AIS CPA and TCPA

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AdrianPetts

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Just venturing into the world of AIS and was wondering what would be sensible values to enter for CPA and TCPA for a unit on a sailing boat in English channel.
 
Uhm.
Those are automatic values generated by details from other vessels.

CPA = Closest Point of Approach
TCPA = Time to CPA

Unless you mean alarms, in which case, I go with the default 2 miles for the safe zone and 12 minutes to safe zone for the alarm.
 
Cardo said:
Oh yeah, my alarm lives turned off, unless we're on an offshore passage!
Click to expand...
For two years now I have had to completely suppress the AIS CPA/TCPA alarms in OCPN due to the expansion of Class B transponders in the Adriatic. This is a real disadvantage in waters populated by fast ferries with little respect for leisure craft and against which an alarm was a significant safety feature, with their high closing speeds.

I am informed that some manufacturers allow alarm settings based on target speed (Vesper Marine) and I did make the proposal to the OpenCPN developers for a similar function (or better, on Class B targets), which all seemed to go down like a lead balloon.
 
Barnac1e said:
For two years now I have had to completely suppress the AIS CPA/TCPA alarms in OCPN due to the expansion of Class B transponders in the Adriatic. This is a real disadvantage in waters populated by fast ferries with little respect for leisure craft and against which an alarm was a significant safety feature, with their high closing speeds.

I am informed that some manufacturers allow alarm settings based on target speed (Vesper Marine) and I did make the proposal to the OpenCPN developers for a similar function (or better, on Class B targets), which all seemed to go down like a lead balloon.
Click to expand...

That's very disappointing. Did they say why they weren't interested? I personally quite like the idea of an AIS alarm dependant on a minimum speed.

So far it appears many plotter manufacturers aren't keen to offer filtering abilities for AIS. It's either all or nothing.
 
Cardo said:
That's very disappointing. Did they say why they weren't interested?
Click to expand...
I first made a 'New Feature Request' for alarm suppression for Class B targets, which didn't even get acknowledged. In retrospect I could understand that it would be a controversial amendment.

Then learning of the Vesper Marine alarm filtering on target speed, thought that a good compromise, all those Class B alarms I was getting were mainly sailing yachts well under the sort of Class A speeds that were likely to kill me. So I wrote again to the Cruiser Forum with that proposal. This time I did get a grudging response that the alarm parameters of OCPN was not very extensive ... but nothing more.

A pity really. In every other way an excellent program that fulfils all my requirements.
 
Barnac1e said:
I first made a 'New Feature Request' for alarm suppression for Class B targets, which didn't even get acknowledged. In retrospect I could understand that it would be a controversial amendment.

Then learning of the Vesper Marine alarm filtering on target speed, thought that a good compromise, all those Class B alarms I was getting were mainly sailing yachts well under the sort of Class A speeds that were likely to kill me. So I wrote again to the Cruiser Forum with that proposal. This time I did get a grudging response that the alarm parameters of OCPN was not very extensive ... but nothing more.

A pity really. In every other way an excellent program that fulfils all my requirements.
Click to expand...

Sounds like they don't want to dedicate the time developing the feature, rather than disagreeing with the idea.
 
AdrianPetts said:
Just venturing into the world of AIS and was wondering what would be sensible values to enter for CPA and TCPA for a unit on a sailing boat in English channel.
Click to expand...

If new to AIS, don't underestimate the value of the bearing and heading readings.
Bearing is like your own, super accurate hand bearing compass. 2 benefits: You are accustomed to making judgements on this data, and also is the information that tells you if the CPA will be ahead or astern of you (if it's a crossing situation)
With heading you can see if the ship has made a small course adjustment. They often adjust a very small amount when quite a few miles from you to just pass behind, the CPS will increase as he does it, but remains small. In this instance though keeping is likely the best solution. The adjustment is so small you wouldn't see it by eye or hand bearing compass.
None of these numbers give you "the answer" in their own right, but certainly can help you make good decisions.
 
Cardo said:
Sounds like they don't want to dedicate the time developing the feature, rather than disagreeing with the idea.
Click to expand...
Before going down to the boat in Spring last year I had downloaded the source files and took them with me on a laptop. Then, while weather-bound in harbour, I did look to see what might be involved - it seemed, to this rusty old software engineer who hasn't looked at any source code for a lifetime or two, rather a trivial change. I made the changes but with only limited on-board PC resources I didn't compile or build so maybe my impression is the result of not understanding the full ramifications.

So I think the change would be a trivial one but the developers are fully involved in much more important upgrades to consider what they may see as a minor inconvenience to few users.
 
Elessar said:
If new to AIS, don't underestimate the value of the bearing and heading readings.
Bearing is like your own, super accurate hand bearing compass. 2 benefits: You are accustomed to making judgements on this data, and also is the information that tells you if the CPA will be ahead or astern of you (if it's a crossing situation)
With heading you can see if the ship has made a small course adjustment. They often adjust a very small amount when quite a few miles from you to just pass behind, the CPS will increase as he does it, but remains small. In this instance though keeping is likely the best solution. The adjustment is so small you wouldn't see it by eye or hand bearing compass.
None of these numbers give you "the answer" in their own right, but certainly can help you make good decisions.
Click to expand...

Absolutely. I rarely use my AIS apart from crossing the Channel. I have watched a big ship adjust their course by 5 degrees to go behind me. Using Mark I eyeball I thought that we would be OK BUT would have probably altered course to go behind him without the info from the AIS. My reassurance was not from the fact that the AIS 'said' he was going to miss me but that he had 'seen ' me.
 
dslittle said:
Absolutely. I rarely use my AIS apart from crossing the Channel. I have watched a big ship adjust their course by 5 degrees to go behind me. Using Mark I eyeball I thought that we would be OK BUT would have probably altered course to go behind him without the info from the AIS. My reassurance was not from the fact that the AIS 'said' he was going to miss me but that he had 'seen ' me.
Click to expand...

that's exactly the point I was trying to make - badly!
 
I use 1 mile for the CPA and 10mins for the TCPA.I am lucky that with my Vesper unit I can have different CPA/TCPA for differing modes i.e. Harbour/Coastal/Offshore which is what I do.I can also filter out slow moving and stationary vessels .This stops all the AIS alarms when passing Felixstowe port on my way out where the ships seem to leave their AIS on.If I was in an area with high speed ferries the CPA would be increased.I sometimes get 30 plus vessels within a 5 mile range but most are filtered leaving 2 or 3 to keep my eye on.
 
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