Potentially dopey question about AIS

[Grinna 2]

OK, so I know that AIS is an automatic identification system transmitted and received via VHF that broadcasts the name, position speed, bearing and a bunch of other details about vessels in real time (well pulses of real time anyway). I also know that thru the magic of the interweb I can look up on a website and see AIS information and see the positions of ships plotted and their tracks etc (e.g. marinetraffic.com).

 

So .... how does the information broadcast via VHF get to the interweb?? Are there land-based receivers that convert the AIS data to web suitable format and post it? If so, who runs and maintains them?

[Grinna 2]

So its true!!! The US Govt is reading my brain waves!!!

[rigger 47]

Most of the AIS data freely available on the web is from ground based receivers, go to www.siitech.com they did have an explanation about it all at one time, aslo www.marinetraffic.com has some info.

They had how you could feed data in as well.

 

Currently I have not found a free site with sat data.

 

FYI there are 2 major areas on the NZ coast that were / are not covered - east coast and parts of fiordland though that area is getting a base station or two that may contribute data

[idlerboat 116]

Hi,

the AIS overlay that you see on Marine trafic dot com....is from land bassed volunteer (VHF) recievers that are connected to the internet. These feeds go to the Argean University.(Greece ?) Where they are processed for the web site.

The overlay is also available as a KLM file for google earth.

The web site matches the AIS data (probably using the vessels MMSI number (maritime mobile service identity) and then adds in the pictures that may have been downloaded to the site of the particular vessel.

 

As an aside...

......a lot of vessel broad cast real time wind details via AIS so if you want to know whats happening in a particular area , see if there is a vessel in the area on marinetraffic....

......Because AIS can give you a vessel of interests MMSI number, you can then call them directly via DSC on VHF radio. This increases the chance of them anwsering as opposed to trying to contact them by vox on channel 16 only. Great if you want them to know that you are nearby.

[rigger 47]

Hi,

As an aside...

......a lot of vessel broad cast real time wind details via AIS so if you want to know whats happening in a particular area , see if there is a vessel in the area on marinetraffic....

 

Never came across this one apart from on the AIS Atons that I worked on, and have never received wind data from another ship's AIS, mind you have not been out of NZ for a few years now.

Would be good to see a screen shot or two

Also do you know what message ID it is transmitted under?

 

Have just done some checking:

I doubt the wind data on Marine traffic.com comes from the vessels - I have checked the details of a number of vessels that I know do not have electronic anemometers fitted and marine traffic has wind data for them - it is not coming from those vessels as they do not have the equipment.

Also vessels in Akl have wind info but none in Tga have it.

Further - the wind readings are identical for a number of vessels scattered about one harbour, and where one of those vessels is it is very sheltered and in no way would the wind be the same there as out mid harbour.

[idlerboat 116]

......OOOOps.

I am sure you are right Rigger. I just did a proper check of the marine traffic site. It still does not state where its wind data comes from but it does mention "port conditions". I wrongly just assumed that it was an extra sting on the class A transponders output. Should have gone and searched the AIS codes first.

 

cheers

[Island Time 1,246]

Pretty certain no AIS has wind data, not even class A. I can post a screen shot of the data from a class A ship AIS if it is interesting to anyone?

Never seen wind data being transmitted though

Matt

[Guest]

We have a wonderful saying in Bislama. Kilim ded finis.

[Megwyn 2]

Emma commanded adult men and women newspaper pictures Rebecca Taylor specialist start in indiana. Emma considers your she's going to struggle to dress the correct way she must therefore for she ages.

 

Translator error?

[otto 31]

Went for a sail to Bundaberg not long ago

Skipper had a new chart plotter with AIS and was all excited

Was great, once i could see the ship it eventually showed up on the screen.

Had one fishing boat we watch approach us from the horizon only to be seen on AIS when we were within a mile.

But then could have been our VHF aerial, just need to know more about it all.

[rigger 47]

Went for a sail to Bundaberg not long ago

Skipper had a new chart plotter with AIS and was all excited

Was great, once i could see the ship it eventually showed up on the screen.

Had one fishing boat we watch approach us from the horizon only to be seen on AIS when we were within a mile.

But then could have been our VHF aerial, just need to know more about it all.

 

Otto it could have been the settings on the unit - Garmin has a setting where it can be set to show targets at 1, 3, 6nm and show all.

Or it could be the antenna on your vessel or the transmitting antenna on the other vessel.

[Island Time 1,246]

We mostly see class B (Voluntary AIS Equipped) from about 4 miles min. Often a lot more. Like all VHF, depends on installation etc. We don't restrict range of target aquisition unless it is very busy!

 

A mile still would give you time to miss him, but it is too close!

[Guest]

We mostly see class B (Voluntary AIS Equipped) from about 4 miles min. Often a lot more. Like all VHF, depends on installation etc. We don't restrict range of target aquisition unless it is very busy!

 

A mile still would give you time to miss him, but it is too close!

Interesting comment IT. I was on the boat with Otto whose owner did seem to expect to see ships as they left the the port of London I think

 

We seemed to pick them up at around 5-6nm most of the time. One at 10nm and another smaller boat at 3 there abouts. In 95% of cases we saw them visually 1st. Both Otto and Owner did spend extensive time tuning the system....... yeah fecking right, they were a pair of damn compulsive button pushers so everything got fondled every 5 frecking minutes it seemed. The poor thing did well to survive those two I reckon

 

But on investigation the last time anyone had a close look at the VHF ariel was around 20 years ago so that may have had some input into the range. It was on top of the Mizzen mast but it did give us real good VHF radio range.

 

Was interesting to note most of the ships info that came up on the AIS was only a position and speed. Very few had a name and even less had all the details filled in. This is mostly in the Coral Sea and coastal Qld Aussie.

[rigger 47]

Was interesting to note most of the ships info that came up on the AIS was only a position and speed. Very few had a name and even less had all the details filled in. This is mostly in the Coral Sea and coastal Qld Aussie.

 

That is most likely a problem with the unit on the yacht not the other units on the other vessels.

[Island Time 1,246]

Yep, definitely an issue with the install.

Airiel to receive, ground plane to transmitt.

 

By the way, several boats I know have had very poor performance on AIS sorted by throwing away the VHF airel splitter and installing a dedicated airel. Our install has a dedicated airel.

 

To get mmsi and course is the first reception, other details are filled in with subsequent receptions - provided - that the first details of the vessel (course and speed) are seen again.

 

On our unit, it takes a couple of minutes to go from mmsi and course to full details. but it does do that most of the time.

 

We also see ships on the AIS (Class A) WAY before we see them in reality. Sometimes more than 50 mile range on class A. We have also had a ship call US from 55 miles, so he could obviously see us on his AIS and had nothing to do!!

 

Class B is different. Often signals are lost at 4-5 miles, depending again on the installation, but I don't think we have ever had a situation where the aquisition range is less than 3-4 miles. Its often hard to see (visually) a sailboat at 5 miles in the daytime, and 3 at night would be very difficult with the lights many display.

 

I really like the AIS!

[rigger 47]

By the way, several boats I know have had very poor performance on AIS sorted by throwing away the VHF airel splitter and installing a dedicated airel. Our install has a dedicated airel.

 

To get mmsi and course is the first reception, other details are filled in with subsequent receptions - provided - that the first details of the vessel (course and speed) are seen again.

 

On our unit, it takes a couple of minutes to go from mmsi and course to full details. but it does do that most of the time.

 

We also see ships on the AIS (Class A) WAY before we see them in reality. Sometimes more than 50 mile range on class A. We have also had a ship call US from 55 miles, so he could obviously see us on his AIS and had nothing to do!!

 

Class B is different. Often signals are lost at 4-5 miles, depending again on the installation, but I don't think we have ever had a situation where the aquisition range is less than 3-4 miles. Its often hard to see (visually) a sailboat at 5 miles in the daytime, and 3 at night would be very difficult with the lights many display.

 

I really like the AIS!

 

Class A units have two VHF antennas - transmit and receive.

 

Class B units may only transmit every 30 seconds depending upon speed and heading changes. Might also contribute to slower / later detection.

[Guest]

Agree IT, I think the instal needs fine tuning. It did get a flash US made splitter installed and all that looked fine, to my knot trained eye, so I think you are probably right, needs it's own aerial. It's knot like there isn't room for one. From some playing prior to leaving it did appear as if the boat was transmitting well and getting picked up a long way off so maybe our unit wasn't having a good receive fortnight??

 

Must say the unit he got was interesting, one of these. http://www.huayang-tech.com/en/ProductView.asp?id=128&sortid=80 US$725 odd delivered to the boat in NZ complete with K-Map, C-Map capable, AIS built in, will interface with radar, wind gear and that stuff. Looks well made, did seem a bit slow on the screen refresh at times and I think could do with a slightly faster processor but it did get heavily fondled a lot but the button pushing twins so it may have just been pissed off Or possibly a K-Map issue as I suspect that's only C-Map put in another dress. All in all knot to shabby and for the price I thought it was well featured.

 

I want AIS for my upcoming races rather than drag a big radar reflector around and are now seriously considering getting one of those just for that and I'll take the chart plotter side as a bonus. Sussing around here and AIS only costs up there with that Onwa. I wasn't considering a plotter but I may end up with one after all.

 

You seen or heard any chat about those units IT?

[Guest]

Class A also has a 25 watt transmitter and Class b only 2 watts (we have ours hooked up to a 1 kilowatt linear amp and can be seen from mars!!... only joking Spectrum Management!!). Antenna gain (a 1/2 wave as opposed to a 5/8) and antenna HEIGHT play an important part in receiving and sending data distance. Interesting enough, you are better off with a 1/2 wave antenna on a yacht than a 5/8th because of heel. Creates a "null" in the radiated pattern. Its also possible to use a simple VHF receiver to get AIS on a PC but you have to avoid the audio filtering and tap directly off the discriminator. Auckland has ONE receiver working (AIS Station No.252 http://www.marinetraffic.com/ais/statio ... ion_id=252) somewhere around Mt Albert which probably limits the amount of contacts it picks up (why not on the Sky Tower!!??) and there's talk of some countries making AIS ( compulsory for all visiting craft!!

[rigger 47]

I want AIS for my upcoming races rather than drag a big radar reflector around

 

Lots of smaller vessels (like small fishing vessels) do not have AIS so you may still want that.

 

Also there are a good number of ships out there that are not transmitting their correct position, not out by huge amounts- mostly less than 0.2nm in error, and often a lot less than that. So if you are relying minimum CPAs then it may be a concern....

 

I'd keep the radar reflector.

[Guest]

(we have ours hooked up to a 1 kilowatt linear amp and can be seen from mars!!... only joking Spectrum Management!!)

Don't go frying the UFO's

 

Yes Rigger that is a work in progress and a reflector sure hasn't been totaly canned, far from it. I was working making sure I'm seen as me hitting ship may, if it do it hard enuff, just scratch the paint but if it's the other way around I think it'll quite nicely bugger my day. Ahhhggggg..... the drag of a reflector, yucky.