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Old 08-01-2018, 00:13   #1
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AIS Closest Point of Approach warning

Hi folks.

I am currently developing the AIS facility of my soon to be launched marine data product and I am at the stage of writing the Closest Point of Approach algorithm. What distance at the CPA would you want to receive a warning from the system ?

I will make it configurable anyway and also take into account the other vessels size but there will also be a default and I am wondering what that default should be.

Suggestions much appreciated.
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Old 08-01-2018, 00:31   #2
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Re: AIS Closest Point of Approach warning

This number varies so much with changing situations that it really does not matter what the default value is. Most users will be changing it repeatedly... I know that I do!

The better AIS systems have multiple sets of alarm values that can be related to different scenarios, and teach of hose values can be user set inside the groups. But I suspect that you already know that... this is hardly a virgin field of programming!

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Old 08-01-2018, 00:32   #3
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Re: AIS Closest Point of Approach warning

Oh , first shot. My sense is to think in terms of a very tired single handed Sailor on a long distance sail! So first warning should be as early as possible. Then it raises itís intensity to the level that would wake a deadman! In other words it should require our dog tired Sailor actually get up to turn it off. Preferably the shut off would be in a distant location where our Sailor can see his beomouth adversary. Like Captain Ronís sez letís take it out and give it tryĒ
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Old 08-01-2018, 00:41   #4
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Re: AIS Closest Point of Approach warning

On the open ocean we track large AIS targets when they appear on AIS which is at about 20 nm. We track them visually when they cross the visual horizon which is about 11 nm. We contact large ships at 8 nm when the CPA is within 1 nm. I would want an alarm to sound anytime the CPA is within 2 nm but at 11 nm distance.

Everything changes in confined waters where we track all targets unlike the US Navy!
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Old 08-01-2018, 00:43   #5
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Re: AIS Closest Point of Approach warning

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Oh , first shot. My sense is to think in terms of a very tired single handed Sailor on a long distance sail! So first warning should be as early as possible. Then it raises itís intensity to the level that would wake a deadman! In other words it should require our dog tired Sailor actually get up to turn it off. Preferably the shut off would be in a distant location where our Sailor can see his beomouth adversary. Like Captain Ronís sez letís take it out and give it tryĒ
Escalating warning levels as the CPA diminishes are a good idea thanks for that. What distance CPA would be the starting point for you at the lowest level given say the other vessel is a 300 meter tanker ? and when would you want it screaming - just so I can add some sensible defaults that people can use without too much configuration effort.

Many thanks
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Old 08-01-2018, 00:58   #6
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Re: AIS Closest Point of Approach warning

Offshore many folks like to maintain a minimum of one mile CPA, and it seems that the standing orders on many ships is to do that as well, so that would be a reasonable alarm point, OFFSHORE. In more crowded waters, this would result in nearly continuous alarms and so is not so reasonable. So, as I tried to point out earlier, cruisers will need to change that setting frequently, and the default isn't terribly important (at least IMO).

Further, I don't particularly care what the size of an oncoming vessel may be... 300 m or 30 m or whatever, I want to be aware of it and it's CPA, speed, range, and bearing... all at first glance. Further details should be available on demand, but the first four factors are what counts in one's early evaluation of the developing situation.

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Old 08-01-2018, 03:09   #7
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Re: AIS Closest Point of Approach warning

Quote:
Originally Posted by damianham View Post
Escalating warning levels as the CPA diminishes are a good idea thanks for that. What distance CPA would be the starting point for you at the lowest level given say the other vessel is a 300 meter tanker ? and when would you want it screaming - just so I can add some sensible defaults that people can use without too much configuration effort.

Many thanks
You should study the implementations in OpenCPN and commercial programs before just making up something yourself.

Don't make it too complicated. There was a discussion recently on VISUAL warnings which can be triggered according to different criteria than audible ones. Whatever warnings you have should be EASILY CONFIGURABLE since as others have said on here, we change this configuration frequently depending on traffic conditions.

It should also be possible to cancel an audio alarm without having to unconfigure the alarm, unlike what many commercial programs do, but with a flashing red target caret or something on the screen.

We also have something called "target attenuation" on my systems, which grays out the target carats of anchored vessels (for example), or vessels which are far from being a threat.

If you read the archives you'll find a lot of practical information about how we use these systems.
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Old 08-01-2018, 08:51   #8
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Re: AIS Closest Point of Approach warning

I have a related issue. The CPA shown by the system does not provide info if the other vessel is passing (at the CPA) in front or behind our vessel. This is an interesting info as it may help to decide how to react. Some systems have a relative bow vector that can be shown for the incoming vessel which provides visual answer to this. However, it would be nice if the CPA will be accompanied by a letter or +/- character to warn about the relative position of the impending CPA
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Old 08-01-2018, 09:02   #9
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Re: AIS Closest Point of Approach warning

Alarms are always a complex topic.
I would want to see the ability to initially alarm on both CPA and Time to CPA.
it would be nice to be able to reconfigure notifications for each approaching vessel separately, for example large boat approaching fast mute alarm until time to CPA is half the initial limit time; small slow boat, mute alarm until CPA is 100 meters or time to CPA is 5 minutes.
These options could be presented in a drop down list or selected with a wheel control.
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Old 08-01-2018, 09:09   #10
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Re: AIS Closest Point of Approach warning

Any thought to some sort of time-based override?

I'm thinking in terms of a singlehander leaving SF en route Hawaii. In the Bay, targets and alerts are going to be constant, but as the sailor get farther offshore, targets will become less frequent, to the point of being quite seldom, if at all. How about a feature that realizes the boat is on a voyage and based upon the time since the last alert ramps up the volume and intensity of the alert signal (if/when it happens) to 'jolt' an exhausted/complacent/whatever singlehander into action?
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Old 08-01-2018, 09:29   #11
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Re: AIS Closest Point of Approach warning

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Originally Posted by meirriba View Post
I have a related issue. The CPA shown by the system does not provide info if the other vessel is passing (at the CPA) in front or behind our vessel. This is an interesting info as it may help to decide how to react. Some systems have a relative bow vector that can be shown for the incoming vessel which provides visual answer to this. However, it would be nice if the CPA will be accompanied by a letter or +/- character to warn about the relative position of the impending CPA
This issue drives me crazy with the AIS implementation on my plotters (B&G Zeus). I have to watch the bearing.

The best solution of this I've seen is the graphic display of the crossing geometry in OpenCPN. You can select vessels and turn it on and off. It's great and really helps you to make decisions -- an instant graphic view of what kind of situation you are dealing with.
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Old 08-01-2018, 09:52   #12
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Re: AIS Closest Point of Approach warning

Quote:
Originally Posted by meirriba View Post
I have a related issue. The CPA shown by the system does not provide info if the other vessel is passing (at the CPA) in front or behind our vessel. This is an interesting info as it may help to decide how to react. Some systems have a relative bow vector that can be shown for the incoming vessel which provides visual answer to this. However, it would be nice if the CPA will be accompanied by a letter or +/- character to warn about the relative position of the impending CPA
I have the same question but i see Dockhead has given a reply of his own (watch the bearings which is what i also do). I have a Raymarine AIS which shows the icon of the passing vessel in pink if there is any danger according to user-adjustable limits. I had a very close CPA with a container ship (<200m) but the icon for the ship did not go pink so i think this answers the question. It appears that even if the CPA is very small, if the icon is not pink it means you are going to pass behind the vessel. It would have been nicer to see the type of MARPA display you get on a radar screen in order to be sure.
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Old 08-01-2018, 10:20   #13
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Re: AIS Closest Point of Approach warning

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Originally Posted by damianham View Post
Escalating warning levels as the CPA diminishes are a good idea thanks for that. What distance CPA would be the starting point for you at the lowest level given say the other vessel is a 300 meter tanker ? and when would you want it screaming - just so I can add some sensible defaults that people can use without too much configuration effort.

Many thanks
That would depend on the TCPA and the relative speeds as per the closing vectors. Obviously, a "worse case" scenario is that of a collision bow-to-bow (a CPA of "zero"!) between (for example) a sailboat making 7 knots due E and a tanker making 22 knots due W to have a closing speed of 29 knots. That allows the least amount of time for action and is, frankly, more important to notice than a CPA of 0.25 NM on an overtaking course with a closing speed of around 15 knots.

I would want a different sort of alarm to alert me of the first condition than the second and I would want that alarm (a different colour on the screen for the approaching vessel, perhaps red for potential collision; yellow for "close approach" and green for "a pass of greater than 1 NM") to have a different audio component. This buzzer or beeper or siren could change once I had made sufficient course corrections to avoid a collision.

Of course, CPA is also a matter of judgement. What is "too close for comfort" for me (I don't want to be heeled by a bow wave of the other guy) may be different for another sailor and would be reflected in my SOG and my basic maneuverability, sea state, etc. That's when you pick up the mic. If I was becalmed, and my motor was out of commission, for instance, I can't easily get out of anyone's way, and so would need to give the approaching "huge ship" sufficient warning to move or to cut speed. On the high seas, this is not a big deal, but on coastal approaches and shipping laness/TSSes, it isn't easy to arrange. My wife was on a delivery in 2011 of a 32 footer, the shaft of which cracked through in calm air just outside of Charleston Harbour in South Carolina at night in the rain. An AIS transceiver would have made a bad situation less dangerous while they waited for a tow vessel to arrive as massive cargo ships churned away on either side of them.

I hope this helps.
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Old 08-01-2018, 10:28   #14
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Re: AIS Closest Point of Approach warning

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It should also be possible to cancel an audio alarm without having to unconfigure the alarm, unlike what many commercial programs do, but with a flashing red target caret or something on the screen.

We also have something called "target attenuation" on my systems, which grays out the target carats of anchored vessels (for example), or vessels which are far from being a threat.
These are my two biggest beefs with systems I've used. Generally, I want to know what's out there, so some sort of alert option is appropriate. But once I know it, please don't keep blasting alarms at me. I'll just end up being forced to shut off all alarms, so I can focus on what's important.

On a similar note, I fully intend to navigate so as to avoid moored ships. I don't really need a countdown, flashing icons or, especially, loud noises to tell me I'm going to pass really, really close to that ship tied up alongside the narrow channel that I have to follow.
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Old 08-01-2018, 11:27   #15
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Re: AIS Closest Point of Approach warning

I don't like different alarms for different AIS conditions. If it's important enough to raise an alarm, then it's important enough to demand my attention. False alarms or annoying alarms when there's no need for a response eventually get permanently turned off. You might as well not even have a quiet "ignorable" alarm.

Visual indications are good.

I don't think I would like an alarm that changed its trigger parameters based on "time since previous alarm", or other adaptive behavior. A couple of different modes (such as "harbor" and "at sea") with configurable CPA/TCPA/Range settings would be useful, but further complication sounds too confusing. I would have to be convinced that complex adaptability would be a good thing.
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