AIS Plotter Alarms

[wooslehunter]

I use a Standard Horizon CP180 connected to an AIS receiver.

The CP180 will sound an alarm if either CPA OR TCPA are within user defined limits. I can also set an activation limit such that targets outside the limit will be ignored.

Here are some examples setting the limits to CPA = 0.5nm, TCPA = 15mins, activation range = 5nm.

1. CPA = 2nm, TCPA = 10mins - I get an alarm. I don't care as CPA is too large.

2. CPA = 0.1nm, TCPA = 1hr - I get an alarm. I don't care as TCPA doesn't happen for an hour. I have other things to worry about before that.

3. CPA = 0.1nm, TCPA = 9mins - I don't get an alarm because I have to set the activation range down to avoid permanent alarms. But this target is a cross channel cat at 35kts which will get way too close in just over 8 mins. Chances are, at 5 miles, I won't even see it.

So, how does other manufacturers' equipment work? It seems more logical that the alarm should sound if CPA AND TCPA are within limits.

 

[Playtime]

Firstly, if you are in crowded waters (like the Solent) then turn the alarms off and use your eyeballs.

For cross channel (where AIS is extremely useful) I set up CPA to be (less than) 0.5 mile and TCPA to (less than) 30 minutes. This gives plenty of time to see the threat and determine if action is required (like calling them up to ensure they have seen you).

Both criteria have to be met for the alarm to sound (this is using OpenCPN on a laptop). I don't think it would make sense if the alarm sounded for only 1 criteria.

 

[pvb]

So, how does other manufacturers' equipment work? It seems more logical that the alarm should sound if CPA AND TCPA are within limits.

I think most plotters work in the same way, ie the alarm is triggered by either CPA or TCPA.

 

[Playtime]

I think most plotters work in the same way, ie the alarm is triggered by either CPA or TCPA.

Please explain how TCPA alone would work as an alarm trigger.

There must be a CPA limit as well to trigger the alarm; every ship has a CPA!

 

[pvb]

Please explain how TCPA alone would work as an alarm trigger.

There must be a CPA limit as well to trigger the alarm; every ship has a CPA!

I think you need to read a plotter manual to understand this. Plotters which display AIS targets calculate the CPA and TCPA for displayed targets. Generally, it is possible to set an alarm either on CPA or on TCPA (or indeed on both). Plotters offer a range of values which can be set to trigger either a CPA or a TCPA alarm. So it's perfectly possible to have an alarm triggered by TCPA alone.

 

[Playtime]

So it's perfectly possible to have an alarm triggered by TCPA alone.

It doesn't sound sensible to me.

If you set the TCPA to 30 minutes (disregarding CPA) on a cross channel trip, every ship (within range) in the channel that has a CPA within half an hour (whether it be 0.5 miles, 5 miles or whatever) will trigger the alarm.

Surely it should only trigger on BOTH CPA and TCPA.

 

[Martin_J]

Hi

I am not at the boat to test but the manual for the CP180_and_CP300_V16.01 states...

>>> CPA Alarm: Occurs if CPA is less or equal to CPA Limit. This test is done for active
targets only.
>>> TCPA Alarm: Occurs if TCPA is less or equal to TCPA Limit. This test is done for active
targets only and if CPA value is less or equal to CPA Limit.


This implies to me that TCPA alarm would only alert if the CPA is within preset alarm limit as well.

 

[Playtime]

This implies to me that TCPA alarm would only alert if the CPA is within preset alarm limit as well.

Thanks - that makes much more sense!

 

[pvb]

I am not at the boat to test but the manual for the CP180_and_CP300_V16.01 states...

>>> CPA Alarm: Occurs if CPA is less or equal to CPA Limit. This test is done for active
targets only.
>>> TCPA Alarm: Occurs if TCPA is less or equal to TCPA Limit. This test is done for active
targets only and if CPA value is less or equal to CPA Limit.


This implies to me that TCPA alarm would only alert if the CPA is within preset alarm limit as well.

That's a relatively recent change to the SH plotters.

 

[wooslehunter]

To perhaps make it a little simpler.

If a target is outside the activation range, then it cannot trigger either alarm - good feature, who cares if something's miles away.

If a target is inside the activation range, then it can trigger either CPA or TCPA alarms - OK feautre.

There is no way to set up so CPA AND TCPA must be violated before the alarm sounds - missing feature.

A few seem to be comming to the same opinion as me. CPA is no use without TCPA, so why raise an alarm on either rather than both.

The only way to have something is to set the activation range down but then fast moving targets only become active at the last minute.

So back to the original question: what do other plotters do?

 

[Martin_J]

Wooslehunter...
I did understand the question (and agree with you about the way the SH should work) but the SH manual appears to say that the SH180 and SH300 units do not work the way you suggest yours does. I would have to test later to see if my unit behaves as you say yours does or as per the quotes I gave from the instruction manual.
If mine behaves as per the manual then perhaps yours is on an older version of firmware.

Agreed - If CPA is just .1 miles then it would be great to know (whenever it's going to be) but if TCPA is in 2 minutes but distance 15 miles away then it's not so relevant.