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patrickbryant 11-16-2012 09:43 PM

Caution: AIS Apps
I recently had a fellow sailor - who is just IN LOVE with his iPhone - show me an app that displays vessel positions received via AIS. He said: "Why should I buy an expensive receiver and chartplotter when I have this?"

Since there may be others who are tempted to forgo the expense and use their iPhone or other mobile device instead of a real AIS receiver and traffic display, here're the answers to that question:

1) The AIS data received on your iPhone, laptop, via the Internet, or by any source other than an actual AIS receiver located in your vessel is posted to the Internet by volunteers who have receivers located in fixed positions (often far from water). I'm one of those volunteers. The vessels you see near Half Moon Bay, CA on and probably came from me (look for "N8QH" as the source). My receiver is in my sailboat, from which I upload a continual data stream to and via the Internet. And when my boat leaves the dock, I disconnect from the Internet and ... POOF! ... all of the data I am supplying, and seemingly all the AIS-equipped vessels in that area, VANISH. There are no other volunteer receivers that cover the area because the surrounding mountains block reception. This is true for many other volunteer receivers: their receivers are only on part-time.
2) If you think you can use AIS data from a service via your iPhone for collision avoidance, then read the fine print. Here's an example warning posted on
"Vessel positions may be up to one hour old or incomplete. Data is provided for informational reasons only and is not related by any means to the safety of navigation."
The author of that warning isn't exaggerating or just trying to avoid some obscure legal liability. For the SF Bay Area, there are only a handful of volunteer receiving stations, and the coverage in the Bay is highly incomplete. Large swaths of the Pacific Coast (and most of the remainder of the planet) have no coverage at all. If you don't see any vessels on your iPhone AIS app - that doesn't mean there aren't any vessels there.

So don't cheap out by trying to use your iPhone along with some external service to avoid getting crunched by a freighter! The data is unreliable! It's false security. The only AIS data you can trust is the data received by an AIS receiver located in your own boat.

TakeFive 11-16-2012 10:45 PM

Re: Caution: AIS Apps
Thank you for the helpful warning. It's good to hear my concerns verified by someone who actually feeds the data into the system. I can't tell you how many times people tell me that their boat is "equipped with AIS," and upon asking further I find out that they are running MarineTraffic app on their cell phones. It happened as recently as yesterday in this thread:


Originally Posted by TakeFive (Post 948924)

Originally Posted by TakeFive (Post 948880)
...I don't think iOS or Android [chartplotter] apps display AIS targets yet, and I doubt they'll ever do split screen like a Windows or Linux laptop. ( doesn't pass muster for real-time navigation.) The split screen is great, because I can just focus on sailing without ever touching the computer to zoom in or out.

[Correction: I realize that Android and iOS have some apps that will duplicate the displays of other computers, so it may be possible to use those apps to display AIS targets.]


Originally Posted by Familycruisers (Post 948892)
Marine Traffic is an AIS app for Android.

As I said in my prior post, MarineTraffic is not a navigation quality tool. MarineTraffic uses a network of land-based stations manned by hobbyists at their homes. There can be blackout zones when the stations go down, and there can be time delays in their updates. When a vessel is moving 15 knots up the river, a delay of 4 minutes equates to a nautical mile. That's the difference between being able to tack across the channel and encountering a very uncomfortable situation.

I have confirmed this from side-by-side comparison of my real-time AIS reception with the MarineTraffic app on my phone.

MarineTraffic a nice novelty for looking up pictures and port history of boats, but do not use MarineTraffic for real time navigation!

SlowButSteady 11-16-2012 11:03 PM

Re: Caution: AIS Apps
I've noticed that my iPhone app often doesn't display all the targets on the MarineTraffic website, AND there are times when neither of them display tankers off El Segundo that I can see with my own eyes.

patrickbryant 11-16-2012 11:19 PM

Re: Caution: AIS Apps
Thank you, TakeFive. You are absolutely right. AIS received from is only intended for people who live on a coast to identify the boats that pass by their windows.

As a licensed Global Maritime Distress and Safety System Maintainer, it just drives me nuts that anyone would use an iPhone or Android on their boat - that's receiving AIS data from any Internet source - for traffic avoidance. It's like driving down the freeway while only looking in the rear view mirror.

I think it's a symptom of what I call the "mobile device cult" and it's bound to eventually lead to the same serious consequences as texting while driving.

mdbee 11-16-2012 11:37 PM

Re: Caution: AIS Apps
Just curious about " posted to the Internet by volunteers who have receivers located in fixed positions (often far from water)." I noticed AIS data showing in the Antarctic, are there volunteers there? It seems they would have better things to do than to be feeding data into the Internet. Curious how that works?

smackdaddy 11-16-2012 11:45 PM

Re: Caution: AIS Apps
78 Attachment(s)
The answer is simple pat...don't leave the dock bro. We're counting on you.

RobGallagher 11-16-2012 11:47 PM

Re: Caution: AIS Apps
OK, so I don't have radar and I avoid sailing in the fog. I do have an iphone. A couple of times a year it happens, the fog rolls in. I do check my phone to see what might be out there. I don't expect it to be 100% up to date or inclusive. At least I know what could be heading my way. I do take other precautions like keeping watch and sending out a securitae call.

It's sometimes accurate, though I don't expect it to be, and it's a whole lot better than nothing.

sww914 11-17-2012 12:01 AM

Re: Caution: AIS Apps
As we sailed from the OC to San Diego we passed an aircraft carrier anchored off of the Marine Base. The AIS said it was a Coast Guard buoy tender. I'm pretty damned sure it was an aircraft carrier, kinda hard to miss those things, especially when there's a swarm of helicopters around it and jet fighters flying over constantly.

smackdaddy 11-17-2012 12:06 AM

Re: Caution: AIS Apps
78 Attachment(s)
I'd say the buoy had nothing to worry about, sw.

TakeFive 11-17-2012 01:00 AM

Re: Caution: AIS Apps

Originally Posted by RobGallagher (Post 949529)
...It's sometimes accurate, though I don't expect it to be, and it's a whole lot better than nothing.

There are times when having no data is safer than having bad data.

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