How does AIS work?

[StuM]

Originally Posted by Cpt Pat

Marinetraffic.com is intended for the "boat spotting" hobbyist.

And for commercial fleet monitoring.
(Take a look their pricing and plans)

[StuM]

Originally Posted by ranger42c

How high is the internet?

About 36,000 km

[StuM]

Originally Posted by ranger42c

As Pat says, you're not receiving an AIS (VHF) signal on your iThing. Marinetraffic.com or something similar has a boatload of store-and-forward receive stations scattered around the globe...

And access to satellites that pick up and relay AIS signals.

[Cpt Pat]

Originally Posted by StuM

And access to satellites that pick up and relay AIS signals.

Notice it just says "tanker" without any other data. You have to pay extra to see any details of vessels received by satellite.

My point is: because of delays and coverage gaps, it isn't safe to rely upon any of the Internet AIS services for the purpose of collision avoidance. "Boat Beacon" gives a very misleading impression in their on-line description that the app is a: "reliable AIS-based collision detection system" but then goes on to say: "Boat Beacon relies on a network of shore based AIS stations for the live ship data." So, no shore station, no data. (https://pocketmariner.com/mobile-apps/boatbeacon/).

I am VERY concerned that some gullible skippers may be duped into believing they can relax their visual watch for traffic by relying on this app. There is no substitute for having an onboard AIS receiver.

If used as a casual advisory aid in situational awareness - fine. Use every tool available for that purpose. But please understand that - if no vessels appear on that app - it doesn't mean a cargo ship isn't about to run you down.

[ranger58sb]

Perhaps a relevant anecdote...

A few years ago, several boats were heading to a centrally-located marina for our annual owners club dock party. One of those boats was using an Internet app to view AIS signals from nearby boats. Happens coverage in this area is usually OK, so he could "see us" on his phone or whatever.

I think he said his app was also sending his own info, some kind of AIS-wannabee. Sending that info somewhere, I guess to other users of the same app but not sure. He was invisible to our onoard AIS equipment, even as I actually watched his boat go by in the binoc.

No big tankers or container ships involved in that particular example, but there were several out in the nearby big-ship staging anchorage that day, and I could see them on our equipment.

-Chris

[StuM]

Originally Posted by Cpt Pat

Notice it just says "tanker" without any other data. You have to pay extra to see any details of vessels received by satellite.



My point is: because of delays and coverage gaps, it isn't safe to rely upon any of the Internet AIS services for the purpose of collision avoidance.

I agree with you 100%. This vessel will now be almost 50 NM from the plotted position if she maintained her speed

[Cpt Pat]

Yes, in addition to errors between the "last known position" of vessels and their actual positions, there are also delays in the data collection process, and in data reliability.

My own AIS reporting station is typical - it stores and then reports positions in batches. The batched reports are sent every 2 minutes. This is typically done to reduce Internet bandwidth (which I have to pay for) by de-duplicating position reports. A class A AIS-equipped vessel can send as many as 60 position reports in 2 minutes, and rather than uploading all those reports (59 of which would already be "stale"), I send only 1 report -- with a delay as long as 2 minutes. There are additional unquantifiable delays at the service provider (marinetraffic, etc.) in processing all that data.

Keep in mind, the AIS receiving stations are operated by volunteers. We aren't paid anything. Consequently, the data reliability is "worth everything you paid for it" -- nothing. My AIS receiving station will go offline entirely when I'm underway, there is a local power failure, or frequently disrupted service from my Internet service provider (ATT).

Finally, there are times when the AIS service providers (marinetraffic, Vessel Tracker, etc.) go off line. I've seen them all experience hours of service unavailability. There is no warning or direct indication when this has happened. And there is no reason why there should be any warnings because their services aren't intended for collision avoidance or navigation.

I've worked in cyber security for 27 years. I am well aware of how fragile Internet-based services can be. You don't want to get hit by a freighter because someone somewhere in the world dug up a fiber cable with a backhoe. Never trust your life to the Internet!

A modest AIS receiver carried on board costs less than a new smartphone. I imagine anyone who uses a service like Boat Beacon as manifesting yet another symptom of smartphone addiction. Using something compulsively, even when it imperils your life, is a classic sign of addictive behavior.

The advertising material pertaining to the Boat Beacon app exploits ignorance of how AIS works. (References in my previous post.) It's claims are unattainable and reliance on those claims creates a life-safety hazard at sea.

[Cpt Pat]

Originally Posted by Cpt Pat

I've worked in cyber security for 27 years. I am well aware of how fragile Internet-based services can be. You don't want to get hit by a freighter because someone somewhere in the world dug up a fiber cable with a backhoe. Never trust your life to the Internet!

I no sooner wrote the above then, 4 days later, Comcast created an example of how unreliable marine traffic reports received via the Internet can be.

Anyone using smartphone apps instead of actual on board AIS receivers would have seen no traffic at all in the Half Moon Bay area and reduced coverage of vessels in the traffic lanes south of the S.F. Bay for 12 hours last night because my volunteer AIS receiving station feeding marinetraffic.com, Vessel Tracker, and AIS Hub was down. Cause: severed fiber cable.

https://www.hmbreview.com/news/sever...ntent=headline

[les22]

Originally Posted by Comix Bay

Unlike radar AIS can see around and over a land mass between you and a target. Radar beams are stopped and reflected back by most solid masses. AIS is transmitted straight up and over. I sit here in my chair and watch on my iPhone or iPad my GSon working up the fjords of the west coast of B.C. mainland or on the far side of Vancouver Island via his installed AIS equipment. This is all done through Marine Traffic. Another good factor of AIS is the capability to make voice contact with other AIS equipped boats. This can not be done with radar.

Ted VA7YQQ

"AIS can see around and over a land mass between you and a target. AIS is transmitted straight up and over." - This is not true.
"Another good factor of AIS is the capability to make voice contact with other AIS equipped boats." - Not through AIS you can't.

[slug]

Originally Posted by Haddock1

All,
I have an AIS B system on my boat which is a transponder which is linked to my plotter. It's a great system and shows closest point of approach and a vector line. Beats hand held compasses like in the old days. Anyway, when in use I can see vessels that are transmitting AIS data via VHF from my masthead aerial which give a range of circa 20 miles. Easy to understand. What I don't understand is that if I zoom out on the plotter I can also see vessels which are over the horizon/line of sight of the aerial (i.e. sailing from Palma, Mallorca, I can see vessels on the coast of Spain). So the question (drum roll) how is this possible???? Also, when on the internet, I can track friends who are still out in the Med and have the same system as me, through services such as Marine Traffic. Clearly tucked up at home in the UK how am I able to see their boats?
Any insight most welcome.
Confused Haddock.

VHF line of sight , power output plus propagation variables

Italian meteomar vhf 68 weather can occasionally be heard at night in Menorca

A cruise ship has its ais ant very high above sea level
Your ant may be masthead