AIS INSTEAD of radar?

[Dockhead]

Originally Posted by El Pinguino

Your AIS gives course and speed over ground.... you should be using course and speed through the water.. . .

Well, your AIS plots range, bearing, CPA, TCPA (and BCR and BCT on commercial sets). These are neither water nor ground referenced -- they are based on relative position and vectors.


Your AIS REPORTS as received (it does not calculate) COG, SOG, and position of the target, and if it's being broadcast, HDG and ROT. Plus all the static data.




Why do you need course (or rather, heading) and speed through the water? This will puzzle many here, I guess. It's not for automatic plotting -- as I said, the four basic calculated facts -- range, bearing, CPA and TCPA are neither water nor ground referenced. The type of data used to calculate these things is not important so long as it is consistent.



HOWEVER, if you set your radar use COG and SOG instead of HDG and STW, you will get a distorted radar picture, like this, when the radar is in relative motion and North Up:





https://www.myseatime.com/blog/detai...ich-one-to-use

[Dolphin460]

AIS plus radar and why. Best way to explain is by example. I was leaving Greece for Sicily. It was dark. I was under full sail on a hard strb point at 32AWA in 18k true doing 8 plus SOG in medium chop and 1 to 1.5m of sea. Here comes a large Moby cruise ship right at me about 6 miles out doing 23k. I was hoping I did not have to change course. So, because of AIS I had all of her information and she had all of mine, I radioed her before I had to change course in order to avoid her. She replied back, Finalmente we have you, continue on your heading, we will be turning to port and will be taking your stern”. That was not the first time I realized how useful AIS was and will not be the last. Just my opinion.

[Uricanejack]

Originally Posted by Dockhead

Well, your AIS plots range, bearing, CPA, TCPA (and BCR and BCT on commercial sets). These are neither water nor ground referenced -- they are based on relative position and vectors.


Your AIS REPORTS as received (it does not calculate) COG, SOG, and position of the target, and if it's being broadcast, HDG and ROT. Plus all the static data.




Why do you need course (or rather, heading) and speed through the water? This will puzzle many here, I guess. It's not for automatic plotting -- as I said, the four basic calculated facts -- range, bearing, CPA and TCPA are neither water nor ground referenced. The type of data used to calculate these things is not important so long as it is consistent.



HOWEVER, if you set your radar use COG and SOG instead of HDG and STW, you will get a distorted radar picture, like this, when the radar is in relative motion and North Up:


Attachment 185605


https://www.myseatime.com/blog/detai...ich-one-to-use

I am still convinced Elpingo is right.

I’ve tried to explain why before but I’m not articulate enough to explain it. But here goes anyway.

The radar pictures are not distorted. They are two different displays. The first is relative motion. The second true motion.

The first is simple easy to understand the second takes a little bit of thought
In this particular situation.
Some people find a crossing situation with relative motion hard to visualize.

The problem with using ground track rather than water track doesn’t effect relative motion. The relative motion will be unaffected.

Most of our little radars (mine furuno 1612) are relative motion heads up displays. Without a high quality log and a gyro compas. Stabilizing a radar without these is can be done with GPS, but it will be ground stabilized.

The true motion is then calculated. But using ground speed not water speed distorts the true motion. The true heading is not the true heading and speed through the water. it’s the course and speed made good.

In the case illustrated on the two pictures. The true display appears to have a heading 40 degrees from the true heading the vessel is steering.

This picture could possibly be miss interperated as a crossing situation when it is actually a head on situation.

This might not be a problem in good visability. Your eyes will tell you what’s happening. If you look. We see the world relative to us.

The EBL or heading line will confirm the relative motion.

Most people will find this difficult to resolve in thier mind if they can’t see the other vessel.
An experienced radar plotter should resolve it quite quickly.

Problem? Expierienced Radar plotters are getting old and retiring. I would be very surprised to see or hear of systematic radar plotting being carried out on a ship today other than as a class room excessive.

The auto matic aids have taken over and nobody makes radars with reflection plotters.

The lecturer is correct like to like can be compared. Or are comparable.
So ground to ground true motion will give a over the ground correct result with the correct CPA.

The ARPA can’t compare ground to ground. The ground stabilized ARPA. Is observing the movement through the water not over the ground.

The relative motion is unaffected. It is accurate. The problem is by applying the co and speed over the ground the calculated true motion is incorrect. So the ARPA will give the correct CPA, with the wrong heading and course displayed.
The displayed true motion will be incorrect.

I expect for a number of reasons most ships will have thier ARPA set up in this mode.
My opinion it’s not a big problem if the operator understands thier is an unknown error in the true motion information they are using for colision avoidance.
In most circumstances it will be a small error.
In this example it’s an unusual large cross current being used to illustrate a point. This big an error will be rare in practice. It can happen.

By using ground to ground. AIS. The problem is reversed your true motion is accurate. Over the ground. Because they are comparing like to like. Current leeway are not a factor in determining CPA.

The problem is to determin the relative motion. To take the steady bearing and put your EBL on the other vessel. Which is in a virtual world not an observed world.
The relative motion has to be calculated. But something is missing from the algorithm.
Again the calculated aspect will be incorrect. Which in some circumstances will not matter.
In some particularly head on or nearly head on or border line overtaking crossing.
This could be misleading.

So you will get correct CPA with incorrect course and speed.

Is it a big problem? Like any error, Not if you know it’s there. You can make an allowance.

The author of the articles mentions big oil audits. I have never seen a big oil audit. So I wouldn’t know what they look for. They have a list with boxes to check. Apparently the ARPA setting is one of them.
The author doesn’t agree with.

Like I said my opinion. Elpingo is right. Water track should be used for colision avoidance.
If not so long as you know the difference. It’s ok.

I could confuse someone trying to resolve the difference in a relative display and a true display. Eg a relative radar and a true plotter.

[Dockhead]

Originally Posted by Uricanejack

I am still convinced Elpingo is right.

I’ve tried to explain why before but I’m not articulate enough to explain it. But here goes anyway.

The radar pictures are not distorted. They are two different displays. The first is relative motion. The second true motion.

The first is simple easy to understand the second takes a little bit of thought
In this particular situation.
Some people find a crossing situation with relative motion hard to visualize.



The problem with using ground track rather than water track doesn’t effect relative motion. The relative motion will be unaffected.

Most of our little radars (mine furuno 1612) are relative motion heads up displays. Without a high quality log and a gyro compas. Stabilizing a radar without these is can be done with GPS, but it will be ground stabilized.

The true motion is then calculated. But using ground speed not water speed distorts the true motion. The true heading is not the true heading and speed through the water. it’s the course and speed made good.

In the case illustrated on the two pictures. The true display appears to have a heading 40 degrees from the true heading the vessel is steering.

This picture could possibly be miss interperated as a crossing situation when it is actually a head on situation.

Well, you explain it quite well, and in fact better than I did. That's exactly what I was saying myself. I was not disagreeing with Ping -- I was explaining why you need to use HDG and STW in the radar rather than COG and SOG.

Originally Posted by Uricanejack

This might not be a problem in good visability. Your eyes will tell you what’s happening. If you look. We see the world relative to us.

The EBL or heading line will confirm the relative motion.

Most people will find this difficult to resolve in thier mind if they can’t see the other vessel.
An experienced radar plotter should resolve it quite quickly.

Problem? Expierienced Radar plotters are getting old and retiring. I would be very surprised to see or hear of systematic radar plotting being carried out on a ship today other than as a class room excessive.

The auto matic aids have taken over and nobody makes radars with reflection plotters.

The lecturer is correct like to like can be compared. Or are comparable.
So ground to ground true motion will give a over the ground correct result with the correct CPA.

Yes.

Originally Posted by Uricanejack

The ARPA can’t compare ground to ground. The ground stabilized ARPA. Is observing the movement through the water not over the ground.




The relative motion is unaffected. It is accurate. The problem is by applying the co and speed over the ground the calculated true motion is incorrect. So the ARPA will give the correct CPA, with the wrong heading and course displayed.
The displayed true motion will be incorrect.

Yes, and as I said -- it's a radar picture problem. It will look like what we see watching AIS target carats overlaid on the chart plotter, and all this incorrectly represents course, speaking graphically -- the numbers, however, will be correct.


A small quibble -- AIS reports include HDG, and most radars (and plotters) will correctly represent HDG whether or not water referenced data are being used as the reference for the radar set. That's why AIS targets are shown as CARATS, rather than blips.


But if the radar is set to Course Up (rather than Head Up), your own heading will be distorted, in relation to AIS targets whose heading is represented.

Originally Posted by Uricanejack

I expect for a number of reasons most ships will have thier ARPA set up in this mode.
My opinion it’s not a big problem if the operator understands thier is an unknown error in the true motion information they are using for colision avoidance.
In most circumstances it will be a small error.
In this example it’s an unusual large cross current being used to illustrate a point. This big an error will be rare in practice. It can happen.

By using ground to ground. AIS. The problem is reversed your true motion is accurate. Over the ground. Because they are comparing like to like. Current leeway are not a factor in determining CPA.

The problem is to determin the relative motion. To take the steady bearing and put your EBL on the other vessel. Which is in a virtual world not an observed world.
The relative motion has to be calculated. But something is missing from the algorithm.
Again the calculated aspect will be incorrect. Which in some circumstances will not matter.
In some particularly head on or nearly head on or border line overtaking crossing.
This could be misleading.

So you will get correct CPA with incorrect course and speed.

Is it a big problem? Like any error, Not if you know it’s there. You can make an allowance.

The author of the articles mentions big oil audits. I have never seen a big oil audit. So I wouldn’t know what they look for. They have a list with boxes to check. Apparently the ARPA setting is one of them.
The author doesn’t agree with.

Like I said my opinion. Elpingo is right. Water track should be used for colision avoidance.
If not so long as you know the difference. It’s ok.

This is a fascinating discussion. I don't think any of us disagree, anywhere, at all.


I thought that was a particularly good observation of yours about "virtual world" vs "real world" -- I think that nails it. I think a decent radar operator doesn't have so much trouble with the virtual picture. The EBL is a powerful tool, but even without the EBL, a constant bearing jumps off the screen, to a reasonably skilled operator, even out of the corner of his eye. The constant bearing is an aspect of the "virtual world", but we learn to go about comfortably in this world.





FWIW, AIS, displayed on a radar screen (rather than plotter as most of us do), on a radar screen set to use water references, will also show water tracks, not ground tracks, and will correctly represent HDG. That's the exact reason why the AIS standard calls for HDG to be reported as well as COG.

Originally Posted by Uricanejack

I could confuse someone trying to resolve the difference in a relative display and a true display. Eg a relative radar and a true plotter.

Ha, ha. Confuse away. That's exactly what I deal with every day -- plotter is inherently true (set to North Up), and radar relative (set to Head Up). It requires a bit of a mental leap going from one to the other, especially sailing in strong tidal waters like I do, with currents up to 6 knots and more, but it's the way I like to work. Actually, the radar picture then corresponds to what you see with your eyes.

[ranger58sb]

Originally Posted by Dockhead

Ha, ha. Confuse away. That's exactly what I deal with every day -- plotter is inherently true (set to North Up), and radar relative (set to Head Up). It requires a bit of a mental leap going from one to the other, especially sailing in strong tidal waters like I do, with currents up to 6 knots and more, but it's the way I like to work. Actually, the radar picture then corresponds to what you see with your eyes.

A thought... that you've engendered:

I can set plotter to Head Up too... so Mk I* eyeballs, RADAR, and plotter would all be on the same wavelength (so to speak).

Haven't ever had to do that, and generally prefer North Up on the plotter, but I think I'll file that idea away for the next time we're out in snotty dark...

-Chris

[Dockhead]

Originally Posted by ranger42c

A thought... that you've engendered:

I can set plotter to Head Up too... so Mk I* eyeballs, RADAR, and plotter would all be on the same wavelength (so to speak).

Haven't ever had to do that, and generally prefer North Up on the plotter, but I think I'll file that idea away for the next time we're out in snotty dark...

-Chris

Some people do that -- my Father used to use Head Up on his plotter, but personally I hate it, and would never use the plotter that way. My Dad used to drive me crazy by setting my helm plotter to Head Up on my boat, whenever he took a watch


Why?


Because the plotter is fundamentally ground referenced -- it's showing your relationship to the Earth. If you use it Head Up, it just perverts the whole function of it, in my opinion. It makes the earth move around according to your heading, and so the orientation of what you see on the screen is no longer tied to the very earth which is being displayed -- you have to find the compass rose to even know where North is, and you can't even recognize the shape of familiar land masses, because they are shown in any which orientation, and then spin around when you change heading. Bleh!


In my opinion, plotters need to be North Up. The Own Ship carat will show you which way you are headed, in the context of this view.


Maybe it's years of experience, but I am very comfortable having plotter North Up and radar Head Up, right next to it. Those are two different views of reality, both of which are highly relevant. Plotter shows where I am in relation to the Earth, and where is North and where is South is immediately obvious, and by the same token, land masses are recognizable because they are presented in consistent orientation. This view is for navigational orientation.



Radar shows what there is out there, IN RELATION TO ME. Just like what eyes show. This is a different view, for a different purpose -- for tactical orientation -- what potential obstacles are there out there, in what relative position to me.



But what about radar navigation, you might ask? Good question! It's a small leap, to translate what you see on a Head Up radar screen, to a chart or other depiction of your navigational situation. But my practice is different -- for radar navigation, I usually just use the radar overlay on the chart plotter. Incredibly useful, because you get instant verification of the chart, plus anything not shown on the chart, is shown in its chart-oriented position.



And since I've bought a Navico Ethernet adapter, I am able to overlay the radar picture even on my OpenCPN chart display, which is what I generally use for a chart plotter at the nav table.

[ranger58sb]

Originally Posted by Dockhead

Some people do that -- my Father used to use Head Up on his plotter, but personally I hate it, and would never use the plotter that way. My Dad used to drive me crazy by setting my helm plotter to Head Up on my boat, whenever he took a watch

Heh... usually drives me crazy, too. I started reading maps when I was 7 or 8 or something, and maps always came North Up... to me.

But then I know there are some people who can only think in Head Up (versus North Up), or Right and Left (versus North, South, East and West)... like wifey... and I've just had to come to terms with that. I think the base argument is that North doesn't much matter when you're focusing on aiming your own movement; what matters is your movement, and what you'll see next. (Or something like that. Maybe it's more relevant to "piloting" -- that whole IN RELATION TO ME thing -- than to "navigation.")

Just seemed like an interesting idea to set everything the same for a while. likely (for me) only for short distances through particularly complicated nav areas. But I might not be able to make the mental transition with the plotter either.

-Chris

[Snore]

Maybe it’s me— “back in the day” before GPS-I folded my charts and held them more or less heads up. When I have a “no electronics” day on the boat I do the same thing. It is just easier for my brain to transfer what I see to the chart.

But on a long passage when I plot position on a paper chart to backup the plotter - the chart below decks is north up.

[zeehag]

have used my radar, not used ais.
if i were you, follow instructions of those telling you to try to track pangas and storms on ais.
add coastlines and ships and rocks...
i am keeping radar not installing any ais.
my gps does that with nmea 2000.
IFF i want it to

[fish53]

You must be kidding? Radar is vastly more useful than an AIS, if you can't have both stick with a radar.

[CapnBazza]

According to Allan and Barbara Pease that’s how most women prefer to read maps.

[CapnBazza]

Originally Posted by Snore

Maybe it’s me— “back in the day” before GPS-I folded my charts and held them more or less heads up. When I have a “no electronics” day on the boat I do the same thing. It is just easier for my brain to transfer what I see to the chart.

But on a long passage when I plot position on a paper chart to backup the plotter - the chart below decks is north up.

According to Allan and Barbara Pease that’s how most women prefer to read maps.

[El Pinguino]

Originally Posted by CapnBazza

According to Allan and Barbara Pease that’s how most women prefer to read maps.

https://nguyenthanhmy.com/courses/2013/WhyMen.pdf Page 118

[CapnBazza]

Originally Posted by El Pinguino

https://nguyenthanhmy.com/courses/2013/WhyMen.pdf Page 118

Thank you squire, you have done us all a great favour by posting the link to this book. All sailors should read this and there would be a lot less solo sailors if we did.

[RaymondR]

I have found that whilst underway and on course it's more convenient to have the plotter set for heading up because one tends to search and identify objects in relation to the boats heading. However since I pretty well always enter an anchorage on the course I would need to run to depart in the dark and get the compass bearings to run clear I set the plotter to north up. I do this because I have found a lag in the indication of heading when it's rapidly changing whereas this is not so much of a problem with boat position and it's much easier to coordinate the boats heading using the compass when the chart heading on the plotter is continuously changing.

I have always wondered how so many women can find there ways all over the oceans and seas by themselves, they just point the chart at where they want to go and set course accordingly.