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Old 06-10-2014, 05:09   #1
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CPA calculation - ship sizes - closer proximity

FS#1490 : CPA calculation doesn't take ship size into account
-Options > Ship > Own ship - Need relative position of GPS sensor in ship
-Options > Ship > AIS - Show targets real size have checkmark
-AIS Signal - Does it locate the GPS relative to the ship?
-CPA Avoidance setting - A new minimum avoidance setting?

Can users advise what do other programs do about AIS closer proximity calcs?
How do they handle close proximity? < 1 nm?
Is < 1 nm the right number?

We are looking for experience/opinions and facts about what other programs do in this situation. What is recommended by Coast Guard and other authorities.

The goal is to have a simple (from programming standpoint) but flexible solution that works for everyone.

Also please note your own experience / perspective (commercial, tankers, ferries, small, etc)
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Old 06-10-2014, 06:07   #2
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Re: CPA calculation - ship sizes - closer proximity

I do not know the answers and worse yet I have a sideloader question:

- is GPS offset broadcast in class A TX sentence???

I know I should know but better to ask a silly question than remain dumb. I hope.

barnakiel
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Old 06-10-2014, 07:27   #3
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Re: CPA calculation - ship sizes - closer proximity

barnakiel...

The offset for the ship's GPS is transmitted as part of Class-A static data. We use this info if we are rendering the ship as a "full-size" vector image on-screen. Try it sometime with a TCP feed.

IMHO, we should take a very conservative approach to CPA incursion warnings.

Dave
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Old 06-10-2014, 09:35   #4
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Re: CPA calculation - ship sizes - closer proximity

Quote:
Originally Posted by bdbcat View Post
barnakiel...

The offset for the ship's GPS is transmitted as part of Class-A static data. We use this info if we are rendering the ship as a "full-size" vector image on-screen. Try it sometime with a TCP feed.

IMHO, we should take a very conservative approach to CPA incursion warnings.

Dave
THX!

Now I know.

I believe that when sailing a smaller craft, the erratic movement of such a craft (large variations of its speed and heading/course) exact judgement of tight CPA/TCPA scenarios may vary from very difficult to impossible. In fact, when it is a close call, we might be better off by switching off (if only mentally) the AIS dictated clue.

Perhaps the above could somehow be incorporated into the real world coding choices by implementing something along the lines of a "too close to say" warning being displayed along the CPA/TCPA of objects that might pass us by within an X mile(/cable) range.

Thank you again for all your work and tuition.
barnakiel
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Old 06-10-2014, 09:39   #5
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Re: CPA calculation - ship sizes - closer proximity

Thanks, I am sure I asked a couple of dumb questions, but what about this one?

-Options > Ship > Own ship - Need relative position of GPS sensor in ship

Once knowing the offsets what makes sense for close proximity calcs. I agree with bdcat about being conservative. I think user needs to know what is considered "conservative" with some explanation or description of how it is handled.
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Old 06-10-2014, 12:09   #6
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Re: CPA calculation - ship sizes - closer proximity

Hi,

I cant understand the original post.

Are you asking if the CPA can be a minimum of 1 nm? If so thats crazy. It needs to be as it is now where a much tighter miinimum can be used.

In the ocean far from land i want to be alerted to any ship coming within a few miles of me. But in ports, congested areas, chanels etc, i need to be able to be warned of those coming only a few hundred meters from me.

A 1 mile CpA is useless in the area below:

But I really want to be warned about some ferry thats just racing around the corner and coming close to me.


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Old 06-10-2014, 12:30   #7
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Re: CPA calculation - ship sizes - closer proximity

Mark please read the FS link it will be clearer to you re additional proximity calcs.
We agree with you but as you get closer there are additional considerations. What do other programs / official bodies do / recommend. It would be good to come up with a reasonably simple solution.


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Old 06-10-2014, 13:30   #8
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Re: CPA calculation - ship sizes - closer proximity

I wouldn't get too clever with using the GPS location in the CPA calculations. I still occasionally see ships with bad or missing heading info, and that would really mess things up.

NavMonPc is not particularly sophisticated in the CPA calculations but for what its worth I just use the GPS position. I do not trigger an alarm is the TCPA is negative (the target is moving away from us). I do have simple filters on the alarm trigger criteria to reduce the transient false alarms caused by maneuvering vessels.

As for close-in AIS alarms, especially in harbors where there is a lot of maneuvering going on, the false-alarm issue makes AIS collision warnings almost useless, at least with NavMon and any chartlotter I've used. If you reduce the CPA / TCPA enough to mitigate false alarms, you are left with very little time to react. I use my eyeballs instead. Even if there were a "magic" setting, I wouldn't trust it.
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Old 06-10-2014, 16:27   #9
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Re: CPA calculation - ship sizes - closer proximity

Quote:
NavMonPc is not particularly sophisticated in the CPA calculations but for what its worth I just use the GPS position. I do not trigger an alarm is the TCPA is negative (the target is moving away from us). I do have simple filters on the alarm trigger criteria to reduce the transient false alarms caused by maneuvering vessels. As for close-in AIS alarms, especially in harbors where there is a lot of maneuvering going on, the false-alarm issue makes AIS collision warnings almost useless, at least with NavMon and any chartlotter I've used. If you reduce the CPA / TCPA enough to mitigate false alarms, you are left with very little time to react. I use my eyeballs instead. Even if there were a "magic" setting, I wouldn't trust it.
Thank you Paul, that is helpful to my understanding of Close Proximity Calcs. Maybe there is some user adjustable distance setting, which does what?
--Doesn't start elaborate calculations
--Messages To look up and out, CPA is too close for AIS?
--Optionally sounds a horrendous DIVE DIVE DIVE! - Just kidding.
The current AIS Options include:
Warn if CPA is less than (Nmi) 2.0
and TCPA is less than (Min) 30
Maybe there should just be a note that "CPA calcs are meaningless < 2nmi, too many variables and unknowns..look up and out"
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Old 06-10-2014, 16:32   #10
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Re: CPA calculation - ship sizes - closer proximity

and warning about the ferry... some flag identifier for Targets over X speed?
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Old 06-10-2014, 18:09   #11
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Re: CPA calculation - ship sizes - closer proximity

I can't tell you how absolutely insane it would be to have the minimum CPA IMPOSED by you at 1 NM.

Its nothing to do with the vessels you can see. Its about the ones you can't easily see, or can't easily determine where they are going.

Its infantile to only rely on this stupid hairy eyeball crap. OpenCpn is electronics and should be allowed to do its best. Of course people use their eyes too. Do you think we are idiots? The only idiots are the people who do not use ALL the nav aids.


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Old 06-10-2014, 20:18   #12
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Re: CPA calculation - ship sizes - closer proximity

Mark...
Please, take a deep breath and read all that text again. Has anyone ever said that there should/will/whatever be a minimum CPA of 1 mile? Not so. All this is about how to warn you better than now when a target is close - when it is 20 miles away it really does not matter it is 300 meters long, not so when it is 2 cables away, at that moment it is kind of a bit more important if it's AIS GPS is mounted on the bow or on the stern and how it is handled in he CPA calculations. Are we clear now?

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Old 07-10-2014, 01:18   #13
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Re: CPA calculation - ship sizes - closer proximity

Thanks Pavel, Mark sorry if the question was unclear. I would like to know what other programs and authorities do / recommend for CPA calcs at closer proximities.

Paul (navmonpc author) was very helpful.







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Old 07-10-2014, 02:03   #14
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Re: CPA calculation - ship sizes - closer proximity

Did search AIS CPA calcs
Fugawi had GPS located on bow of boat when there is no offset signal
Fugawi calcs from the previous gps signal.

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Old 07-10-2014, 02:21   #15
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Re: CPA calculation - ship sizes - closer proximity

This statement agrees fully with COLREGS requirements for good seamanship and proper look-out. Possibly the reader should also be aware that, in interpreting AIS data by ECDIS systems, the good practice is to use the target positions only and not trust the SOG (Speed Over Ground) and COG (Course Over Ground) computed and sent by the target’s AIS transmitter. Those parameters are calculated separately using the Kalman algorithm, as is customary with ARPA systems.
Decision Support
Item 40 of the AIS guidelines clearly state that “AIS information may be used to assist in collision avoidance decision making”. Decision making is precisely the process where TOTEM ECDIS provides unique Decision Support Tools.

What is "the Kalman algorythm"?
http://www.hellenicshippingnews.com/...-ce918b407e1a/

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