AIS tracks

[Bathdave]

To avoid thread drift I have started a new one spinning off from the one about the reward for a missing yacht.

There is a comprehensive thread in the cruisers forum which is linked from that thread, to which the boat owner himself is a regular contributor. It explains in a much more comprehensive way than the YBW thread how the mishap unfolded. I have also read the family blog (again linked from the CA forum thread) which is compelling and brutally honest. No way is my wife going to countanance a TranAt after reading that...it is recommended reading for all

My interest was piqued by the fact the guy left his engine running in neutral to maximise the time the batteries were charged, and left his AIS running when he abandoned ship, to enable him to track it and recover it in due course.

Despite this only intermittent pings, several days apart, were detected by satellites monitoring AIS. One of the CA forum members has a job that give him access to commercial/professional sites picking up AIS (ie not relying on 'retail ' sites like Marine Traffic.

One of the narratives spinning out of the CA forum thread is that Cat A AIS is captured by sats, Cat B is much more variable.

I had assumed AIS tracks were captured,...Marine Traffic has load of boats locally around here showing, pleasure boats with Cat B (presumably) as well as commercial vessels. My wife tracked me crossing the channel on someone else's boat on Marine Traffic last year.

I am planning to put AIS on my boat for the new season, partly to dip between vessels in the TSS and partly to allow family to see where we are as we go further afield.

Thinking about it I can see that AIS is actually a circle at sea level for other vessels to 'see', presumably broadcast over VHF ?

How does it get picked up by satellite and reported on, e.g. marine Traffic?

Does the location of this missing boat (500 miles NE of Antigue initially, who knows where now) have a bearing on the lack of pings, e.g lower satellite footprint compared with Channel waters, where many boost are continually visible?

 

[RAI]

Thinking about it I can see that AIS is actually a circle at sea level for other vessels to 'see', presumably broadcast over VHF ?

How does it get picked up by satellite and reported on, e.g. marine Traffic?

Does the location of this missing boat (500 miles NE of Antigue initially, who knows where now) have a bearing on the lack of pings, e.g lower satellite footprint compared with Channel waters, where many boost are continually visible?

Class A transmits with more power than Class B, so the reception range is greater.
Reception range is nominally limited by line of sight and signal to noise ratio.(Although refraction also occurs). Satellite have great line of sight to large areas.
For the satellites, which transpond the VHF AIS signals they receive, the big issue is the number of signals received from a given area (mutual interference). This problem is worse near coasts and harbours than it is mid ocean. Consequently, the satellites do a good job of filling in the oceanic areas.
The satellites do not yet provide continuous global coverage, but will in time as more are launched.
To get access to the satellite AIS data, a subscription service is currently required (satellites cost a lot more than coastal ground stations).

 

[Habebty]

Marine Traffic has yet to pick up my AIS signal, yet Vessel Finder shows me within about 6 minutes of switching the AIS on. There are lots of limitations to web apps that rely on shore station receivers presumably due to VHF reception.

I used to rate Marine Traffic, but all the sites(Vessel Finder included) seem to show a different selection of Class B targets. Class A seems consistent across the board though.

 

[newtothis]

We have access to military-strength satellite AIS at another part of the business where I work [US Navy is a client]. I could ask one of the techy guys to take a look if anyone can tell me what to ask for.

 

[jonjo]

Satellite AIS tracking is just a technical curiosity and intermittent fringe benefit, it should have no bearing on the decision of a small boat sailor whether to install an AIS class B. device. There are much better alternatives for long term automated remote voyage tracking.

 

[RichardS]

Satellite AIS tracking is just a technical curiosity and intermittent fringe benefit, it should have no bearing on the decision of a small boat sailor whether to install an AIS class B. device. There are much better alternatives for long term automated remote voyage tracking.

+1

The missing yacht thread is irrelevant to any decision about whether to install a class B transceiver for collision avoidance. Class B signals are picked up by suficient land receivers (and in parts of the Adriatic are re-broadcast a hundred miles down the Italian coast) that for most coastal sailing you can be tracked by family but this is an incidental benefit. If you want blue water tracking you need a dedicated satellite tracker.

Class B transmissions could be very helpful for yacht recovery and would have helped the missing yacht recovery but you do need a continuous battery power source to keep the transmitter operating using solar panels or wind gen. However, the targeted "recovery" mechanism is much more likely to be from another boat or a plane with an AIS receiver rather than by satellite location.

Richard

 

[RAI]

I can vouch personally for the effectiveness of satellite AIS. On a passage from the Azores to Lagos, we got stuck on a little doldrum that followed us along. We took 11 days to complete a 7 day trip. Unfortunately, SWMBO had told an American friend of our planned arrival date and when landfall did not occur as planned, the American lass got onto the US coast guard. All she had was the boat's name. The coast guard found us with satellite AIS, found the nearest commercial ship with a satcom, called them, a tanker, which called us on channel 16. He could see us on AIS directly. Our answers were relayed back. Our American friend received assurances of our well being, position and new ETA in Lagos.
So I'm inclined to believe that satellite AIS does work with Class B transceivers out of range of land and is used by the safety authorities.
Whether a subscription to a satellite AIS website is more cost effective than a subscription to SPOT or InReach depends on whether it is for safety or more general applications. However, the former can run off ship's batteries and solar panels while the latter devices can be taken into a life-raft. Both is what I believe in, belt and braces. (Must not forget the EPIRB and PLB either).

 

[[70521]]

I understand there has been a lot of work done on AIS signals being received by satellite and being able to filter out the noise so that the packet is readable. However it would not be my first choice to track a boat over an ocean. Something like a Yellowbrick device has a battery life of several months.

 

[JumbleDuck]

How does it get picked up by satellite and reported on, e.g. marine Traffic?

I hadn't realised that MarineTraffic used satellite reports until you prompted me to look and, yes, the Atlantic Ocean is full of things. However for these things all you get is the vessel type, like "tanker". You have to pay another $14.98 per month to track a single vessel by satellite, $400 per month to track everything with all but 15 delayed by 12 hours or $800 for all but 50 delayed by 6 hours. Their satellite tracking pages notes: Note that Satellite AIS tracking may be less effective for vessels equipped with Class B AIS transponders due to their lower transmission power.

I had assumed AIS tracks were captured,...Marine Traffic has load of boats locally around here showing, pleasure boats with Cat B (presumably) as well as commercial vessels. My wife tracked me crossing the channel on someone else's boat on Marine Traffic last year.

The channel is relatively titchy, and a reasonable lofty aerial should be able to see at least half way across just about anywhere along it. I can't see any satellite reports on Marine Traffic until well to the west of Ushant/Land's End. When I first installed AIS I found that I showed up at once on neighbouring boats but it took about a day before Marine Traffic showed me. I now appear at once. I wonder if they introduce some sort of initial delay into their system to avoid short-term spoofing?

 

[rogerthebodger]

I understand there has been a lot of work done on AIS signals being received by satellite and being able to filter out the noise so that the packet is readable. However it would not be my first choice to track a boat over an ocean. Something like a Yellowbrick device has a battery life of several months.

With the right equipment you can even track across the south atlantic

http://cape2rio2017.com/tracking/

 

[[70521]]

With the right equipment you can even track across the south atlantic

http://cape2rio2017.com/tracking/

Is there anything unique about the South Atlantic?

 

[rogerthebodger]

Is there anything unique about the South Atlantic?

Alot less shipping and pleasure crossing than in the north Atlantic.

Also sailing west to east you have to go quite far south even in the summer or you have to beat all the way. Not very nice sailing.

 

[Greenheart]

I went to Hythe in Hampshire lately and saw a vessel being escorted up Southampton Water by tugs...terrible photo, apologies..

...but when I tried to identify what she was, she seemed invisible to AIS...only the tugs were shown...

...is it likely such a vessel wouldn't use AIS? Or maybe the tugs were accompanying her because her systems were out of action?

 

[[70521]]

Alot less shipping and pleasure crossing than in the north Atlantic.

Also sailing west to east you have to go quite far south even in the summer or you have to beat all the way. Not very nice sailing.

I see your point. Satellites just pick up signals and pass them the the ground station where they are processed.