Thoughts about AIS

[colemj]

Originally Posted by donradcliffe

When a situation like this is developing, you should consider doing what 3rd world fishermen do--whistle into the mike on Channel 16. This may wake up a somnolent WK, and the clarity of your transmission lets him know that someone is close by.

But using the dsc function makes his radio ring loudly directly on the bridge until it is answered. Most just get louder the longer it goes unanswered.

Mark

[goboatingnow]

What you get from this is the direction of change of bearing, which tells you definitely whether you're going ahead or behind, and it tells you the speed of change. From a series of bearings you will know without any doubt whether you are passing ahead or behind.

I fail to see , unless you actually construct the vector, how you can determine passing ahead or astern from merely observing bearings, you can make the vector calculations in you head to "get an idea", but unless you do the chart work you can't really answer much with the bearings ( other then the collision case)

dave

[Dockhead]

Originally Posted by goboatingnow

not trying to convince you, but consider this

(a) at least with AIS, you have more( I stress only "more") reliable CPA's, CPAs are the biggest issue

(b) One ignores all CPAs,that fall outside what you regard as dangerous


(c) The key with channel crossings as you said is that you want asymmetric CPAs. close once you are astern and greater if you are crossing ahead.

(d) Where you have a close call, I don't see any method being better then another, because personally I distrust my own COG vector ( and hence CPA) more then anything else, This is not fixed by using either Vesper or Open CPN. ( it was awful using MARPA)

(e) Personally I have not ever tried to run the TSS with marginal CPAs, too date I have found there is "adequate" windows once you don't try and just run across. The great thing about AIS is I can see the "bunches" of ships and try and time my crossing to slip into the "gaps", that was never really possible before.

(f) So the issue boils down to evaluating the CPAs that are inside ones own "danger zone", I take simple decision, vectors look to close in time, thats not a crossing I will risk. Usually the CPAs are screaming this anyway.

The other thing I find is with the transponder , I see ships in the TSS opening the CPAs themselves anyway.

I don't see what help plotting bearings is, all you are really doing is re-constructing something the electronics are doing for you anyway.

That's very similar to what I do and how I think about it.

By the way, I almost never cross in a TSS and avoid it as much as possible. I usually cross where there is no TSS so TSS rules don't apply, which is much easier.

If ships are bunched up and I can choose a gap where there are no close crossings -- then that's exactly what I do, just as you do. I think it's clearly the right thing to do. For this you don't need any exact determination of the crossings.


But it's when there are no gaps and you have to thread the needle which is the real challenge. Here writing down the bearings allows you to identify between which ships a given course will take you. Even with a single ship it's helpful with a close crossing because it tells you which way you need to alter course to increase the CPA.

For me it's a matter of habit, because before AIS I always did this, with a hand bearing compass, or with radar. Didn't you? I bet you did. It's a standard technique which I was taught -- I didn't invent it. As I said in my post above responding to Mark, it tells you without any doubt and just about nil chance of error whether you are passing ahead or behind in a close crossing situation, and is capable of telling you this with regard to multiple targets at once. In this it is very powerful, and tells you something vital which our chart plotters don't tell us.

And so we get back to the original theme of this thread. Vesper and OpenCPN display this information in a very clever way, and I wish my chart plotter did it, so that I didn't have to do all this hand work.

[Dockhead]

Originally Posted by goboatingnow

I fail to see , unless you actually construct the vector, how you can determine passing ahead or astern from merely observing bearings, you can make the vector calculations in you head to "get an idea", but unless you do the chart work you can't really answer much with the bearings ( other then the collision case)

dave

What do you mean?? Of course you can! This is navigation 101! If the bearing is steady, you're on a collision course. If the bearing is increasing, the ship will end up to starboard of you (will pass ahead of you if it is coming from your port side, or behind you if it's coming from starboard). And vice versa. This is elementary, my dear boy! No vectors needed.

[goboatingnow]

For me it's a matter of habit, because before AIS I always did this, with a hand bearing compass, or with radar. Didn't you? I bet you did. It's a standard technique which I was taught -- I didn't invent it. As I said in my post above responding to Mark, it tells you without any doubt and just about nil chance of error whether you are passing ahead or behind in a close crossing situation, and is capable of telling you this with regard to multiple targets at once. In this it is very powerful, and tells you something vital which our chart plotters don't tell us.

before AIS, I just did quick manual radar plots. i.e. I constructed the vector.


as to HBC bearings, , mostly I used two objects to construct a simple relative bearing, If that worried me, then I powered up the radar and placed a EBL on it ( thats assuming I had a radar). other wise Id use a HBC. If after a few bearings I had a constant or slowly changing bearing, then in the day time , I would visually determine aspect, or at night , as you say its easier.

What I never did was make assumptions of exactly where I would cross the ship, unless the readings were very obvious. , i.e. a rapidly open bearing ("in front of me") meant I was passing astern and one open rapidly ( "behind me ") meant I was clear crossed ahead.

the killer was in-between those case, that was buttock clenching as The error in HBC readings meant I couldn't really draw much conclusion, usually I bailed , picked a safe course and confirmed I had opened the CPA.

Once I did a TSS in fog and no radar/AIS, I found that being an atheist allowed me to choose many gods, "old and new" to pray to.( PS I wasn't the skipper)

dave

[Dockhead]

Originally Posted by goboatingnow

before AIS, I just did quick manual radar plots. i.e. I constructed the vector.


as to HBC bearings, , mostly I used two objects to construct a simple relative bearing, If that worried me, then I powered up the radar and placed a EBL on it ( thats assuming I had a radar). other wise Id use a HBC. If after a few bearings I had a constant or slowly changing bearing, then in the day time , I would visually determine aspect, or at night , as you say its easier.

What I never did was make assumptions of exactly where I would cross the ship, unless the readings were very obvious. , i.e. a rapidly open bearing ("in front of me") meant I was passing astern and one open rapidly ( "behind me ") meant I was clear crossed ahead.

the killer was in-between those case, that was buttock clenching as The error in HBC readings meant I couldn't really draw much conclusion, usually I bailed , picked a safe course and confirmed I had opened the CPA.

Once I did a TSS in fog and no radar/AIS, I found that being an atheist allowed me to choose many gods, "old and new" to pray to.( PS I wasn't the skipper)

dave

Agreed.

But just one thing -- with a series of bearings, you don't need a "rapidly opening bearing" to tell you that you are definitely passing clear or not. You can tell when it's dodgy because of a lack of consistency -- not necessary speed of change. Bearing changes a little, then doesn't change, and most dangerous, a little change in the other direction -- this shows that the little course errors are giving you an average relative bearing which shows a collision risk.

On the other hand, if the bearings are very consistent, even if they are moving only slowly, you know you are passing clear. You don't just know that you're passing clear, you know that you know you're passing clear, because this method accounts for both errors of measurements and deviations of course of course and speed. It is an extremely powerful technique, and I commend it to you. It was especially useful in the hand bearing compass days as it dealt with the inherent inaccuracy of the data you get from a HBC.

I have also used the EBL on the radar in the olden days, and this is a much handier but much weaker technique then taking a data series of bearings. You only see the current bearing in relation to the very first one (when you placed the EBL); you don't see the development of all the bearings in between. Much, much weaker.

Radar plotting with vectors is already obsolete, as it merely does laboriously and by hand what MARPA does relatively quickly and automatically, giving you the same information. A great exercise for for understanding what's behind the process, however.

[goboatingnow]

On the other hand, if the bearings are very consistent, even if they are moving only slowly, you know you are passing clear. You don't just know that you're passing clear, you know that you know you're passing clear, because this method accounts for both errors of measurements and deviations of course of course and speed. It is an extremely powerful technique, and I commend it to you. It was especially useful in the hand bearing compass days as it dealt with the inherent inaccuracy of the data you get from a HBC.

hmmm. I fully agree, a constant change in bearing is a sign things are looking good. But I never assumed that slowly opening bearings was a good sign. If I got what you described, then I usually did my best to determine aspect and sought to open rate of bearing change.

Like you I wrote down bearings, but I did this in the cockpit. Id sack a skipper that went below in a potential collision sitiuation ( and so would most YM examiners )

dave

[Dockhead]

Originally Posted by goboatingnow

hmmm. I fully agree, a constant change in bearing is a sign things are looking good. But I never assumed that slowly opening bearings was a good sign. If I got what you described, then I usually did my best to determine aspect and sought to open rate of bearing change.

Like you I wrote down bearings, but I did this in the cockpit. Id sack a skipper that went below in a potential collision sitiuation ( and so would most YM examiners )

dave

I don't think anyone would sack a skipper who had given the con to a qualified crewman in order to do intense radar or chart work below! This is miles away from potential collisions remember -- the whole point of doing this exercise right.

A slow but consistent change of bearings means you're passing clear, and it means -- importantly! -- that your data is exact enough and our course and speed is steady enough to know that for sure. This is really powerful.

If the change is inconsistent, it means EITHER you have a potential collision or you don't have exact enough data to know for sure that you're passing clear.

Of course depending on the aspect of the ship and its distance and speed, a slow change of bearing can also mean a close CPA. But we don't have to worry about that nowadays because AIS tells us this separately. We should be careful in interpreting it (as discussed above), but we don't need precision to the cable.

[Dockhead]

Originally Posted by goboatingnow

. . . If I got what you described, then I usually did my best to determine aspect and sought to open rate of bearing change.

By the way, why would you need to determine aspect?

If you are writing down the bearings you know instantly which way to turn. If the bearings are increasing, you turn to port in order to make them increase faster and thus open up the CPA, and vice versa. This works for any possible aspect.

[goboatingnow]

Originally Posted by Dockhead

By the way, why would you need to determine aspect?

If you are writing down the bearings you know instantly which way to turn. If the bearings are increasing, you turn to port in order to make them increase faster and thus open up the CPA, and vice versa. This works for any possible aspect.

because (a) its a safeguard, (b) I like to work out his course even roughly and (c) I think very carefully before I turn to port !!!!!

[Dockhead]

Originally Posted by goboatingnow

because (a) its a safeguard, (b) I like to work out his course even roughly and (c) I think very carefully before I turn to port !!!!!

Amen to (c)!!!!! For anyone else reading this -- all of this discussion relates to the second, "analysis" phase of collision avoidance, between the first phase "detection" and the third phase "avoidance". Once you are in the avoidance phase, everything is different, and in many cases you must not turn to port.

But you can see the rough course and everything on your plotter with AIS. I guess you're talking about how you did it pre-AIS.

[goboatingnow]

But you can see the rough course and everything on your plotter with AIS. I guess you're talking about how you did it pre-AIS.

yes , today with marpa , ais, better stabilisation , its much easier to determine course of the ship

[Pelagic]

Originally Posted by goboatingnow

yes , today with marpa , ais, better stabilisation , its much easier to determine course of the ship

I think stabilization is the key in getting the best out of our electronic equipment and assuring situational awareness when a number of targets are making avoidance actions simultaneously in the 4 to 2nm range.

Assuming not all are transmitting AIS, you are back to relying on radar as your primary avoidance tool and AIS to confirm wherever you can.

Mark mentioned his Radar had a very stable picture but is it truly "stabilized"?

Is it stabilized by compass heading only or also "ground stabilized" using speed log/ island references/ or GPS data as well?

Once I know how it is done on the latest yacht electronics, I can suggest a few bridge management tricks to keep the awareness level consistent and stable as the ranges decrease and targets make alterations.

[colemj]

Originally Posted by Pelagic

Mark mentioned his Radar had a very stable picture but is it truly "stabilized"?

I mentioned it was stable compared to how you had described your very unstable experience with your unit.

It is an ARPA system fed with a 5hz GPS and 20Hz rate compass. I wouldn't call it commercially "stabilized", and it certainly does not provide the "stability" of class A AIS reporting from other ships.

However, for non-AIS transmitting targets, it seems to do a good job of giving us enough information to navigate safely.

For fun, I often let it acquire AIS targets and compare the data presented. I am impressed in how closely they do compare as long as the target COG and SOG are steady. When ships are changing course or speeds, the ARPA takes time to "catch up" to those changes, of course.

One "gotcha" with ARPA, is that if it is tracking a weak target, that happens to pass closely by a strong untracked target, it sometimes transfers its acquisition assignment over to that strong target.

Mark

[Pelagic]

Thanks Mark....that was very helpful.

Your radar is "heading stabilized" which cancels out slewing errors but does not allow you to operated in "True presentation"

Downside is that you are limited to only "relative" info on targets since it is fed no ground stabilizing data which would give you TRUE info on targets heading/speed that takes into account current-wind. (That accounts for the discrepancies with AIS since that info is GPS fed)

Before GPS, ship radars could be "Ground stabilized" with an ARPA feature where you identified and aquired a fixed target like island or nav bouy and selected "Land lock " so that radar would stabilize for drift when it used your speed and heading inputs.

I am surprised that today's better yacht radars don't have land lock features and only display GPS data rather than use it to ground stabilize for true motion....maybe they do but I have only seen radar overlay offsets in plotters ??

True Motion North-Up Stabilized presentation is fantastic to use in the multilateral situation I described above when inside the 5 to 2nm zone.

You have a birds eye view of the situation where the land stays put....you and all the other moving targets move in true fashion.

I set target vectors to relative and extend track histories to show previous course headings.

In True Motion course changes are instantly identified with their heading lines and the birds eye view helps to see the intent of others who may not be following the Rules...allowing you to take evasive action if a domino effect occurs.

Highly recommend you find out what is needed to make your radar work in TM and practice using it in daylight when coasting.

Then your AIS overlays would be exactly on the radar Target's rather than slightly offset

Originally Posted by colemj

One "gotcha" with ARPA, is that if it is tracking a weak target, that happens to pass closely by a strong untracked target, it sometimes transfers its acquisition assignment over to that strong target.

Mark

In SE Asia... since many targets don't use AIS I find that in congested waters I need to concentrate on getting the best out of my radar.

The kind of problem Mark describes is usually because the Radar settings are left on Automatic and it does not compensate quickly enough for too strong a radar return and the ARPA jumps from Bridge to Bow in target calculations or jumps to another ship as Mark warned

Just when you need it most at close range...ARPA on Automatic settings fail you!

Practice this in daylight.
Set Gain/rain/sea/ tuning to manual.

Play with these manual adjustments in different sea/weather conditions to see how they interact.

The goal is to reduce the very strong close target to the same return value as the inherently weaker targets further out without loosing them in the clutter. Reduce the size of a big ship target to a steady small return

Practice at 3nm range switching down to 1.5nm without target loss in different weather conditions.

Then ARPA info stays consistent without loss or jump as you monitor passing