AIS Alarms

[boatmike]

From a previous post I have a "senior citizen" question regarding AIS alarms.
I have a new Standard Horizon 500 plotter. From the book of destructions with it I had assumed that there were two ways to set an alarm.
Lets call a potential collision the scene of the accident or SOA
If I set the distance alarm to 2 miles then will it go off if a vessel is within 2 miles of the SOA or 2 miles of where I am?
If I then set the time alarm to 20minutes, is this 20 minutes before the SOA or 20 minutes before the vessel gets to the 2 mile limit?
/forums/images/graemlins/confused.gif

 

[LadyInBed]

Test it!
Set the collision course alarms at a 'safe' distance then set a collision course.
Simple! /forums/images/graemlins/shocked.gif

 

[deep denial]

I believe the answer is that the alarm will go off if the CPA (closest point of approach) is projected to be less than 2 miles

 

[boatmike]

Yeah well I could have worked that out for myself even being as old and technologically challenged as I am (Doh!)
What I was hoping was someone might know without me deliberately stopping in front of a tanker to find out see??? /forums/images/graemlins/mad.gif

 

[LadyInBed]

I don't know, some people just can't be bothered to experiment these days
/forums/images/graemlins/laugh.gif

 

[boatmike]

What like playing marbles on the motorway to see if you get run over? /forums/images/graemlins/tongue.gif

 

[fireball]

[ QUOTE ]
If I set the distance alarm to 2 miles then will it go off if a vessel is within 2 miles of the SOA or 2 miles of where I am?
If I then set the time alarm to 20minutes, is this 20 minutes before the SOA or 20 minutes before the vessel gets to the 2 mile limit?

[/ QUOTE ]
If you set the distance alarm to 2 miles then it will ALARM if the predicted courses mean the vessels will be INSIDE 2Nm at any point during their crossing - to achieve this, the CP will predict your course/time and that of the target vessel and evaluate the distance between them

If you set the time alarm to 20 minutes then it will do the above calculation for the predicted positions in the next 20 minutes - ie if the SOA is 25 minutes away you would have 5 minutes of no alarm followed by 20 minutes of alarm (assuming no course change)

That's my reading of it anyway.

 

[jackh]

I assumed that an alarm of 2 miles and 20 minutes means -

If another vessel will pass within 2 miles within the next 20 minutes the alarm will sound.

 

[fireball]

Yes - but in 20 minutes time you will not be where you were ....

 

[jackh]

By pass within i meant closest point of approach based on the projected courses of your own and the target vessels.

I was trying to agree with Fireballs explanation, but possibly just confused the issue...

 

[fireball]

/forums/images/graemlins/grin.gif

 

[davidfox]

Reading the replies, I suggest you have another go at that instruction book!

 

[boatmike]

Yeah! I had hoped that someone on the fulorium would have a SH chartplotter and know the answer because the destructions in the book are not clear..... I live in hope that someone will know rather than guessing like me! /forums/images/graemlins/frown.gif

 

[boatmike]

I just downloaded the book from their website. Previously I was sitting at home waiting for a delivery, bored and wondering because I was confused about the other post and the book is on the boat. I think it is clear actually. However for those who want to know here is the "oracle"


14.0 AIS SYSTEM DEFINITIONS
· Target: It is a vessel equipped with AIS. Information about the targets is being received
by AIS Receiver and displayed on the screen.
· CPA: Closest Point of Approach is the closest distance that will be achieved between
your vessel and the tracked target, based on your vessel’s speed and direction and the
target’s speed and direction.
· CPA Limit: This is the distance from your vessel that a target may reach before a target
is deemed a threat.
· CPA Alarm: Occurs if CPA is less or equal to CPA Limit. This test is done for active
targets only.
· TCPA: Time to closest Point of Approach is the time remaining until the CPA will occur.
· TCPA Limit: This is the time remaining before the CPA is reached.
· 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.
· Radio Call Sign: International call sign assigned to vessel, often used on voice radio.
· Name: Name of ship, 20 characters.
· MMSI: Maritime Mobile Service Identity.
· MMSI number: A unique 9 digit number that is assigned to a DSC radio station. It
primarily registers the boat information in the U.S. Coast Guard’s national distress
database for use in emergency situations.
· Active Target: Target located within the Activation Range. Active target is represented
by oriented triangle with COG and Heading vectors. Rate of turn may also be displayed.
· Dangerous Target: Target detected by CPA or TCPA Alarm. Dangerous target is
Active Target by definition. For better visibility Dangerous Target symbol is flashing.
· Sleeping Target: Target located outside the Activation Range. Sleeping target is
represented by a small oriented triangle.
· Lost Target: When the AIS info is not received from that vessel for 3.5 minutes. The
presentation will be a flashing black triangle with a cross through.
· Activation Range: Range around your boat where targets become active. AIS target
become active within this range. Activation Range should be greater than CPA Limit by definition.

So I think you set the activation range (say 15 miles?) and it will show all targets in that range.
Then if you set the CPA limit to 2 miles it will sound a CPA alarm if a vessel enters that zone. So if you get a CPA alarm the other guy is already within 2 miles.

Then if you set the Time to Closest Point of Approach Limit (TCPA Limit) to 20 minutes a TCPA alarm will be activated when the calculations show you will be within 2 miles of one another in 20 minutes time. So if you don't do anything about the TCPA Alarm, when he gets 2 miles away you will get a CPA alarm to say he has got there.
I think I get it now. Does anyone dissagree? I appear to be having a thicko day today!

 

[Phoenix of Hamble]

I disagree.....

But only with your CPA statement

I read that as the alarm will sound when a vessel is within 15miles of you, but its CPA will be 2miles or less, not as you state, which is when the vessel get within 2 miles of you...

 

[fireball]

I slightly disagree with the TCPA statement - a TCPA alarm will be activated when the calculations show you will be within 2 miles of one another in 20 minutes time. - I think this is UP TO 20 minutes time - but you probably knew that /forums/images/graemlins/wink.gif

 

[boatmike]

Well thats another way of reading it, and I am not sure which is the correct reading... that's where we came in.

 

[Phoenix of Hamble]

Yup.... but the interpretation i've given would make much more sense in terms of safety management and is in line with the same logic as the TCPA alarm....

Having spent my entire working life in or around software development environments, it just 'feels' like I am right..... but willing to be proved wrong....

 

[boatmike]

Nit picker!
Actually if we are going to be like that the alarm WILL be activated when he is 20 minutes away and will continue as he gets closer..... smarty pants!

But do we agree with Morgana? Dunno! I don't think so because it would not make sense. If the CPA alarm went off for any vessel with a PREDICTED CPA of 2 miles you could get an alarm from a ship 15 miles away that would take say 40 minutes to get there which seems to make a nonsense of the TCPA alarm dunnit? Gawd I don't know! Surely some of you guys who have had AIS for more than a season know? Don't you? /forums/images/graemlins/confused.gif

 

[Phoenix of Hamble]

Urm.... I'd rather get the alarm with 40 mins notice where a small course adjustment would do, that when it was at 2miles when I may need to alter quite a bit....

And with my logic, if I ignored the CPA alarm, then i'd get a reminder when the TCPA hit 20 mins....