AIS INSTEAD of radar?

[daletournier]

I'm in the both camp, two devices on watch 24/7, both relatively inexpensive.

I just purchased a new B&G 3g radar that connects via ethernet to open cpn $1050 usd, , that's alot of watch keeping for little money.

[MartinR]

Originally Posted by Sailor647

Not having radar is like driving your car in the dark - WITHOUT HEADLIGHTS.

Bad comparison. On the ocean, you have no road to follow. You can use your night vision to navigate, like people have done for thousands of years. Other boats, ships have navigation lights (mostly). This is not unsafe. Yes, there are circumstances when RADAR is nice to have, but it is not essential in any way, unless you sail most of the time in foggy waters. Then you need to use you hearing. Actually, screens of any kind destroy night vision. So if you have your RADAR running in the cockpit you are basically navigating half blind.

To the question. AIS is not to be used for collision avoidance. RADAR fills this function, and is also a helpful tool for navigation,

[Suijin]

Originally Posted by fgd3

I have found AIS to be something of a fun toy. Updates just don't come fast enough for small boat handling. With big ships that are hard to turn AIS is valuable for collision avoidance but my experience with it on a small boat is the information isn't sufficiently up to date to be useful.


Fabbian

Again, it’s a matter of where you sail.

Going in or out of the ocean to Norfolk at night can be a fraught experience. Lines of big ships steaming through as closely spaced as the harbor master will allow, and you need to slip in to squeeze through the narrow opening in the causeway across the mouth of the Chesapeake and then deal with in the traffic separation zones and track who’s going where. There are lights everywhere and before AIS it was a white knuckle affair no matter how many times you’d done it.

AIS has transformed the challenge. You can instantly see everyone’s speed, bearing, direction and they can see you. It’s almost easy now. MUCH safer for everyone involved.

[Jim Cate]

To the question. AIS is not to be used for collision avoidance. RADAR fills this function, and is also a helpful tool for navigation,

Who says that AIS isn't to be used for collision avoidance? I use it for t his all the time. I also use radar, but for merchant shipping, AIS is a far better tool in my usage. Tracking multiple targets with a simple, non-MARPA radar is pretty demanding on a short handed vessel and AIS makes it easy. And, of course, the "other guy" now can track us as well... if they are watching their own AIS, and my observations in this part of the world show that most ships appear to be altering course to generate CPAs of 1+ miles.

Jim

[Dockhead]

Originally Posted by MartinR

. . . To the question. AIS is not to be used for collision avoidance. RADAR fills this function, and is also a helpful tool for navigation,

If AIS is not to be used for collisions avoidance, then what IS it for?


Of course it's for collision avoidance. Commercial vessels use ARPA as their primary collision avoidance tool, not AIS, but our radars with their puny antennae don't have the bearing discrimination to get really accurate, or even reasonably accurate automatic plots. So for us, AIS is the only way to get a quick automatic plot and calculation of CPA, TCPA, speed and course of the the vessel, and this is absolutely invaluable, especially when you're dealing with multiple targets at the same time.

[tanglewood]

Originally Posted by Dockhead

If AIS is not to be used for collisions avoidance, then what IS it for?

Identification. That's why it's called "Automated Identification System", not "Automated Collision Avoidance System"

[Dockhead]

Originally Posted by tanglewood

Identification. That's why it's called "Automated Identification System", not "Automated Collision Avoidance System"

No one said it automates collision avoidance.

But it does do plotting of collision risks - those that are broadcasting, not all of them - far better than any other means we have.

That's what we use it for.

If it were only for identifying vessels, it wouldn't give us calculated CPA, TCPA, COG, SOG of the targets. And we wouldn't buy them.

But I know you know all of that.


"The original purpose of AIS was solely collision avoidance, but many other applications have since developed and continue to be developed. . .

"AIS was developed by the IMO technical committees as a technology to avoid collisions among large vessels at sea that are not within range of shore-based systems."


https://en.wikipedia.org/wiki/Automa...ication_system

[Uricanejack]

Originally Posted by Dockhead

No one said it automates collision avoidance.

But it does do plotting of collision risks - those that are broadcasting, not all of them - far better than any other means we have.

That's what we use it for.

If it were only for identifying vessels, it wouldn't give us calculated CPA, TCPA, COG, SOG of the targets. And we wouldn't buy them.

But I know you know all of that.

The AIS signal gives ship identification, GPS position, Gyro, speed.

Your unit calculates the colision info based on the info it receives. And comparing it to it’s own position information.
The accuracy may appear to be a great deal better than it actually is.

AIS is very good, even so it is not intended to be a stand alone or primary system.
Even with the limitatations of my little heads up unstabalised furuno 1612. And HBC.

A steady bearing is a steady bearing.

[Dockhead]

Originally Posted by Uricanejack

The AIS signal gives ship identification, GPS position, Gyro, speed.

Your unit calculates the colision info based on the info it receives. And comparing it to it’s own position information.
The accuracy may appear to be a great deal better than it actually is.

This is trivial, because it is true of every means you have to assess collision risk. No single source of information should ever be treated as infallible.



The big advantage of AIS is that the speed and course of the other vessel is not calculated, based on a series of radar sweeps, so based on data subject to the limitations of the radar bearing discrimination and range accuracy, but is REPORTED by the other vessel subject only to the limitation of the accuracy of the ship's own GNSS device, typically just a few meters. The data is time stamped with accuracy of like a millionth of a second, and as a result, AIS calculations of target vectors and CPA and TCPA are much more accurate than even ARPA using data from the best big ship radars, and more accurate by an order of magnitude or more, than what our little boat radars can do.


So AIS, for recreational boats, is THE killer app for plotting collision risks. It is not infallible, of course, but no other method is infallible, either, and no other method we have even comes close in speed, accuracy, and reliability.


It's good, however, to have radar, besides AIS, and to KNOW HOW TO USE IT, as many people in this thread have said. It's pretty straightforward to verify that AIS targets are at least where they say they are, by overlaying AIS target carats on the radar screen. There should be a radar blip where the target carat is! (You should also be cross-checking all your conclusions with your eyes, or with your helmsman's eyes). You'll also need the radar for those targets, which don't broadcast AIS (North Sea fishing boats! ).

[Dockhead]

Originally Posted by Uricanejack

. . . AIS is very good, even so it is not intended to be a stand alone or primary system.
Even with the limitatations of my little heads up unstabalised furuno 1612. And HBC.

A steady bearing is a steady bearing.

Yes, I certainly agree with all of this, other than the question of what should be the primary system.


What's really important here is not forgetting the basics -- steady bearings. Whether you're just looking at the radar picture, or using a HBC, or just looking with your eyes -- lack of perceptible relative motion should set off instinctive alarm bells.


There's always a HBC in my cockpit. I'm surprised at how many younger sailors don't even know what it is.



And I can set up to four EBL's on my radar. I actually think the EBL is more useful on small boat radars, than M/ARPA.

[El Pinguino]

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

Its a mystery to me why many yachts even feel the need to turn their AIS on....
OK on an ocean passage or in the North Sea/English Channel or but what do this lot below think they are achieving? Yes I know ... there was a bit of smoke haze there the other day...

Is it a 'look at me... look at me' thing? I know some that then complain - arms akimbo - that they don't like people knowing where they are.

Came as a bit of shock a few years ago to find my sister... most internet illiterate person on the planet... was getting notifications every time we sailed from or arrived in a port.

For the OP..... radar... and ask them to throw in a free AIS... not like they are very dear.

[CarlF]

Radar is a great tool if a well trained radar operator is keeping a continuous proper watch - but that's not happening these days. Bridge crews use AIS because it's the quickest way to check the CPA. And if a collision does occurs, it will be the AIS track that will be used in hearings to determine fault.

In Congressional testimony on the collisions involving the USS McCain and Fitzgerald, Admiral Richardson testified

"There is this Automatic Identification System, AIS. We had, I think, a distorted perception of operational security that we kept that system secure – off – on our warships. One of the immediate actions following these incidents is that, particularly in heavily trafficked areas, we’re just going to turn it on.”

If you read the report, it seems highly likely that - despite multiple crew/training failures - had AIS been transmitted by the Navy ships, the collisions would not have occurred.

And the USCG also calls AIS an collision avoidance system

"...the Automatic Identification System (AIS) is a ship-to-ship collision avoidance system that allows for communication of position, speed, and other ship data...
https://www.navcen.uscg.gov/?pageName=NAISvsAIS

[Uricanejack]

Originally Posted by Dockhead

Yes, I certainly agree with all of this, other than the question of what should be the primary system.


What's really important here is not forgetting the basics -- steady bearings. Whether you're just looking at the radar picture, or using a HBC, or just looking with your eyes -- lack of perceptible relative motion should set off instinctive alarm bells.


There's always a HBC in my cockpit. I'm surprised at how many younger sailors don't even know what it is.

And I can set up to four EBL's on my radar. I actually think the EBL is more useful on small boat radars, than M/ARPA.

I guess we agree on some of the fundamentals.
With a difference of opinion of what is primary. Which in a way goes directly to the fundamental question asked here. AIS instead of Radar.

My answer. Not AIS instead of radar. AIS in addition to Radar. We might be agreeing on this due to different thought process or priorities. Yet coming to similar conclusions.

Many, maybe even most sailors are leaning towards instead off. Not nessessarly on this thread.

Getting back to accuracy. There is a few big difference between the information from an ARPA and the information from an AIS.
ARPA is observed information even though it has been calculated by a computer.
Even a good ARPA requires 3 minutes to get reasonable accuracy preferably 6 minutes.
This means the course and speed of both vessels has been averaged over 3 or 6 minutes.
Of course this accuracy goes back out the window if ither vessel changes speed or course.

With AIS you will probably get quicker initial accuracy and information and quicker accuracy after a change of course or speed by ither vessel.

The AIS information from a ship will probably have reasonably high accuracy over a longer period of time. They tend to have very steady course and speed.
Steering to within 1 dog and decimles of a knot.

The course and speed of a small vessel will vary much more.

AIS as far as I know uses GPS position to GPS position to determin Co and Speed made good. Over the ground.
How long does a particular AIS average the positions over. To determin course and speed. I don’t know. It appears to be quite short.

Comparing a small vessels ground track to a large vessels ground track may be mathematically accurate.

Due to the shorter time frame may vary quite a bit due to the variation in speed or course. The predicted CPA may change quite a bit. Particularly with fluctuations in speed when crossing.

Just on fundamental principle.
I always prioritize observation and information from observation. Over ellectronic derived position or information from electronically derived positions.
Even though they are probably more accurate.

One of the early observations of the use of ARPA by mariners.

Mariners instead of using the increased accuracy to increase and improve colision avoidance and reduce close quarters situations from developing.
A significant no mariners used thier perceived better ARPA information to reduce margins and minimize alterations resulting in a greater number of close quarters situations.

The smaller CPA predicted by ARPA often becomes acceptable. Where the less accurate traditional plotting and taking of bearings would lead to course or speed alteration.

Overall several studies concluded the introduction of ARPA increased the tolerance of risk.

Which is interesting.

[daletournier]

Originally Posted by tanglewood

Identification. That's why it's called "Automated Identification System", not "Automated Collision Avoidance System"

I use mine specifically for collision avoidance. The word collision is used in the setting of parameters, identification (name) is a bonus.

[Dockhead]

Originally Posted by Uricanejack

I guess we agree on some of the fundamentals.
With a difference of opinion of what is primary. Which in a way goes directly to the fundamental question asked here. AIS instead of Radar.

My answer. Not AIS instead of radar. AIS in addition to Radar. We might be agreeing on this due to different thought process or priorities. Yet coming to similar conclusions.

Many, maybe even most sailors are leaning towards instead off. Not nessessarly on this thread.

Getting back to accuracy. There is a few big difference between the information from an ARPA and the information from an AIS.
ARPA is observed information even though it has been calculated by a computer.
Even a good ARPA requires 3 minutes to get reasonable accuracy preferably 6 minutes.
This means the course and speed of both vessels has been averaged over 3 or 6 minutes.
Of course this accuracy goes back out the window if ither vessel changes speed or course.

With AIS you will probably get quicker initial accuracy and information and quicker accuracy after a change of course or speed by ither vessel.

The AIS information from a ship will probably have reasonably high accuracy over a longer period of time. They tend to have very steady course and speed.
Steering to within 1 dog and decimles of a knot.

The course and speed of a small vessel will vary much more.

AIS as far as I know uses GPS position to GPS position to determin Co and Speed made good. Over the ground.
How long does a particular AIS average the positions over. To determin course and speed. I don’t know. It appears to be quite short.

Comparing a small vessels ground track to a large vessels ground track may be mathematically accurate.

Due to the shorter time frame may vary quite a bit due to the variation in speed or course. The predicted CPA may change quite a bit. Particularly with fluctuations in speed when crossing.

Just on fundamental principle.
I always prioritize observation and information from observation. Over ellectronic derived position or information from electronically derived positions.
Even though they are probably more accurate.

One of the early observations of the use of ARPA by mariners.

Mariners instead of using the increased accuracy to increase and improve colision avoidance and reduce close quarters situations from developing.
A significant no mariners used thier perceived better ARPA information to reduce margins and minimize alterations resulting in a greater number of close quarters situations.

The smaller CPA predicted by ARPA often becomes acceptable. Where the less accurate traditional plotting and taking of bearings would lead to course or speed alteration.

Overall several studies concluded the introduction of ARPA increased the tolerance of risk.

Which is interesting.

This is an interesting post, which raises a number of interesting questions.

Concerning accuracy – I’ve got nothing to add to what you wrote, except that you shouldn’t forget the inherently greater accuracy of AIS, especially in this day of differential GNSS systems which can resolve position to a couple of meters, and which can make extremely accurate measurements of speed and course. (Is this important? Maybe not -- you can argue that a calculation of CPA with accuracy of more than a cable, or anyway a few hundred meters, is overkill and not needed.) Also, there are many fewer things to go wrong in an AIS system, which does not need any other instrument data at all to do CPA and TCPA. A professionally maintained nav system on a commercial vessel with a large radar will certainly give target plotting results which are perfectly adequate. HOWEVER, you can’t say the same thing about small boat radars, so here the accuracy of AIS may be very important.

As to the accuracy of plotting small targets, especially sailboats – there is nothing in the world, which can predict something which is inherently unpredictable, like the course and speed of a sailboat sailing hard on the wind in gusty conditions. But here AIS is extremely valuable, because of its much greater responsiveness – it’s easier to define the cone of uncertainty and see the RANGE of possible results, and it's easier to see the LIMITS. So it’s much easier, with AIS, to choose a manuever which will give you a safe CPA to the LIMIT of the varying path of the sailboat, than with ARPA, which updates too slowly.

But AIS can only work if the other vessel is transmitting. It’s a passive means. Radar is an active means – radar sees, like our eyes do, except radar sees in darkness and fog too. For this reason, going back to the original question, if I were forced to choose, I would always choose radar over AIS. And if by “primary”, you are simply saying that you look at radar first – then I totally agree with you. The first job of collision avoidance is to be aware that the target even exists (where the Fitzgerald bridge screwed the pooch). Radar is a completely superior means for this. AIS should be used only for plotting and should never be relied upon to tell you whether there is anything out there, or not.