.

Wish my Zeuses would do it that way.
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Old 12-06-2014, 14:10   #87
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Re: Thoughts about AIS

i guess i am having trouble with all this -- when we get a cpa that we find is a bit worrisome we simpy pick up the radio and work out a passing - most ships will talk to you as they want to be safe -- fast ferrys on the other hand may not as we found out yesterday --

by call the ship by name and knowing they can see us it works --
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Old 12-06-2014, 15:16   #88
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Re: Thoughts about AIS

There is a vast difference between using the COG line projections for a user selected time period, and the computer generated projection of where each vessel will be at CPA. I think that you have to experience it to understand it.

I have used Nobletec Odyssey with this feature with many crossing situations, and by observing it under good visibility conditions I am confident that it is very valuable. I agree with the previous post making the point that manipulating COG line projections is dangerous. It is time consuming, and you could easily have yours and theirs set to different times and act on incorrect information. On the other hand the computer generated lines to CPA are done so quickly that it creates a moving display depicting changes made by each vessel in almost real time. If you do ever try it, you would not want to be without it in my opinion.

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Attachment 82725

This display provides the needed information.
  1. Your boat is the red one. The target boat is going at high speed in a traffic lane.
  2. You can quickly learn that the target is going to continue in the lane, and expect him to turn slightly to starboard to avoid the separation zone.
  3. Your boat is safely out of the traffic lane.
  4. Note that your heading is about 15 degrees to starboard of your projected track. This is due to current flowing starboard to port, and you need to be aware of it to avoid shallow water. The displayed soundings, and approaching land display allow making proper decisions.
This information is all the more necessary when running in fog. Quick situational awareness of your position, and AIS targets gives you more time to look for non-AIS boats on your radar.
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Old 12-06-2014, 15:28   #89
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Re: Thoughts about AIS

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i guess i am having trouble with all this -- when we get a cpa that we find is a bit worrisome we simpy pick up the radio and work out a passing - most ships will talk to you as they want to be safe -- fast ferrys on the other hand may not as we found out yesterday --

by call the ship by name and knowing they can see us it works --

Try doing that is a TSS in the middle of the English Channel. !!!! , their reply. I'm holding my course, what are you doing ?

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Old 12-06-2014, 21:37   #90
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Re: Thoughts about AIS

In Asia..they generally don't bother to answer
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