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olympia 14-09-2013 03:32

AIS on OpenCPN showing ships over 1000 nm away?
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Hi there,

I installed a GME AISR120 AIS receiver this year and find it works very well. In fact, it seems to work impossibly well! At the moment, anchored at Magnetic Island near Townsville (Queensland, Australia) my AIS is showing ships up to 1000nm away (in Noumea). Obviously I am not receiving VHF radio signals over such a large distance, so there must be another explanation for receiving this information.

As my Macbook Pro was not connected to the internet at the time, these signals could only be received over VHF so I assume there must be some type of repeater that "relays" AIS packets over long distances, possibly receiving packets via satellite? Does anyone know what's going on here? I've look at Wikipedia and Googled "AIS range" but I can't seem to find any info. If someone knows whats happening here, I would really appreciate an explanation.

A screenshot is attached showing my OpenCPN screen.

Thanks in advance!




bcn 14-09-2013 03:52

Re: AIS on OpenCPN showing ships over 1000 nm away?
Receptions of this type are happening sporadicly depending on the conditions of propagation.
But as you might have seen the ship is rendered in yellow, so no static data are available - and most probably you won't see those as they are sent much less frequently and the chance to spot a message ducted over those distances is very remote, so only one out of a bunch of messages will get to you. In this cases just observe when the data have been updated last time ("Report Age").

There are also cases where erroneous AIS data are sent. Perhaps you will see some day a ship that wants to sell you that it is at Ayers Rock :p

Relaying is foreseen in the AIS standards, but for sure not to sent data from 1000nm.

Viking Sailor 14-09-2013 12:46

Re: AIS on OpenCPN showing ships over 1000 nm away?
Ham radio operators have sent and received messages between Calif. and Hawaii via ducting for years.

Years ago, as a ham, I talked for about 10 mins between Calif. and Brazil on VHF with just 5 watts. This was most likely via a meteor ionization path. Although meteor showers are normally associated with nighttime, and this particular event was at night, there are also daytime meteor showers that can have a similar effect.

Also, when working for NASA, I did a number of successful communications experiments using 25 watts VHF transceivers via the Advanced Technology Satellites (ATS). These experiments where in support of Air Traffic Control and emergence communications. The paths to and from the satellites were on the order of 23,000 miles. So, transmitter power and receive sensitive is not a limiting factor in long distance VHF communications.


Paul Elliott 14-09-2013 13:12

Re: AIS on OpenCPN showing ships over 1000 nm away?
The error-checking on the AIS signals is pretty good, but not perfect. Sometimes a signal will get a little garbled, and as a result some of the received data is bad. This can result in erroneous positions (or as other bad data). In these cases the vessel will either quickly jump back to the correct position, or in the case of truly marginal signals the vessel will time-out and disappear.

If the target is regularly updated with the questionable position, there are two probable reasons:
1) Most likely there is VHF ducting propagation. This is commonly seen up to a few hundred miles, and occasionally up to two thousand miles.

2) Unlikely, but the vessel could be transmitting bogus data. I see this on the internet feeds where I get signals from thousands of vessels all over the world, but I suspect that these signals are actually computer-generated test data, not actual vessels.

MarkJ 14-09-2013 14:35

Re: AIS on OpenCPN showing ships over 1000 nm away?
Hi Paul,

I think one gets these great ducting signals when their kit has an excelent antenna instalation.

Whats the range you are getting on those green ships north east of you? They appear to be over 100nms fromyou. And how many targets are you getting in Bowen? Is that normal?


olympia 14-09-2013 20:34

Re: AIS on OpenCPN showing ships over 1000 nm away?
1 Attachment(s)
Hi All,

Thanks for all you replies and suggestions. I guess I really am receiving VHF packets from 1000nm away. I didn't think it was possible, as VHF is generally regarded as "line of sight" so this has been very educational for me. Thanks again for the explanation.

I have been watching the screen today and even in daytime, I see vessels at anchor in Noumea and steaming around the Pacific. The report age for some of the vessels stays under 10 seconds and for others, every few minutes. I don't see static data (vessel name etc) for most of them - the range for receiving static messages seems to max out at 80nm but on average about 40nm.

The masses of ships at anchor around Gladstone, Mackay and Abbot Point are coal vessels waiting to be loaded - sometimes there are dozens waiting for their turn. The reason I bought an AIS unit was to be able to navigate those areas at night - it can be a bit scary sailing through a fleet of anchored ships, especially when some of them start moving!

My AIS has gone from being a purely safety driven feature to a ship watching past-time! Its very therapeutic watching all these vessels going up and down the coast. It also reminds me just how many ships are out there all the time.

No please excuse me while I go back to watching these ships go by...




Paul Elliott 15-09-2013 00:02

Re: AIS on OpenCPN showing ships over 1000 nm away?
The reason that you aren't seeing the static messages is purely statistical. The dynamic messages are sent every ten seconds (usually, more or less), while the static messages are sent every six minutes. If a significant percentage of the messages are garbled and discarded due to being distant/weak, then it may take a very long time before a static message makes it through.

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