AIS Closest Point of Approach warning

[Olddave]

I don’t like audible alarms full stop. So the alarm needs to be deactivated easily.
Good watchkeepinh means keeping an eye on developing situations without relying on an alarm, and the alarm always demands your attention when you are busy with a more immediate situation.
Dave

[Paul L]

Check the Vesper AIS manual. I think they do CPA/TCPA alarms and position when crossing displays about as well as you can.
You can setup 4 profile scenarios, anchored, harbor, coastal, and offshore alarms, each with different alarm criteria. One button to see the passing diagram. All done on a super low power display available in the cockpit.

[patprice]

My tired befuddled mind when at sea likes to keep things simple. Very simple. I like my AIS to have a loud alarm and for me it's prime purpose is to tell me something is close.

On reacting to the alarm, bring into play any other resources available to identify the risk. If daytime probably eyeball and object bearing change. And more eyeball. And VHF if need be.

If at night, again eyeball but distance judgement is difficult. Radar is very useful. So is again object bearing change.

Keep it simple. All the fancy electronic stuff is best left to those guys on the bridge of a ship. And while they are computing all sorts of data they are probably not keeping it simple and will likely run you over.

Last paragraph tongue in cheek.

[hamburking]

If CPA is 2 miles or less, sound the alarm immediately. To be clear...don't wait till the vessel is 2 miles away...if the vessel is 20 miles away and the CPA is 2 miles or less, blast that alarm.

Many boaters (present company excepted) don't know how to change settings or alarms...or don't bother. I've seen this first hand. So whatever is set "out of the box" is likely to stay for a long time. Kudos for seeking data and input on this. Too often, non-sailors sitting in an office make bad decisions regarding nautical matters. Examples would be a whole nother thread.

Good luck with your project.

[Olddave]

If I was designing an AIS, the Target List would take priority over alarms.

I like to see the most relevant vessels at the top of the list. This seems to be a challenge for AIS designers, presumably because of the imperfect nature of GPS positions and CPA’s.
Furuno has the most stable and relevant Target List I have seen. My Vesper AIS is ok too.
Dave

[Paul Elliott]

Originally Posted by Olddave

[...] presumably because of the imperfect nature of GPS positions and CPA’s.

I'm not sure what you mean by "imperfect nature". Care to elaborate?

GPS positions / SOG / COG and the resulting calculated CPA are as close to perfect as we are likely to see. There is some uncertainty when it comes to ship dimensions and heading, but the situation has become quite bad by the time these factors start to matter in an open ocean encounter.

[Stu Jackson]

Unless and until you learn who the reliable skippers here are, you may be missing their input.

Most of the replies are from them.

Stick around some more and you'll KNOW.

Jim & Ann, Paul, and Dockhead are the ones you need to listen to.

Reduce complications.

Defaults are meant to be adjusted. Make that EMINENTLY clear. F'rinstance, they aren't on simple things like external voltage regulators for alternators. We hear it all the time, over and over.

KISS. Make it USER friendly, NOT "designer" friendly.

[hzcruiser]

I'm also quite happy with my Vesper WatchMate class B transceiver:

• low power consumption
• simple setup
• audible alarm

In closed waters the alarm is off as I would have to keep pushing the "ignore" button all the time, off shore it's set to beep if CPA is closer than 1nm, that's it.


All big ships I've seen on the screen so far made course changes "automatically" to keep a minimum distance of said 1nm.



As a side note: One cruise ship actually hailed me on VHF while I was bopping around in Bass Strait testing the new sea anchor. They asked if I needed any assistance (because they saw on AIS that I was making no headway). Very nice touch.

BTW, the VesperMate has additional "gimmicks" like an anchor watch that seemed like an afterthought. You can push the "set" button and give it a radius, nothing else. If your anchor drags temporarily you cannot change that original position, only extend the radius.

[Olddave]

Originally Posted by Paul Elliott

I'm not sure what you mean by "imperfect nature". Care to elaborate?

GPS positions / SOG / COG and the resulting calculated CPA are as close to perfect as we are likely to see. There is some uncertainty when it comes to ship dimensions and heading, but the situation has become quite bad by the time these factors start to matter in an open ocean encounter.

Hi Paul
I’m not a tech expert so I have used the term “imperfect nature of GPS and CPA” to describe how CPA keeps changing when displayed on AIS units. You have two vessels sending each other their own Lat and Long, course and speed via VHF. Speed and course are changing slightly all time and Lat & Long are subject to some degree of the jitters also. Therefore the CPA calculated by AIS units jumps around a little or is “imperfect”. The Class B units transmit less frequently than Class A, adding to the imperfection of the CPA. Now throw in several other vessels within range of your VHF antennae, and the AIS unit begins to have trouble showing the vessels in order of priority with regard to their CPA.
My Vesper unit is quite hard to read when the Target List is set to display targets by priority. The order in which the targets are listed keeps changing, to the extent that it can be difficult to select the desired target.
As a result I always have the priority setting turned off, and scroll through the list to find CPA’s that are a concern.
Having said all that, I would recommend the Vesper Watchmate 850 to anyone, it’s a good piece of equipment. I have it connected to my computer and Maxsea software. The AIS is displayed on my plotter, which is a 24 inch flat screen TV, a very similar setup to the commercial vessels I operated when I was working.
I hope all that explains what I consider to be the “ imperfect nature” of gps and CPA in relation to AIS

[David M]

Perhaps let the user define the default CPA after he or she boots up the nav system for the first time? Make it a part of the initial setup.

Let them define a TCPA as well if they wish.

[CaptTom]

Originally Posted by Olddave

I like to see the most relevant vessels at the top of the list.

Excellent point.

I don't really care if the CPA is well BEHIND me. For that matter, I don't really care if the CPA is well behind the other vessel. And as I mentioned above, I don't really care about the CPA to a vessel moored alongside the channel.

These are not threats, and shouldn't be treated as such. They should be at the bottom of the list.

Before anyone freaks out, yes, I need to know about overtaking vessels, and yes, in the fog the location of a moored vessel is relevant. So are buoys, piers, islands and bridges. All of which show up on the display I should be already monitoring.

Think about it this way; AIS is being extended to things like virtual buoys. Do you really need an alarm telling you that red nun #8 is a half-mile astern?

[David M]

I'm sure the OP means contacts that one has chosen to activate with a range that is closing obviously and not just any contact that appears on the display.

Alerting you to new contacts is what a guard zone is used for. If one is not paying constant attention to the radar, and I hope they are not, and a contact enters the guard zone, that will set off an alarm and then one will choose whether or not to activate the contact. That is how ARPA is supposed to work anyway.

[CaptTom]

Originally Posted by David M

I'm sure the OP means contacts that one has chosen to activate with a range that is closing obviously and not any contact that appears on the display.

Maybe. I took the OP's question at face value: "What distance at the CPA would you want to receive a warning from the system?"

I think the point is, there is no one number. It depends on the location (harbor or open water, for example) and the bearing to that CPA (ahead or astern) and a bunch of other variables including traffic density and our own piloting style.

Perhaps the more relevant point is, it will take some clever design, with some real thought given to real-world use. These things are rare in programming nowadays.

[David M]

I agree. And I suggested to the OP in post 25 that perhaps the default CPA value should be set by the user in the options menu before using the nav system for the first time. Give the user the choice of which CPA is going to be the default value each time he or she boots the nav system.

[Olddave]

Originally Posted by damianham

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.

In addition to other useful suggestions amongst the posts here, I would suggest that you drop any consideration of vessel size.
We all operate under the international seafarers laws known as the Colregs. To cut a long story short, I am obliged to avoid a collision with a smaller vessel exactly the same as I would avoid being run down by a larger vessel.
Dave