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Old 28-09-2012, 12:12   #466
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Re: Freighters vs. sailboats

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Originally Posted by cappy208 View Post
Let me put it this way. When your vessel is heading 045 The compass reads (well, should read!) 045. When you yaw, it will read whatever course the head of the vessel is on.

There is no alteration of the heading output for any nema output that somehow slows the swing down.

ARPA requires by definition a certain number of target sweeps to lock onto a target for tracking. Then it requires more sweeps to finalize the computers program. Each time the heading it altered, the computer has to restart the computation.
A traditional flux gate compass even if perfectly gymbled will not read a steady 045 if you could maintain this heading exactly. The compass will respond and deviate in response to pitch and roll even if the heading does not change.
Pitch and roll create acceleration and magnetic component effects in the fluxgate compass with a horizontal component ( because the compass mechanism has mass and the earths magnetic field has a vertical component)
A (perfect) gyro, or gyrostabilised compass would maintain a constant 045 in the same conditions.
While no gyro stabilisation is perfect, the more accurate heading data provides for a more accurate MARPA calculation.
The more accurate the boats heading (and hence the direction the radar is pointing) is know the more accurate is the MARPA data. As heading data is is typically much less accurate than the targets bearing, improvements in heading data have a noticeable effect on MARPA accuarcy.
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Old 28-09-2012, 12:13   #467
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Re: Freighters vs. sailboats

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There is no specific warning for tethering. Just as there is no different signal for a tug working along side. But, this is part of the boaters overall responsibility to study up on these things. Far too many boaters have a wider wallet than they do an expanse of knowledge about the ecosystem they are floating and boating in.

I can't think of a better reason to take a boating course. Can you?
Situational awareness is key, obviously. That being said, tethers are not exactly significantly visible, and may not been seen until too late. If the tehtered tug is close, then of course it would be foolish to cross its path. If it is farhter than 200 meters, the situation may be different, especially if it is a power vessel cutting across the path.

A nice big orange ball in the middle of the tether would be useful. In most of North America, they are visible where power lines cross highways, or on approaches to airports, for VFR purpose for planes.
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Old 28-09-2012, 12:29   #468
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Re: Freighters vs. sailboats

It is not the heading source that is effecting the ARPA data, it is the vessels heading gyrations caused by the seas. The degree or three heading difference between fluxgate, gyro, GPS, or magnetic is pretty much irrelevant. It is the wild fluctuations from getting batted around which effect the whole computer processing of data.

For this reason, I keep saying the solution is to return to old fashioned basics. Eyeballs on target. To do that, a steady helm is needed. Lacking a steady helm, being able to relay heading information to compare past observations is key.
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Old 28-09-2012, 12:40   #469
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Re: Freighters vs. sailboats

Put it this way. Go out on a nice flat day under power. steer a steady course. Plot a target. count how many minutes you have to wait until you get a steady COG. SOG, and CPA. Now, when that is accomplished, start turning at approximately the same speed/tempo as you felt you were out in the rough seas. Over the same time span as it took to get good ARPA info, you will see the same degradation of data as at sea.

The term stabilized is in relation to the gyro INSIDE its cage, in relation to the heading of the vessel. It is NOT in relation to how fast, or how wild the vessel is moving. The old gyros used to be stabilized within their cages on a series of rubber gaskets. Then they went to stainless springs. Now they are soft rubber mounted with software to stabilize them. This has nothing to do with the data out.
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Old 28-09-2012, 12:46   #470
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Re: Freighters vs. sailboats

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Originally Posted by cappy208 View Post
It is not the heading source that is effecting the ARPA data, it is the vessels heading gyrations caused by the seas. The degree or three heading difference between fluxgate, gyro, GPS, or magnetic is pretty much irrelevant. It is the wild fluctuations from getting batted around which effect the whole computer processing of data.

For this reason, I keep saying the solution is to return to old fashioned basics. Eyeballs on target. To do that, a steady helm is needed. Lacking a steady helm, being able to relay heading information to compare past observations is key.
The heading source needs to accurately and perfectly report the "heading gyrations caused by the seas". A flux gate compass on its own does not do this well.

I agree eyeball avoidance , or with the addition of basic tools like a HBC, should always be the mainstay of collision avoidance, but tools like MARPA and AIS can be very useful in situations like reduced visability. The more accurate the heading data the better the MARPA data.
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Old 28-09-2012, 12:48   #471
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Re: Freighters vs. sailboats

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Situational awareness is key, obviously. That being said, tethers are not exactly significantly visible, and may not been seen until too late. If the tehtered tug is close, then of course it would be foolish to cross its path.
A nice big orange ball in the middle of the tether would be useful.

You are talking about two distinctly different types of towing. The towing that is dozens and dozens of meters behind is already covered in the Colregs. Towing over 200 M astern is already covered.

What is being shown on the video is assisting, or escorting with a tractor tug, on a tether that is (AT MOST) 15 to 40 meters long. It actually depends upon the width of the channel, and which particular method of work the assist tug is going to do. If I as a sailboater cannot tell if I should go between (or around) this, I do think they should not pass go, do not collect their pay, and should just fold up and go home! But then again, I have seen some spectacularly stupid things out here!
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Old 28-09-2012, 12:52   #472
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Re: Freighters vs. sailboats

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The heading source needs to accurately and perfectly report the "heading gyrations caused by the seas".
You should read up on how long a ARPA radar must be on the same heading to get accurate information. It does not state 'accurate heading showing gyration' is looking for 'accurate consistent heading' Huge difference.

When I get back on watch I will read up and see if I can find the specific page and warning in my ARPA manual about heading info.
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Old 28-09-2012, 13:00   #473
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Re: Freighters vs. sailboats

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Originally Posted by cappy208 View Post
You are talking about two distinctly different types of towing. The towing that is dozens and dozens of meters behind is already covered in the Colregs. Towing over 200 M astern is already covered.

What is being shown on the video is assisting, or escorting with a tractor tug, on a tether that is (AT MOST) 15 to 40 meters long. It actually depends upon the width of the channel, and which particular method of work the assist tug is going to do. If I as a sailboater cannot tell if I should go between (or around) this, I do think they should not pass go, do not collect their pay, and should just fold up and go home! But then again, I have seen some spectacularly stupid things out here!
It was hard to tell (it usuall is) how far the tug was away from the stern. Agreed, the length you are referring to... suicidal to attempt to cut between the two, tether or not.
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Old 28-09-2012, 13:01   #474
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Re: Freighters vs. sailboats

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When I get back on watch I will read up and see if I can find the specific page and warning in my ARPA manual about heading info.
My Raymarine radar handbook says on MARPA.
QUOTE:

"The better the accuracy of your heading data , the better the performance of MARPA.
A gyro compass provides the best performance in all conditions. Alternatively you could use a flux gate compass with gyro stabilisation
."
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Old 28-09-2012, 13:16   #475
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Re: Freighters vs. sailboats

Sorry, Raymarine is garbage. U an referring to the technical specifications that actually determine the qualifications of ARPA. Unfortunately, there is ARPA and there is evrything else! Marpa has degraded ARPA with oversight, and lousy controls. JMHO.

There's yacht quality and there's commercial. N'er the twain shall meet. As I like to call it, D(s)imrad tried to market this type of garbage a couple years ago. They put out all kinds of fancy fliers out, and brochures. But when you actually needed help, or needed someone to talk to the website they pointed you to was simradyachting.bleech. That was ALL I needed to see~~
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Old 28-09-2012, 13:51   #476
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Re: Freighters vs. sailboats

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Originally Posted by cappy208 View Post
It is not the heading source that is effecting the ARPA data, it is the vessels heading gyrations caused by the seas. The degree or three heading difference between fluxgate, gyro, GPS, or magnetic is pretty much irrelevant. It is the wild fluctuations from getting batted around which effect the whole computer processing of data.

For this reason, I keep saying the solution is to return to old fashioned basics. Eyeballs on target. To do that, a steady helm is needed. Lacking a steady helm, being able to relay heading information to compare past observations is key.
The beauty of the HBC is that, unlike your radar, it directly tells you the compass bearing to the target.

Concerning ARPA -- I'm afraid to say it, but it seems you really don't know how it works. Heading at the moment of the target sweep is an essential data point, without which ARPA will not make any calculation at all. The position of the target is calculated by ARPA based on:

1. Relative bearing to the target (directly provided by the radar)
2. Your vessel's heading at the moment of sweep (provided by heading sensor and processed by your nav system)
3. Range of the target at the moment of the sweep (directly provided by radar)
4. Position of your vessel at the moment of sweep (provided by GPS)
5. Time at the moment of sweep (provided by GPS)

Based on these data, ARPA calculates a position in lat long of the vessel. ARPA accumulates a number of these position points and produces a predicted track of the target over time.

ARPA produces a predicted track over time of your own vessel based on a series of positions provided directly by your vessel's GPS.

Then, it compares these two predicted tracks and calculates predicted CPA and TCPA.

That's how ARPA works. If any one of the five data used to calculate the position of the target is missing, no position at all can be calculated -- every one is essential. If any one of the five data is inaccurate, then the position will be correspondingly inaccurate, and the predictions about the targets track will be correspondingly inaccurate. Radar and GPS provide very accurate data -- the big question mark is with heading, which is hard to measure on a pitching, yawing vessel at sea.

Every degree of error in heading is a mile of error at 60 miles, so at 6 miles is that a cable (can't remember if it's linear)? If the calculated positions of the target used to calculate the predicted track of the target is off by cables in any direction, the track of the target will be way off, and so the CPA will be off.

Now since ARPA uses a set of calculated target positions -- it's not using just two target positions -- and has sophisticated algorithms to identify and throw out outlyers, a constant error may not be too bad. And that's why gyro compasses are so much vastly better -- they provide rate of turn data as well as heading, and your nav system will refine the heading data by referring the change in heading to rate of turn data, dramatically increasing the dynamic accuracy. And that makes ARPA sing.

The problem with our own vessels varying heading is an entirely different problem -- you have them confused. If ARPA has accurate heading data, it can determine accurate positions of the target regardless of what your own heading is doing -- you can even be steaming around in circles if you like. The problem with your own vessel's varying heading is not calculating the predicted track of the target, but predicting your own track.

That's how ARPA works, and why it is utterly dependant on accurate heading data.
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Old 28-09-2012, 14:39   #477
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Re: Freighters vs. sailboats

The radar will tell the distance to the opposite vessel. A GPS will give the exact location, and will calculate your position and the other vessels indicating precise distance, course and track of the two vessels.

That is what I have seen lately.
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Old 28-09-2012, 15:00   #478
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Re: Freighters vs. sailboats

Nope. The data derived from the RADARs calculations will be error filled if the vessel does not stay on a course (and IIRC, it is a pretty narrow window that is specified) Of course the ARPA will show A course. It will show A CPA. But the data will be wrong.

Will the data occasionally show the correct CPA, COG and SOG? Yes. Will there be inaccurate display of data. Yes. But it will be just as inaccurate as the relative trail shown on the radar. It will look like a wobbly line, showing a narrower wobble as it gets closer, but an inaccurate wobbe nevertheless.

I am trying to find a course slide show to grab a plate from to show that in the ARPA course one of the facts they drill into you is that if either vessel changes heading it invalidates the arpa info. A small course change does effect it, but a large course change especially screws it up.

A RADAR is range gating when it is in use. It looks for the target in the last
'box' it saw it in. After a few rotations when it has seen the same target it locks onto it. Once it has locked on to it, then it begins the computations on plotting. The rougher the seas the longer it can take. I have seen locks in less than 10 seconds. Some never take?! Alternately, sometimes after a locked target has been tracking, it will loose the lock. This happens when the target is not 'found' in the box again. This is all effected by the heading input.

The accuracy of the ARPA is dependent on 4 things. the target maintaining a steady course and speed. and the ARPA vessel maintaining a steady course and speed.

Course changes simply screw up the data. Does it average out? Yes. Would I trust the data? NO. Feel free though.
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Old 28-09-2012, 15:03   #479
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Re: Freighters vs. sailboats

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Originally Posted by MacG View Post
The radar will tell the distance to the opposite vessel. A GPS will give the exact location, and will calculate your position and the other vessels indicating precise distance, course and track of the two vessels.

That is what I have seen lately.
Without heading, you cannot calculate the position of another vessel knowing only your own position, range, and its relative bearing to yours. Think about it for a second and you'll understand why. If you can't calculate its position, then you don't know its course or speed.

You can calculate a rough CPA based on the relative motion of the other vessel and yours -- that's how traditional radar plotting works. You don't need heading for the CPA or speed, but you do need it for course or any calculation of position of the other vessel.

But the systems you have seen use heading data from a fluxgate compass to make the calculations you are talking about.
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Old 28-09-2012, 15:05   #480
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Re: Freighters vs. sailboats

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The problem with your own vessel's varying heading is not calculating the predicted track of the target, but predicting your own track.
ERGO the issue of completing the RM triangle. You (or a computer) needs Your course, the objects course and your speed. If the RM triangle can't be completed, then you really cant solve for solution!
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