AIS RADAR - again!

francis39

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I've started again because the previous thread on this topic was getting silly IMHO. For example:
[ QUOTE ]
... terrible promotion of what could be nice kit, with dreadful terminology. I hope the product dies, and they are reading this thread.


[/ QUOTE ]They probably are, but are too grown up to respond. How childish can you get? And what's in a name anyway? You could argue that NASA's done a brilliant marketing job by getting all these posts from tiresome pedants (like me? It takes one to know one!) who "know it all", and publicising their product free of charge /forums/images/graemlins/wink.gif

And someone else thinks boats can turn their AIS off when they near a harbour /forums/images/graemlins/crazy.gif! I read an article which pointed out that the <u>compulsory</u> introduction of AIS was brought forward from 2008 because of worries about terrorist threats. Any vessel obliged to carry an AIS transmitter (NOT a transponder, BTW!) and not doing so would receive heavy fines from the authorities. They have to transmit their status even when aground, moored, anchored, not under command, etc.

But I can't understand what the fuss is all about: the NASA kit looks the business, whether it is used in conjunction with a twirler or not. The 16-sample tracks (which can be up to 15-minutes'-worth of targets' relative positions) show where they are moving relative to you. If the track is pointing directly towards you, then you're going to collide if neither vessel changes the situation by altering course, slowing down, or speeding up. Twirly radars in the £1000 price range don't offer that facility, and they don't show the MMSI's either. As I said previously, it's horses for courses.
 

Swagman

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Ahhhhhhhhhhhhhhh.........

We must indeed be sad cases to pick this up again, but Francis has my vote.

Suggest we need not worry.

Most who read this are astute enough to pick up good advice, and the 'quote' Francis listed probably did more to advance the cause of AIS than dissuade others from buying it.

I say long may others innovate kit to help yachtsmen travel safer. As this NASA AIS kit will do that at what seems to be a sensible price - it also has my vote (even if I have yet to buy one).

Cheers
JOHN
 

crossbones

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Well, I'm currently tracking 29 ships on AIS, which would kind of overload the Nasa reciever, wouldn't it?
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No it wouldn't. The nasa radar will continuously track up to 30 vessels. I dont think there is any upper limit to their ais engine.
 

francis39

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[ QUOTE ]
The nasa radar will continuously track up to 30 vessels.

[/ QUOTE ] Yes, and they are the 30 <u>nearest</u> vessels, not just a random selection of 30 detectable vessels. If there are only 29, it must be because there are only 29 within detection range, surely?
 

fireball

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Before you lot get nominated for the oscars .... can I just remind you that AIS relies on the acurate electronic positioning of both the transmit and receive vessels - via GPS.
I don't think there is anyone that disagrees that AIS is a good development, however there is strong opinion about the use of the name and completeness of information in reduced visibility.
Yes - it can track 30 vessels ... fine, but they are mostly 300+tons and transmitting AIS - or ferries..... I believe AIS now needs to be further developed to include smaller vessels and then the engine/display developed to be able to handle this extra information.
 

Robin

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Frank

I may be unduly suspicious here and perhaps you can enlighten us.

You have no information in your bio as to what and where you sail or motor. You have made a total of 10 posts it seems and ALL of these are on AIS including a comment that you had seen prototype NASA sets in operation.

I would be very interested to know what connection of any kind you have (or had since your bio says 'retired') with NASA or NASA personnel?

I would like to think people would have open minds on this subject. Personally my views are that AIS on small boats is a useful ADDITION to radar but not a substitute and that there are some caveats in it's use that are not being understood by (some) potential buyers. The comments from Homa on the other post are very relevant and from someone who sails but also uses big ship radar and AIS in anger as part of his job, ie fact based opinion not wishful thinking.

See:-http://www.ybw.com/forums/showthreaded.php/Cat/0/Number/719110/page/0/vc/1

Robin
 

Graham_Wright

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So a couple of points cleared up for me (i.e. not transponder and 300 tons).

One advantage to me not mentioned (I think) was the identity of a vessel, maybe in visual range, but unidentifiable. Having lost steering crossing the channel and seen a vessel bearing down on me, the only calling option was an "All ships". This was followed by a terrifying silence. With an identity on the screen, a direct ship to ship call could be initiated.

Surely no one is suggesting navigation by AIS alone? We are hopefully all sufficiently intelligent to use ALL means available to us to navigate safely and to interpret and cross refer to best assessment?

Please continue my education by explaining if the Nasa AIS system has a built in "engine" which can forward data to slave devices of if an "engine" is a different beast entirely or that can be bought as a stand-alone item.
 

boatmike

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Francis,
I read your post with interest. As is usual there are no shortage of opinions from people with no real experience and I think this is the key here. In truth no one can claim to know how useful this kit is until they have tried it and it is fairly new so no one really knows. My own opinion as one who has used a conventional radar for a while is that it really is a bit like ARPA which is superb when connected to a very accurate compass but can be misleading in its cheaper forms connected to a rather dubious one. The idea of AIS is less complex that ARPA and should be more accurate. Or should I say "as accurate as a good ARPA without spending a lot of money" As someone said it relies on GPS and can only resolve within the limits of the signals received, but as these are pretty accurate these days I see no problem there.
The kit is certainly cheap enough and I am seriously thinking about fitting one as an adjunct to my radar. Would I fit one "instead" of radar? Probably not. But hey any information recieved is good provided that it is treated as an aid and not viewed as gospel so if you can't afford a good radar set, one of these will help I guess so why not?.
One word of caution though. Don't assume every ship has their transponder turned on all the time regardless of the regs. The regs also call for lots of other things like coasters etc. always having a watch on the bridge using mark one eyeball but many don't I am afraid.....
 

Oldhand

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<If the track is pointing directly towards you, then you're going to collide if neither vessel changes the situation by altering course, slowing down, or speeding up.>

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Having just read the NASA AIS "Radar" manual on-line, I conclude you have made a very dangerous statement, as copied above.

The AIS "Radar" produces its display by plotting the ships' positions as input by GPS to their AIS transponders. Thus any ships' "wakes" displayed are the ships' progress over the GROUND and not their tracks through the mass of water of the area. However, all vessels are progressiing in a mass of water, which itself may be moving relative to the ground. Assuming one's own vessel is positioned at the centre of the display, ships' tracks subtended towards you passing through the centre of the screen are only an accurate guide to a collision situation if the tracks represent the ships' velocity through the water mass and not their velocity over the ground.

One thus has to conclude that the ships' tracks displayed by the NASA AIS "Radar" are misleading in terms of collision situations and can only be used as a guide that a "close encounter" situation may exist. Whereas a proper radar, used properly in relative motion mode, shows a much more certain picture of collision situation potential as both your own position at the centre of the display and target echo's, wakes (and ARPA tracks if available) are dispalying the relative situation in the mass of water around you.

I would thus suggest that to use an AIS plot developed in the manner of the NASA AIS "Radar" alone could lead to a dangerous misinterpretation of a situation. It should only be used in addition to proper radar to provide additional information and ship recognition.

In addtion, I see the NASA AIS "Radar" manual states that ship's tracks or wakes are lost every time one changes display range scale, thus one could instantly loose the "picture". I am also concerned that to get any decent length of wake the slowest display update rate needs to be employed, thus diminishing accuracy in terms of the dalay to show a change of course made either by yourself or a vessel being "tracked". I can't understand why the display update isn't continuous and employing all data decoded from the receiver but I guess the designer decide to use minimal processing power for reasons of cost and power consumption.
 

fireball

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repeat 100 times:
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.
IT IS NOT A TRANSPONDER. IT IS A TRANSMITTER.

Sorry to all those that know this, and for those that accidentally put the wrong term in. But I feel there is enough confusion with this technology as it is without adding this to it.
 
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[ QUOTE ]
I would thus suggest that to use an AIS plot developed in the manner of the NASA AIS "Radar" alone could lead to a dangerous misinterpretation

[/ QUOTE ]
You got me really worried for 5 minutes so I had to resort to graph paper to understand the implication of your post.

I think you are wrong providing everything is computed relative to COG the collision vectors are accurate. Phew!

Try a simple plot on graph paper showing two vessels doing 2 knots colliding at right angles. Then add a 3knot tidal current up the rear end of one vessel and hence on the beam of the second.

Assuming AIS ship's course information is transmitted based on GPS COG all the maths hangs together and the NASA box should be able to plot a reliable collision vector.
 

Oldhand

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Only if all vessels are experiencing an identical current rate & direction, which is highly unlikely with any separation in coastal waters, English Channel included. Your conclusion is unsafe.
 

ParaHandy

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i think this is getting out of hand ... firstly, i don't think oldhand needs lecturing on the efficacy of pen and paper, secondly, the NASA AIS will not calculate collision vectors but merely displays your track as determined by your COG ...
 
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[ QUOTE ]
i think this is getting out of hand

[/ QUOTE ]
What's getting out of hand? Someone posts a complex message that would cause many leisure yachtsmen to conclude there is a fundamental mathematical deficiency that prevents a NASA AIS set from reliably warning of a collision.

I post a counter view and state I had to resort to a paper plot to understand the situation.
 

bedouin

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I am very interested in AIS - while being fully aware of it's limitations I would say that it will probably be more accurate in predicting a close-quarters situation that I would be likely to achieve using the sort of active radar I might install on my boat. I think it would also be useful when crossing shipping lanes or TSS even in goo visibility

I'm interested in how much information the set calculates for you - does it give you a calculated distance of closest approach based on the SOG/COG of both boats?
 

bedouin

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[ QUOTE ]
Only if all vessels are experiencing an identical current rate & direction, which is highly unlikely with any separation in coastal waters, English Channel included. Your conclusion is unsafe.

[/ QUOTE ]
Would you like to quantify that statement? If we assume that we are sufficiently far off shore and away from obstacles that we don't expect any alteration in course/speed for any navigational reason, what sort of differential in current would you anticipate seeing over say a 4 mile separation?
 

srm

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At the risk of being an old pedant we are discussing two very different pieces of equipment here.

AIS uses info transmitted from other ships (and also spoof info created with a PC and a VHF transmitter according to one industry source).

Radar transmitts signals in order to receive reflections and detect targets or trigger transponders.

NASA has done us all a great diservice by confusing the two with their incorrect use of the term RADAR. Current ITU Radio Regulations define radar as: "A radiodetermination system based on the comparison of reference signals with radio signals reflected, or retransmitted, from the position to be determined".

That said AIS is very useful when near big ships and radar is useful amongst all sorts of obstructions including big ships and those that are not obliging enough to transmit AIS information such as rocks, fishing boats and buoys.

Both sustems require a level of operator skill in interpreting the data. As part of my job includes responsibility for running Merchant Navy Navigation Radar and Simulator Training I would suggest that the RYA 1 day Radar course is an absolute minimum for anyone who operates a leisure radar set, and this should be backed up with operating the set in clear weather to gain confidence in interpreting the display.

I can see the AIS receiver as being useful on its own in a small boat, especially in areas of heavy commercial traffic. It does not and can not substitute for a real radar set though.

Sean.
 

wiggy

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surely if both you and the AIS transmitting ship are in the same piece of water you will both move together, therefore if you are going to colide on AIS you are going to colide unless action is taken.
 
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