O/T Who knows about AIS on ships?

[Griff]

If there are any maritime experts out there, I've got a couple of questions about AIS on ships. Specifically I'm trying to find out:-

 

- How accurate is the AIS position/speed/rate of turn (same/better/worse than you could get from an uncorrected GPS signal)?

- Does AIS use the same satellites as GPS?

- Whether the nominal AIS location of a ship is in any way corrected relative to the bow/stern or is it just "wherever they happen to bolt the antenna"?

- If the latter (as I've been told elsewhere) this builds in a pretty big potential error for a ship 400m or so long!

 

Cheers,

Mike (Griff)

 

It's not pink, it's medium red violet...!

[susser]

Yes,

It must be a great comfort to know if you are about to get chomped by the Blunt end or Pointy End.

To use the nautical term.

There's been some good articles on it in Practical Boat Owner but I'm sure there are some people who'll be along in a minute.

Or six bells O'clock as we say in the Andrew.

 

[Richard H]

From my limited understanding, (I don't really "do" the marine stuff any more, just manage the chaps that do)

 

1) Same as bog standard GPS, cos thats what it uses

 

2) Yes, as above.

 

3) AFAIK, just where the GPS antenna is bolted.

 

4) Good Point

 

I'll happily discuss any specifics with my GPS guru when I see him on Wednesday if you would like 🤔

 

Richard......................

 

 

 

 

Photos here

 

[Griff]

Richard - the last point is the most critical for what we're doing, though it may be a case of getting the data on a vessel by vessel basis before they come to port (there is advance good notice of arrivals). We can then add this info to the other data kept on the ships like manifold positions etc for future reference. But I do believe it's a valid point - have you just run over that yacht or are you just about to? 400m could make a lot of difference to the yacht

 

It's not pink, it's medium red violet...!

[Richard H]

Hi Mike,

 

The following data is contained in a standard AIS packet,

 

Ship's position.

Ship's name.

Call sign.

MMSI number for tracking.

Speed, course, and heading.

Navigational status.

Type of ship.

Position sensor indication.

Antenna location.

Rate of turn.

Rudder angle.

Vessel's maximum draught.

Air draught.

Ship's length and breadth.

Angle of heel.

Angle of roll.

List of ports of call.

List of hazardous cargo.

 

Considerably more than I expected the only system I have seen was receive only, the vessel we were consulting on isn't keen on being seen.

 

Trust that this is of some help 🤔

 

Richard..................

 

 

Photos here

 

[susser]

Slightly O/T but;

Wasn't there a thing a little while ago about people not needing to pay that old charge that was designed to support the Lighthouses ?

Light dues I think it was called. The Idea was "we don't need your lights anymore 'cos we got GPS". Well I guess it's gone full circle if the ships can now see each other and what they are and what they are doing, and now the coastguard are coordinating it all they need an income to do it with.

Maybe Light dues should be renamed "Automatic Identification System Coordination Dues."

I'm amazed.

 

[Griff]

So they do transmit antenna location..!! Though that may not be relevant if they give a fairly true position of, say, the bow and also length/breadth as we can work out what we need from these inputs.

 

I've seen AIS displayed on some of the higher precision DGPS & RTK pilot berthing aids. One sceenshot in Italy showed a ship "berthed" at 90 degrees to the narrow channel so it appeared to be sitting on top of the quays either side - apparently lots of ships have their AIS badly set-up.

 

Richard - I wonder if you're referring to certain tubs being built in Cumbria? Other parts of our company have supplied lots to keep this invisible too - who do you work for?

 

It's not pink, it's medium red violet...!

[bluenose]

Griff

 

AIS uses standard GPS although most ships now have some form of differential GPS. The position of the aerial is loaded into the AIS transponder on installation by having coordinates A,B,C and D where A is distance from the bow, B is distance from the stern , C is the distance from the port side and D is distance froim the starboard side. A + B = the length of the vessel and C + D is the width. Most large ships are fitted with a turning indicator which feeds into the AIS to give an accurate rate of turn.

 

More info on all this is available from the ITU-R 1371 -1 specifications.

 

Cheers

 

Nick

[Griff]

I never cease to be amazed by the collective pool of knowledge on this forum. Thanks all - off to do some more homework now!

 

And Nick, appreciate the spec number - found tons of useful links on Google.

 

It's not pink, it's medium red violet...!