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?

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.

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.

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.

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 I’m 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.

seattleboatguy said:

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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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.

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.

Just a small public service message, brought to you by Kellog's Raisin Bran, now with 8 scoops of raisins in every box! (bran flakes sold separately)

dhays said:

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 I’m 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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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 don’t want to pay for it.

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.

Wayfarer said:

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.
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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.

dhays said:

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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ARPA recalculates after every sweep of the antenna, so every second or so.

Wayfarer said:

ARPA recalculates after every sweep of the antenna, so every second or so.

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Thanks!

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.

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.

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.

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.

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...

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.

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.

dhays said:

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.

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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.

The upside to AIS around the Chesapeake is being able to see the boat names. Kinda fun seeing the various names. It's annoying seeing just an MMSI number. I have checked the info for some AIS targets, but mainly just for curiosity. I'll have to remember to check on the ones squawking just an MMSI to see if they're actually sending useful data (length, beam, heading, etc).

wkearney99 said:

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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I didn’t use the radar on my sailboat unless there were visibility issues; running at night, fog, heavy rain, etc... Being out in the cockpit, open to the elements, I always had very good situational awareness of other boats. However, over the past couple years I am finding that I am using the radar most of the time now. Sitting in the pilothouse I’m not as aware of traffic coming from behind me. The radar is really handy for that. I don’t use the alarms because there is usually so much traffic that it would always be going off. However I do select targets frequently and use that information.

I do love the AIS though. I only have a receiver but want a transceiver some day.

I think that an AIS transponder is a necessary piece of equipment when in the vicinity of commercial traffic. I want them to be able to call me by name. We travel in the fog. Just a couple of weeks ago in Buzzards Bay, crossing the shipping lane, fog, and a knot and 1/2 of current I heard a tug and barge hailing the “vessel at such and such a location”. The tug was coming down stream. Not a place for the tug to be guessing what your intentions are. They are always happy to tell you what they want you to do and I am most happy to obey. Wasn’t me they were contacting as I was over a mile upstream from the tug. The other guy never responded and maybe never knew he was in the tugs “sights”

Cigatoo said:

I think that an AIS transponder is a necessary piece of equipment when in the vicinity of commercial traffic. I want them to be able to call me by name. We travel in the fog. Just a couple of weeks ago in Buzzards Bay, crossing the shipping lane, fog, and a knot and 1/2 of current I heard a tug and barge hailing the “vessel at such and such a location”. The tug was coming down stream. Not a place for the tug to be guessing what your intentions are. They are always happy to tell you what they want you to do and I am most happy to obey. Wasn’t me they were contacting as I was over a mile upstream from the tug. The other guy never responded and maybe never knew he was in the tugs “sights”

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I've heard a few similar announcements, and wondered if the target vessels were likewise aware. That and it made me aware of the times I didn't have my VHF on and if I wasn't likewise unaware. I always try to adjust my course to be effective for everyone, that means doing it ahead of time and thus to CPA.

Many AIS systems will give you the age of the latest position report. On a newish Raymarine, this is shown as the "most recently seen" time in the AIS data. Not ideal as you must subtract the current time from it to see the age. OpenCPN shows it as a live running age in the AIS data, counting up in seconds. This is much more useful. Class A will rarely get older than a few seconds before being refreshed provided you are within good range. Class B will go to 30 seconds or even several minutes as the transmissions are not as frequent or as powerful. Chart plotters often depict the target, even if the data is old. Some will shade the target to indicate old data.

Yes, AIS input can be abused, just as radar input can be. As TT suggested, it is the operator that is the weak link. i'd prefer good AIS data over any radar data, but you must take, understand, and make use of what you get.

AIS data may be more informative, but RADAR is real-time constant bearing , decreasing range ...

How does it get better than that for avoiding collision? More info, but none better.


Don't want to be guessing on how current the AIS info is when rounding a bend and the CPA is in seconds.

psneeld said:

AIS data may be more informative, but RADAR is real-time constant bearing , decreasing range ...

How does it get better than that for avoiding collision? More info, but none better.


Don't want to be guessing on how current the AIS info is when rounding a bend and the CPA is in seconds.

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No argument here. Radar is near real time and doesn’t lie. But the AIS data adds to the benefit. Given a choice it would be Radar.

We were crossing Lake Ontario in an extremely heavy rain, the worst I have ever been underway in. Our 4kw radar was in white out mode due to the rain. There were 2 freighters crossing our course and the only way I knew they were there was the AIS. I could see them on the plotter but I couldn’t see anything on the radar. One obviously would cross well ahead of us. The other was on a collision course with us. We were stand on and they were give way. As I was reaching for the mic the freighter called us by name. I don’t know if he was able to see us on radar or not but he could see us via AIS since he knew our name. He stated his intention to turn to cross behind us. Without AIS I would have not known that he was there and maybe he would have not known we were there either. His radar may have been able to punch through the rain, I don’t know. But this one time paid for the AIS.

psneeld said:

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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Yep, have seen that often enough around here, too. Nice to have AIS, but RADAR way more useful for time-sensitive info. Especially given maybe 1% (or less?) of boats here in the Chesapeake transmit an AIS signal...

wkearney99 said:

The upside to AIS around the Chesapeake is being able to see the boat names. Kinda fun seeing the various names. It's annoying seeing just an MMSI number. I have checked the info for some AIS targets, but mainly just for curiosity. I'll have to remember to check on the ones squawking just an MMSI to see if they're actually sending useful data (length, beam, heading, etc).

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Sometimes it takes a while for our system to resolve an MMSI into a name. Dunno why... Maybe the MMSI transmits every time, name only every other sometimes?

-Chris

I agree that radar is critical, except for when it doesn’t work as in my post above.