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Old 03-21-2018, 11:36 AM   #1
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Question AIS Relative or True

I have installed a new Raymarine eS127 with a 12" screen.

I am setting up the AIS "Collision Avoidance" and under this tab another tab called "Target Vectors."

I have the choice of "Relative" or "True."

Can someone please explain the difference between these terms?

Which should I use?
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Old 03-21-2018, 11:40 AM   #2
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wow....

True is whether the guy is north, south east, west or in between of you in relation to the north pole....

Relative is 000 off your bow, 180 stern, 270 of your port side, 090 off your starboard side and interpolate for in between.

choice? depends on how you perceive things...
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Old 03-21-2018, 11:54 AM   #3
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wow....

True is whether the guy is north, south east, west or in between of you in relation to the north pole....

Relative is 000 off your bow, 180 stern, 270 of your port side, 090 off your starboard side and interpolate for in between.

choice? depends on how you perceive things...
Hey sometimes the simplest questions are hard. Easy explanation.

Thank you for taking the time to explain it.
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Old 03-21-2018, 12:17 PM   #4
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In WWII, American and British ships used the systems of relative or true bearings in different ways.

On the US ship, there might be a report of a U-boat "bearing 080 degrees," which meant that the enemy was located ten degrees north of east (true).

On the British ship, the report would be of a target "bearing red twenty," which meant that the target was twenty degrees off the port bow, or at 340 degrees relative. Of course, the true compass bearing of the target was dependent upon the ship's present course.

Yes, I'm full of useless trivia.
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Old 03-21-2018, 02:10 PM   #5
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In WWII, American and British ships used the systems of relative or true bearings in different ways.

On the US ship, there might be a report of a U-boat "bearing 080 degrees," which meant that the enemy was located ten degrees north of east (true).

On the British ship, the report would be of a target "bearing red twenty," which meant that the target was twenty degrees off the port bow, or at 340 degrees relative. Of course, the true compass bearing of the target was dependent upon the ship's present course.

Yes, I'm full of useless trivia.
This system is still used by the Navy.

A lookout reports a contact to the bridge- "Bridge, port lookout, red signal flare on the horizon, 340 relative"

It's a faster way of getting a handle on what is around the vessel.
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Old 03-21-2018, 02:21 PM   #6
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unless you have a gyro compass in front of you, usually relative is best anyhow.
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Old 03-21-2018, 02:33 PM   #7
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unless you have a gyro compass in front of you, usually relative is best anyhow.
Speaking as someone with thirty years of daily experience in collision avoidance as an air traffic controller, I agree with one exception and that is if you, not the target, are maneuvering, in which case the true...well, not really true, but magnetic bearing becomes more useful.

Actually, rereading your qualification about a compass, I think I agree completely.
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Old 03-21-2018, 03:35 PM   #8
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I can see your point, controller to pilots, but crew to pilots would be challenging.... as it is hard to give a magnetic bearing or true bearing without some kind of compass in front of you....

most people I boat with on give targets as a relative bearing in degrees, points, or just slang jargon even if mauevering...

like "target passing astern to porrt 170 degrees relative"....

again only useful in mag ir true if everyone is on compasses.

now a lookout using one will pass true bearing to conn who will plot, but that takes time to get into everyones head....maybe why a few Navy ships have had a bad day lately.

you get too tech with too mamy people and the things get screwed up too often.
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Old 03-21-2018, 04:34 PM   #9
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I can see your point, controller to pilots, but crew to pilots would be challenging.... as it is hard to give a magnetic bearing or true bearing without some kind of compass in front of you....
Yeah, I may have overdrawn the parallel, and I can feel the thread drifting right under my feet, but the controller situation is more analogous to that of the crew, as I think about it and that's why they use the "relative clock" "10 o'clock" would be comparable to 60 degrees on the port bow or perhaps similar to the English system: "red 60".

But even in this context, if the aircraft being worked is maneuvering, traffic is given in the 8 cardinal points: "Traffic NW of your position, 5 miles, E-bound."

All ATC radar is "north up" and magnetic, and traffic is easy as long as the subject is on a northerly heading. Turn him around and it gets more complex, analogous to flying an RC model toward you; not easy. You have to project your brain into the cockpit.

Then do it simultaneously with 10 or 15 aircraft on diverse headings, it is an admirable skill. Even after doing it for a dozen years, I was occasionally amazed that it was possible at all.

10 o'clock and 2 o'clock seemed for many people to be particularly vexatious. In fact, as a pilot, I developed the habit of checking both. And I did work with one guy who had to lay his wrist watch on the console...

But I digress...pedantically yours,

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Old 03-21-2018, 06:36 PM   #10
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RELATIVE IS MY VOTE ----

Makes it simple
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Old 03-21-2018, 07:10 PM   #11
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RELATIVE IS MY VOTE ----

Makes it simple
Yup, bow is 0, everything else is relative to your bow. Of course as you swing clockwise, you think of your bow as also 360.
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Old 03-21-2018, 08:48 PM   #12
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No, this is not what it means in this context.

For motion vectors, true vectors show the targets movement relative to ground. More specifically, it shows COG and SOG. This is how we generally think of movement, so it makes the most sense to everyone.

A relative motion vector shows how the target's position is moving relative to you, and is the basis for collision avoidance with radar and AIS.

By way of example, if the target is on a parallel course to yours, and traveling at the same speed, their relative motion vector will be zero showing no movement. If you sight the boat visually, it will always be in exactly the same position relative to you. It has no movement relative to you.

Now remember back to your collision avoidance basics. A boat at a constant bearing with closing distance is on a collision course, right? Such a target will have a relative motion vector that points directly at you. If their relative motion vector points ahead of you, they will pass ahead of you, and the gap between the vector's closest point to you is the CPA. If the vector is pointing behind you, then the target will pass behind you.

Ray is to be commended for being one of the only consumer nav equipment vendors to support relative motion vectors. Furuno used to support it, but dropped it in the TZ series. All commercial radars have relative motion vectors and use of them is std practice for collision avoidance.
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Old 03-21-2018, 10:22 PM   #13
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Speaking as someone with thirty years of daily experience in collision avoidance as an air traffic controller, I agree with one exception and that is if you, not the target, are maneuvering, in which case the true...well, not really true, but magnetic bearing becomes more useful.

Actually, rereading your qualification about a compass, I think I agree completely.
My son is ATC in Anchorage........
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Old 03-21-2018, 11:08 PM   #14
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My son is ATC in Anchorage........
Which facility? I was a manager in the TRACON 91-94 and then went to the Aviation Technology faculty at UAA until 2010.
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Old 03-22-2018, 11:01 AM   #15
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He is at the Anchorage Center just outside of Elmendorf AFB.
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Old 03-22-2018, 11:30 AM   #16
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He is at the Anchorage Center just outside of Elmendorf AFB.
I left FAA in '94, wouldn't know him in that context, but he could very well have been one of my students at UAA aviation. Why don't you PM me his name.
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Old 03-23-2018, 10:07 PM   #17
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Yeah, I may have overdrawn the parallel, and I can feel the thread drifting right under my feet...

But I digress...pedantically yours,

'prof
Yup...you sure stepped in it, Professor!

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My son is ATC in Anchorage........
I didn't know that! Ask him if he ever rode on an FAA LJ60 on a fam trip. I took several ANC ARTCC controllers flying during my stints through there. I've taken many dozens of controllers flying in my days. Always made a point of it.
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