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Old 09-02-2019, 11:52 AM   #1
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Closest Point of Approach

I was watching the AIS target of a ferry boat pass by yesterday on my Gatmin 741 chartplotter, and started wondering about Closest Point of Approach (CPA). Do the commercial ships have some sort of computer display that says "Your CPA will be 2.7 miles in 3 minutes 21 seconds", or do they still need to plot that stuff by hand?

It would seem like my Garmin 741 has enough data to do that kind of calculation for AIS targets, but I'm not sure if it actually does it. I do sometimes see a dotted line to a ship wreck symbol. Could that be a CPA?
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Old 09-02-2019, 12:02 PM   #2
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If you hover over targets on computer plotter screen, it will give CPA and time of CPA (TCPA) so the operator can take appropriate action. This is assuming your AIS is linked with your Plotter. Radar usually has a similar function. Many list on a side of screen so you don't need to hover.
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Old 09-02-2019, 12:13 PM   #3
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My AIS shows a small symbol for a target. If I touch the symbol it will pull up a small info block about the target. Then I can click on more info and everything about the target comes up, name, course, speed, length, beam, where they are going, CPA and a bunch more info if the target has the data entered into his AIS. Really helpful.
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Old 09-02-2019, 12:48 PM   #4
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Laptop based systems provide data as previously noted as well as crossing point lines on the screen. Lesser target data and AARPA show up on our Furuno NN3. Both laptop and MFDs provide us multitudes of AIS and radar CPA data. The ferries and ships have even better data compilers, often being fed additional target data from shore based and cloud systems.

Assuming your AIS interfacing is correct with your Garmin, there should be good target data available.
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Old 09-02-2019, 01:42 PM   #5
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On my Raymarine system which is a couple of generations old now, the AIS target info will give CPA information. If I create a target on a radar return, it will also give me CPA information. So Iím pretty sure your system will give that information once you become familiar with it.

FWIW, I also have a simple hand sighting compass that is pretty handy when checking if a crossing target will pass in front, behind, or too close for comfort.
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Old 09-02-2019, 02:56 PM   #6
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It would seem like my Garmin 741 has enough data to do that kind of calculation for AIS targets, but I'm not sure if it actually does it. I do sometimes see a dotted line to a ship wreck symbol. Could that be a CPA?
Yes, that's a CPA. It's pretty much stock functionality with any device receiving AIS.

I have a similar plotter and it's worth exploring and understanding the capabilities. There's lots you can do with displays and alarms.
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Old 09-02-2019, 05:46 PM   #7
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Just a slight side note. I've sailed with a few new 'kids' right out of the academy that have a bad habit of ONLY using the AIS data to determine CPA. They're not using ARPA, and are just using those big fancy radars as slave screens for AIS data If you use the ARPA functionality of your radar, the integrated software will determine all of the data you need on it's own, without any information from the AIS. It will give you CPA, Time of CPA, Target's true speed and course, etc... In my opinion, both tools are useful, and both should be used for collision avoidance.

I think a lot of people think that the information on both machines is the same, but it's not. Each piece of equipment comes up with it's information from different methods. The radar actually tracks that echo that you see on the screen, and does the plotting math for you, based on what it 'sees.' The AIS uses the data it receives from the target, as well as the data from your gps to calculate CPA.

If either one of those GPS units are faulty, or being hacked, or spoofed, or whatever, the data will be wrong, and your CPA will be wrong.

The radar won't be fazed by any of that. The radar doesn't care where you are. It just knows what it sees.

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Old 09-02-2019, 05:58 PM   #8
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On my Raymarine system which is a couple of generations old now, the AIS target info will give CPA information. If I create a target on a radar return, it will also give me CPA information. So Iím pretty sure your system will give that information once you become familiar with it.

FWIW, I also have a simple hand sighting compass that is pretty handy when checking if a crossing target will pass in front, behind, or too close for comfort.
Your Raymarine system was a couple of generations old by the time you got it installed, at least that is how I felt. I put the then current system in my boat when we bought it in September. By the time we ran it home in May the system was outdated. I still like it but I would like to have the new stuff, just donít want to pay for it.
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Old 09-02-2019, 07:37 PM   #9
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You are asking about the ECDIS (Electronic Chart Display and Information System) many if not most ships have. We call them chart plotters in the recreational and small commercial vessel world. Everything can be displayed there; charts, radar, sonar, AIS can be overlayed to give a complete picture. They are more powerful systems that we small craft use. In one of the recent collisions of Navy destroyers it was brought out that AIS was not integrated into their ECDIS (may be by now, I don't ride them anymore to personally confirm), and they use a laptop with AIS info to follow that data.
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Old 09-02-2019, 11:07 PM   #10
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I think a lot of people think that the information on both machines is the same, but it's not. Each piece of equipment comes up with it's information from different methods. The radar actually tracks that echo that you see on the screen, and does the plotting math for you, based on what it 'sees.' The AIS uses the data it receives from the target, as well as the data from your gps to calculate CPA.
[/I]

Great reminder. If I recall, Class B transceivers only send out information every 30 seconds. 30 seconds is a very long time when two boats are converging. Not sure what the frequency is of the Radar scans but I think it is a lot more frequent than that.
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Old 09-03-2019, 04:21 PM   #11
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Great reminder. If I recall, Class B transceivers only send out information every 30 seconds. 30 seconds is a very long time when two boats are converging. Not sure what the frequency is of the Radar scans but I think it is a lot more frequent than that.
ARPA recalculates after every sweep of the antenna, so every second or so.
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Old 09-03-2019, 04:26 PM   #12
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ARPA recalculates after every sweep of the antenna, so every second or so.

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Old 09-03-2019, 07:46 PM   #13
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A Class B transmits only every 3 MINUTES if you are stopped or moving < 2 knots. Otherwise 30 sec. Class B AIS transceivers will not transmit over a class A unit, they wait for the class A. Class A equipped ships can choose to not display class B units if the waters are crowded with them.
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Old 09-03-2019, 08:09 PM   #14
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Class B SOTDMA units can transmit more frequently though, up to every 5 seconds if you're moving faster. And they use the same protocol as Class A to find transmission slots, so they're more likely to actually get a transmission out in busy waters.
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Old 09-03-2019, 08:21 PM   #15
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I saw where class A has a SOTDMA capability but did not see that for class B. I imagine that that feature comes with a price increase too.
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Old 09-03-2019, 08:24 PM   #16
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It's only supported by some of the newer Class B units, far from all of them. And yes, it's typically not found on the cheapest units. But if you're buying one of the higher end units for some of the features a lot of them include, there's not much price difference to get one that does SOTDMA.
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Old 09-03-2019, 08:32 PM   #17
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One too many times a transponding boat was already past me and the AIS target was still ahead....I don't trust it worth spit compared to RADAR for close aboard relative position info...
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Old 09-04-2019, 05:39 AM   #18
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Great post Wayfarer. Another excellent reason to run one's radar all the time when underway.

I never got around to installing AIS on our boat and that was my main excuse. With a good radar system, given our cruising grounds, it just wasn't needed. If we spent a lot of time in the waterways in the central US with lots of barge traffic, then I would have moved it way to the top of priorities on The List.
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Old 09-04-2019, 06:21 AM   #19
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It's interesting how we talk a lot about outdated electronic equipment. But I think almost always WE are the weak link in the system, not the equipment. Most people don't know how to use the most basic features that are included in electronics that are very, very old. ARPA is a good example. Even EBL and VRM lines/rings which themselves are the basics of collision avoidance, i.e. monitoring range and bearing and how that changes over time.
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Old 09-04-2019, 08:31 AM   #20
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Great reminder. If I recall, Class B transceivers only send out information every 30 seconds. 30 seconds is a very long time when two boats are converging.
Yep, following some friends on a moon-less night and it was unnerving seeing us over-taking their AIS marker... and then having it jump forward again, over and over.

Radar didn't have that problem as the sweep is generally every second.

But I'll confess to not having learned anywhere near enough about the Furuno setup in our boat. We just don't boat that often at night or in conditions with limited visibility.
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