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Old 08-01-2013, 09:25   #1
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100 mile AIS range?

In a recent cruise from California to Cabo Baja, I found I was getting AIS indications from vessels more than 100 miles away. Additionally I also found that I was able to communicate with the US Coastguard in San Diego via VHF at a distance of 450 miles. Now I don't believe this was some kind of freak "skip" nor do I believe that my installation had super powers. I am guessing that some kind of relaying is going on. Does anybody know anything about this?

I might add that our new transponder (AiS) was from Em-trac and works wonderfully, I had confirmation from several large ships that they had us on their systems up to a estimated 12 miles away, maybe more. For the low cost going cruising without a AIS transponder these days is just plain stupid ! All of the vessels involved changed course to give us a 2 mile CPA. Believe you me at night 2 miles and a 250ft ship looks awfully close.

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Old 08-01-2013, 09:32   #2
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Re: 100 mile AIS range?

The atmospheric phenomenon that occasionally allows VHF radio waves to travel these unusually long distances is called "tropospheric ducting". This is apparently not too uncommon. I've heard accounts from a number of people who have picked up AIS targets or had VHF conversations at those ranges.

According to Wikipedia: "Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (1,300 km) (though some people have received "tropo" beyond 1,000 miles / 1,600 km), while with tropospheric-bending, stable signals with good signal strength from 500+ miles (800+ km) away are not common when the HA JA JA index of the atmosphere is fairly high."

That clears it up, doesn't it?
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Old 08-01-2013, 09:35   #3
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Re: 100 mile AIS range?

No it's not crazy at all.


The Range Rings are 10 nm apart.
The ship I have selected is 56 nms and isn't the furtherest ship by any means.
Note the ship in Nassau that's 93 nms

You can see by the tracks of all the ships in the New Providence Channel that they are all sending a series of signals, not just one skip signal or aboration.

I think it means you have done a good set up with your antenna and everything else. Your VSWR must be very close to 1:1 if you have software to check it.

The screenshot may be in good weather conditions, but I can rely on it to 40 nms for decent size ships. In mist, drizzle or rain its less.

Your 12 nm transmit power is good for Class B. some class B will surprise you only appearing 5 miles off or so. They must have low set antennas etc.
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Old 08-01-2013, 09:38   #4
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Not really, the effect that you mention which I shall call " skip" doesn't normally happen over a day and night period and was present both on the way down and on the way back. Besides that the Coastguard seemed to indicate it was normal
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Old 08-01-2013, 09:40   #5
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Re: 100 mile AIS range?

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Originally Posted by bazzer View Post
In a recent cruise from California to Cabo Baja, I found I was getting AIS indications from vessels more than 100 miles away. Additionally I also found that I was able to communicate with the US Coastguard in San Diego via VHF at a distance of 450 miles.
I'm not familiar with the area or the climate there at this time of year but Tropospheric ducting can result in propagation of VHF over those kinds of distances.

Edit: oops: beaten to it in post #2 I see
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Old 08-01-2013, 09:45   #6
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Re: 100 mile AIS range?

I regularly saw ships 100+ miles away when down in the Sea of Cortez. And the VHF nets regularly had interference from nets on the other side of the Sea of Cortez. I don't know what it is that causes it, but it happens with regularity.

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Old 08-01-2013, 09:49   #7
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Re: 100 mile AIS range?

I do think the Coast Guard has VHF repeater stations along the coast because they can talk on it for miles. But I doubt it extends to the AIS frequencies. If it was you would have been getting AIS targets at that 400nm range.

I just turned my AIS on here in Key West and am getting a ship at 76 nms. So the digital reach of AIS on a good set up is extraordinary
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Old 08-01-2013, 09:53   #8
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I accept they relaying VHF along the USA coast is probable but down the Mexican coast is unlikely.
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Old 08-01-2013, 10:04   #9
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Re: 100 mile AIS range?

Please don't call it "skip". That is an entirely different thing from ducting. It is fairly common as any HAM will tell you. Skip is when a signal actually bounces off the ionosphere, and this happens every day, pretty much around the clock, depending on the wavelength transmitted. VHF will seldom give you any true skip. It doesn't bounce back off the ionosphere. HF does skip, depending on time of day, solar activity, and frequency.

AIS as well as other digital communications over VHF/UHF do particularly well when marginal ducting conditions exist, since it only takes a brief moment to send a packet of data.
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Old 08-01-2013, 10:30   #10
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Re: 100 mile AIS range?

Usually I look here for a forecast of troposheric ducting:
Tropospheric Ducting Forecast for VHF & UHF Radio & TV

Here in the mediterranean I often get VHF/AIS signals from 250 miles away.

But they do have such forecasts also for other regions.
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Old 08-01-2013, 11:14   #11
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Re: 100 mile AIS range?

While the AIS units don't retransmit signals, there is mention of repeater stations in the docs and perhaps that is why you were seeing a signal far beyond the normal VHF range.
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Old 08-01-2013, 11:53   #12
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Re: 100 mile AIS range?

Like CarCode mentioned, its very common in the Med.
Can be good thing, on the other hand, instead of getting to listen to the idiots within 50 miles, you get to listen to all of them within 500 miles
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Old 08-01-2013, 12:20   #13
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Re: 100 mile AIS range?

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Note the ship in Nassau that's 93 nms

On our passage to the Capverdes we had a timeframe of several hours where we had many targets reliably over 200-250nm. And the one off Senegal was even 450 nm away.


Happens from time to time.
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Old 08-01-2013, 12:24   #14
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Re: 100 mile AIS range?

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While the AIS units don't retransmit signals, there is mention of repeater stations in the docs and perhaps that is why you were seeing a signal far beyond the normal VHF range.
We always received the spanish CG station located in the mediterranean while being moored in the Canary Islands, somtimes almost 1000nm away. Reliably, but only the CG stations. So there must be some kind of repeater mechanism in place for these signals.
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Old 08-01-2013, 13:09   #15
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Re: 100 mile AIS range?

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VHF will seldom give you any true skip. It doesn't bounce back off the ionosphere.
Most any ham can tell you about sporadic E propagation using VHF frequencies. It is skip, and while not reliable (that's why it's called sporadic), it is quite common and many hams look for it daily in their quest for the longest distance and number of, communications using this method.

Eric
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