A bit about AIS

[tome]

I've had the pleasure (!) in the past week or so of studying in detail the AIS technical specifications as part of a work project. I can now see why the passive AIS for yachts costs £800 and the full system costs £2,000 plus.

Looking at the full system, this requires two independent receivers and a switchable dual-channel transmitter. The transmitter is required to have a very fast switch on/off time (<1 ms) - switch on/off times of around 10-20 ms are more the norm.

The system has to respond to shore-based VTS channel management control when within area, yet has to self-organise transmission slots with other ships when operating out of range of shore stations (high seas).

The standard is very well thought out, but complex to implement. It aims to optimise bandwidth by dynamically altering the reporting rate. For example, a ship at anchor reports at 3 min intervals whilst a ship moving at >23 knots and changing course reports at 2 sec intervals.

Even a passive system has to simultaneously monitor 2 channels with the ability to change frequencies under base station control, and has to respond to a complex set of protocols.

Hats off to the ITU who wrote the specification, and to companies like Comar and Euronav who have implemented passive receivers. Clever stuff, looking forward to a price reduction and full integration with radar. I think I might just put up with my existing radar for another year or so to await developments.

Integration with radar and DSC would provide improved ARPA vectors and the ability to select a target and call it automatically on VHF using DSC. The target MMSI number is part of the AIS transmission. Imagine this in the context of a channel crossing in fog.

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[Chris_Rayner]

Coo

So when do you think it'll be integrated with your hand-held GPS/DSC radio/chartplotter then? Theoretically possible I think.

Seriously, I can see Raymarine, Simrad etc integrating it into their systems in the not too distant future. With a bit of programming it should be able to get you across the TSSs while you take a kip. Just as long as everybody has their set switched on and working.

Chris Rayner

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[cameronke]

I am getting a wee bit overwhelmed with all the new gadgets becoming available with their associated acronyms

Can someone explain briefly what AIS is please

Regards

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[bumblefish]

Re: Coo

You can have it hooked up to your PC based chart plotter now. Not sure which device though!

<hr width=100% size=1>...If you can find the simple solution to a complex problem, you've got it wrong!.........

 

[tome]

Automatic Identification System, a bit like secondary radar. There's an article in the current YM which explains it reasonably well.

It's a method by which ships communicate their identities, positions and courses/speeds with each other or a shore station.

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[DeeGee]

<font color=red>A</font color=red>nother <font color=red>I</font color=red>ndespensible <font color=red>S</font color=red>hip-thingy.

Something else for poor yotties to spend their dosh on....

<hr width=100% size=1>Black Sugar - the sweetest of all

 

[jonjo]

> I think I might just put up with my existing radar for another year or so
> to await developments.

I foresee receive-only AIS sets hurting radar sales within a couple of years.

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[Robin]

Why? You can only see someone who a) has an AIS fitted (ie not most small boats and not buoys, land, rocks) and b) has the AIS switched on AND connected to GPS. That leaves a lot of stuff to hit or be hit by IMO!

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[BlueSkyNick]

That's very interesting and helpful Tom, particularly as I am also still shopping around for radar, and take note of your own thinking.

However, as is often the case, there are now more questions than answers!

Maybe I'm missing the point somewhere but here goes:

1. Why does it need to be dual channel, if the system is dynamically allocating bandwidth, self-organising transmission slots etc? It could do this with just one transmitter, couldn't it?

2. Why does the transmitter need a very fast switch on/off time? I suspect this isn't relevant to the average yachtie except that it adds to the price which is your original point!

3. What is the value in a passive, ie receive-only, system? If the system picks up a large vessel which is of concern, I would want them to be concerned about me too, not just a traditional radar target. Is this where the £2k package comes in?

Grateful in advance for your help as usual - keep it brief though!



<hr width=100% size=1>I'm average size, Its just that everybody else is short.

 

[tome]

Nick

1) It's dual channel for a number of reasons, one being that different frequencies are used by adjacent areas. When moving from one area to another only one frequency needs to change (overlap). Other reasons are a faulty transmitter jamming one freq or loss of one receiver channel, ie redundancy.

2) Each slot is only 26.67 milliseconds long. Normal switch on/off times would take up nearly 20 ms leaving little left for data hence the need for fast switching. Fast switching=cost.

3) Aircraft radar have what's known as SSR (civilian) or IFF (military) transponders. These provide additional data to the radar displays, eg flight level etc. The same technique could easily be applied using AIS. You would still see your radar picture, but with the advantage that you could ID all AIS equipped targets. You'd also see their actual COG and SOG instead of relying on radar calcs.

It's my conjecture but perfectly feasible to link the DSC VHF to a selected target and make a call direct to the bridge.

Cheers
Tom



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[BlueSkyNick]

"It's my conjecture but perfectly feasible to link the DSC VHF to a selected target and make a call direct to the bridge."

Cor - you will have re-invented the telephone soon !

Thanks for your other answers, very helpful as usual.

<hr width=100% size=1>I'm average size, Its just that everybody else is short.

 

[jonjo]

> You can only see someone who a) has an AIS fitted

By the end of the year this will be all commercial vessel over 300 tons. No doubt we will follow the US lead where AIS has to be fitted to vessels over 65ft as well.

> ie … not buoys, land, rocks

Hasn’t GPS made this traditional pilotage role of radar redundant? Ok so GPS or AIS won't spot a line squall but this is not a big issue in our waters.

My comment was directed at the majority of yachts that do not currently have radar. In two years time yachtsmen will have a choice between fitting an unobtrusive £400 box that will automatically warn of collision risk with high accuracy or fit a large £1500 lump up the mast that paints a pretty picture but in practice does little to reduce risk.

AIS integrated Radar will be a big step forward but like today it will be the province of the larger big budget yacht.

AIS is radar for masses.

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[Twister_Ken]

Presumably though a receiver doesn't need expensive fast switching because it's only ever on RX mode. Does it need twin RX channels?

Are the £2K TX boxes RX as well, or TX only?

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[tome]

No the receivers don't need to switch at high speed, but does need twin channels to handle graceful area transitions and for the other reasons mentioned. I should add that if one RX channel is changed, the system will delay at least 9 mins before changing the other one.

The £2K boxes are both TX/RX and my guess is that this price will plummet as R&E investments are reclaimed from the big boys and they start looking for additional markets.

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[tome]

Interesting theory, though I'm inclined to believe that it will be the combination of low cost radar and AIS which will win the day. I'd still want radar for showing that my tired brain hadn't entered the wrong GPS datum as I approached a landfall after a long passage, and for picking up non AIS targets.

It will still be some time before Mr Fish figures out how to fit an AIS transponder to an approaching line squall.

Ships can include dimensions and the position of their GPS antenna so a low cost radar could even use this data to sharpen up it's image. Lots of possibilities.

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[jonjo]

The time slots are small because the system is designed to handle 2000 AIS transmitting stations typically broadcasting an update every 10 seconds.

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[ParaHandy]

tome

d'you know how much data is transmitted in 26msecs and the data transmission rate?

douglas

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[Robin]

I think one of us is missing the point here. Used alone without radar AIS is no more use than one of the devices that detects that radar is being used and gives a direction for it. Very nice aid for missing the big stuff but it doesn't help them miss me if my AIS is passive only and I would still be vulnerable to being hit by or to hitting anything smaller than say 65ft if that becomes the figure. That still leaves an awful lot of potential for collisions with some fairly solid objects!

Do you actually have radar? If so I doubt you would be so keen to swap being able to 'see' for such blind faith in a black box (pun definitely intended).

Pilotage is very much better with GPS as you say (and we have dGPS and a plotter) but it has not made radar redundant by any means, used together though they are a powerful tool.










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[tome]

Over air rate is 9600 bps and there are 168 bits available for data in each slot. The total number of bits is 256 but some of these are used in ramp-up, synchronisation, start and end flags and CRC (checksum) plus a buffer to allow for 100 Mile propagation delays and jitter.

This is getting a bit technical innit?

BTW, I couldn't help wonder why YM didn't make the connection with radar in the article. They seem to regard the high speed NMEA sentences as some sort of new technology, whereas it's only a faster (38,400 versus 4,800) version of the existing 183 standard.

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[Rich_F]

I'm afraid that I'm a little less impressed with the AIS specs. It strikes me that AIS could be much simpler, and therefore available to more leisure users.

For example, how about if all the information went onto channel 70, along with the DSC traffic. Obviously, the data would need to be thinned, perhaps by broadcasting shorter messages, plus not sending an update until one of the following happened.

- 5 minute timeout expires
- vessel is >50yds from the position predicted by its last broadcast

This could be incorporated into VHFs without needing any new broadcasting circuitry. And would therefore be more useful, as small boats would be able to broadcast their positions.

Or is that just too simple?

Rich


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