AIS does the tracking work by ships relaying AIS signals

[Chiliblue]

Hi,

Just curiosity really but, I just completed a trip from Hamble to BilBao on a yacht with a class b AIS transponder.

It proved to be a great asset as we hit a band of thick fog as we passed from the channel and into the Biscay. However for those left at home tracking us via MarineTraffic it didn't work out so well as we disappeared about 36 hours in as we left the channel and didn't appear again until we were about 12 hours from BilBao. There was very little traffic about in the Biscay with time when we had no targets at all but normally we just had the few that were in vhf line of site. Earlier on in the channel we could see AIS targets all over, way out of our VHF range.

So can a AIS expert explain how the tracking of AIS works?

Is there a relay of information from the larger vessels?

I understand there are land based transponders dotted around, so did we disappear because we were out of their range.

We also used a Spot Connect to send check in messages as a back up so it wasn't really much of a problem but it would be good to have a better understanding of how AIS works so I can set expectations to those following us ashore as the boat moves round to the med.

Thanks in advance

 

[Bru]

I understand there are land based transponders dotted around, so did we disappear because we were out of their range.

Yes

That's about it really!


No, there is no relay from other vessels (at least not at present and I'd think it unlikely there ever will be)

There are satellite receivers coming on stream offering offshore coverage beyond the range of the land based receivers but these tend to be a premium service (at least they were the last time I looked on MarineTraffic)

 

[Chiliblue]

Thanks,

I looked at MarineTraffic before we left and was always amazed to see how much of the Biscay it displayed traffic for!

Lesson learnt, AIS is a great extra tool but not really reliable for tracking.

Have to say it was really helpful at night crossing the shipping lanes, I could identify which way the flood lit fishing boats were actually going.

 

[prv]

Bear in mind that a land station on high ground with a big antenna tweaked by an obsessive radio geek will pick up signals from much further away than you're used to seeing from your own AIS receiver on board.

Pete

 

[LadyInBed]

Ships AIS transmits at 12w, yachts at 2w, so a ship will get picked up across Biscay but a yacht won't.

 

[Martin_J]

As Erbas mentions.. Certain satellites now can store and repeat the VHF AIS information back to ground which is how vessels can be tracked mid-ocean.

It's not the reception via Satellite that is the premium service but you will need to pay the premium in order for family at home to be able so see what the satellites are seeing.

Probably not as much use paying for this is you are sailing in coastal waters if you have Class B but might be worthwhile if you wanted a backup (albeit) costly backup to the SPOT tracker.

An interesting comparision of the differences in position information from Satellite AIS vs. Terrestrial AIS here...
http://www.scotland.gov.uk/Resource/0045/00452579.pdf

 

[Bilgediver]

As Erbas mentions.. Certain satellites now can store and repeat the VHF AIS information back to ground which is how vessels can be tracked mid-ocean.

It's not the reception via Satellite that is the premium service but you will need to pay the premium in order for family at home to be able so see what the satellites are seeing.

Probably not as much use paying for this is you are sailing in coastal waters if you have Class B but might be worthwhile if you wanted a backup (albeit) costly backup to the SPOT tracker.

An interesting comparision of the differences in position information from Satellite AIS vs. Terrestrial AIS here...
http://www.scotland.gov.uk/Resource/0045/00452579.pdf

Thanks. Very interesting.

 

[Bilgediver]

Bear in mind that a land station on high ground with a big antenna tweaked by an obsessive radio geek will pick up signals from much further away than you're used to seeing from your own AIS receiver on board.

Pete

Bear in mind also that quite a proportion of A is shoreside reporting stations operate on the same principal as those doing service for Flightradar24. They are volunteer stations who obtain receivers in exchange for providing data by broadband to a service provider. There may be areas even in coastal waters where no stations exist or they have a blinckered reception due to local topography. The Bay of Biscay is not an ideal place as vessels not hugging the shore could easily be out of range at some part of the passage.? I suspect also that while there are many class A in a certain area that Class B come lower in the pecking order when it comes to time slot what's it!

The most reliable way to keep family updated is to send a position e mail to one. Of the service providers who offer the service and relatives can monitor you across the oceans.You do need either Iridium or SSB.

 

[Koeketiene]

Lesson learnt, AIS is a great extra tool but not really reliable for tracking.

Don't agree.
Through work, I have access to a website run by the US DoT (reserved for the military and law enforcement), which relays/displays AIS signals picked up by satellites.
When friends did the ARC last year, I was able to relay course/speed/position to relatives all the way across the Atlantic.
Similarly, my wife got position updates every 5 minutes the first week of May when I crossed singlehanded from Ostend to Harwich in a F7-8.
When, at home, I later checked on Marine Traffic there were gaps in my track in the middle of the Channel, whilst the US DoT had tracked me all the way across.

A service which includes satellite tracking is available from Marine Traffic - at a price - but there is a time delay of several hours.

 

[Woodlouse]

There are more and more satellites being launched all the time to relay the AIS of vessels offshore. It's not really designed with families tracking yachts in mind though, more to do with regulating shipping and combating piracy.

There was this quite interesting article on the BBC not long ago. http://www.bbc.co.uk/news/science-environment-28372461