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Old 14-09-2019, 02:02   #1
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How can I see AIS targets over the horizon?

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.
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Old 14-09-2019, 04:24   #2
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Re: How does AIS work?

It's not the horizon line of your aerial, it is the line of sight from aerial to aerial. If their aerial is tall enough you will still see them. When I notice this the other vessel is usually quite large. Commercial vessels such as container ships may have their aerial over 150 feet in the air. Also VHF signals are basically line of sight but they will curve over the horizon a little bit. There is also the possibility that you are getting some tropospheric channeling which can propagate VHF signals hundred and sometimes even a couple of thousand miles. This is caused by certain weather conditions and it can somewhat common in certain locations. Marine traffic uses land based receivers on tall towers as well as some satellite services and simply rebroadcasts the information on the internet.
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Old 14-09-2019, 04:36   #3
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Re: How does AIS work?

Thanks for the explanation for ship to ship locally but it still does not answer the Marine Traffic conundrum which I assume must be satellite based systems (ie ship - satellite - base station) as you suggest. Also it seems seems the ship to ship view when looking at the coast of Spain from Mallorca on my plotter are commercial vessels which use Class A AIS. Does AIS Class B also receive not only VHF but also satellite data which would explain the reason I can see vessels many miles away (commercial that is)?
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Old 14-09-2019, 04:56   #4
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Re: How does AIS work?

AIS signals are received by satellites and relayed to services such as MarineTraffic. So you can see world wide AIS traffic over the internet.

As for long range reception by an AIS receiver, this has been discussed at length in another thread over the last few days. Atmospheric conditions can result in "tropospheric ducting"

https://en.wikipedia.org/wiki/Tropospheric_propagation

"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 uncommon when the refractive index of the atmosphere is fairly high."
...
"
Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, Strait of Florida, and Bahrain and Pakistan, has produced VHF/UHF reception ranging from 1000 to 3,000 miles (1,600 – 4,800 km). A US listening post was built in Ethiopia to exploit a common ducting of signals from southern Russia."
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Old 14-09-2019, 05:08   #5
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Re: How does AIS work?

A quick look up and find there are 2 watt and 5 watt AIS transmitters. https://www.milltechmarine.com/Class...ders_c_43.html

The 5 watt are designed to give reliable coverage to 15 miles away. The 2 watt is not as far.

Now I do not pretend to know any of the AIS specs. I have experience in aircraft collision avoidance systems. They have wattage to be able to reliably track out 20 miles. That being said you can see targets up to 100 miles away, just not reliably.

What happens is the magic of RF. You have a transmitter that has to be well coupled to an antenna. Then you have the direct line of sight of the antenna (VHF propagation rules). The on the receive side you have the antenna, cable, and receiver.

So if you have a transmit system that has good output power at the box, a well made low loss cable and a good well installed antenna then it will put a nice strong signal out. It will go a longer distance.

On the receive end you need a good antenna, low loss cable and a receiver that is well tuned. Plus you need a good computer as most are digitized and use math to pull the data out of the signal.

Then what kind of interference do you have in your boat. Maybe you have a noisy power inverter too close to your AIS. You could have a little loose cable that lets in noise. Then you would need louder signals to be able to decode them.

So you are likely to see a wide range of signals as the installs are all over the place. If you only see ships up close to you then you likely have a bad cable or a noise issue. Try turning off things that have inverters and see if your range improves.

If you ever happen to wander across a Ham radio guy ask him if wants to look at your install. He might have the tools to tune your antenna system and get you extra range.

The RF stuff can be 'magic' and a guy who has some experience can look at a system and just sort of know what is not right. It can be some weird things like a tight bend at the wrong place on the antenna cable causing a reflection point.
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Old 14-09-2019, 05:38   #6
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Re: How does AIS work?

I do not know much of anything about AIS, but a search of in the old internet coms up with a WI-Fi enabled transponder, and equipment that is a sharing system, that will share your target, but will also pick up the information shared by others and display it on your system. Long range AIS stations, and such can go for over 1000 miles. This info is shared with those who want/use it. Your system might be connected to this network. You don't mention what system you have on board.

Some receivers from global sharing networks, sharing stations....

https://www.panbo.com/easy-marine-tr...thub-is-first/

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Old 14-09-2019, 05:41   #7
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Re: How does AIS work?

Quote:
Originally Posted by StuM View Post
AIS signals are received by satellites and relayed to services such as MarineTraffic. So you can see world wide AIS traffic over the internet.

As for long range reception by an AIS receiver, this has been discussed at length in another thread over the last few days. Atmospheric conditions can result in "tropospheric ducting"

https://en.wikipedia.org/wiki/Tropospheric_propagation

"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 uncommon when the refractive index of the atmosphere is fairly high."
...
"
Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, Strait of Florida, and Bahrain and Pakistan, has produced VHF/UHF reception ranging from 1000 to 3,000 miles (1,600 Ė 4,800 km). A US listening post was built in Ethiopia to exploit a common ducting of signals from southern Russia."

Tropospheric ducting is a special case and doesn't explain typical AIS reception distances.



As you said, the antennae of larger ships are at considerable height, and that helps. Then on top of that, VHF signals do bend over the horizon to some extent.


But I think the main reason why we get surprised about typical AIS reception range is the much greater efficiency of transmission of this type of signal, compared to "phone" signals (voice) by FM, which is how marine VHF "phone" works. A phone signal has to be pretty strong to break through the background noise, because of the nature of FM phone modulation, where the digital signals of AIS use GMSK, which is exceptionally efficient. See: https://en.wikipedia.org/wiki/Minimu...m-shift_keying.


A good antenna and feedline help a lot too. I normally see ships from at least 50 miles away.
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Old 14-09-2019, 06:38   #8
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Re: How does AIS work?

Quote:
Originally Posted by Dockhead View Post
A good antenna and feedline help a lot too. I normally see ships from at least 50 miles away.
Might well be there's some sort of enhancement going on there, marinetraffic has useful graphs showing receiving stations data which shows what the propagation has been like >

This one has 105m antenna elevation >
https://www.marinetraffic.com/en/ais...2ba76e529a1ec4



Good post re propagation >
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range
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Old 14-09-2019, 07:19   #9
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Re: How can I see AIS targets over the horizon?

It looks like there is also a special AIS Message Type, 27, designed to facilitate satellite reception of AIS transmissions.

https://www.navcen.uscg.gov/?pageName=AISMessage27

From memory, I donít think all AIS devices send this message type.
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Old 14-09-2019, 07:20   #10
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Re: How does AIS work?

Quote:
Originally Posted by conachair View Post
Might well be there's some sort of enhancement going on there, marinetraffic has useful graphs showing receiving stations data which shows what the propagation has been like >

This one has 105m antenna elevation >
https://www.marinetraffic.com/en/ais...2ba76e529a1ec4



Good post re propagation >
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range



Great information.


John's post (the last link) has more information on the topic than anyone could possibly every want to know! Thanks; I hadn't seen that.


And this:


"Wide-bandwidth modes such as FM Voice are not the usual means of utilizing 'tropo-scatter, due to their inherent noise-limited receivers and need to attain threshold in order to have any real useful signal-noise performance."

Was what I was attempting to describe. Data modulation schemes like GMSK can get the signal through pretty far over the horizon via tropo-scatter, which as John says is a normal means of propagation which does not depend on special atmospheric conditions. Even when FM-modulated marine VHF phone signals can't.
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Old 14-09-2019, 08:23   #11
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Re: How does AIS work?

To confuse you more, you can see AIS carrying vessels on your smartphone...
This in info that systems like Marinetrafic collect through long range coast stations plus long range ship transmitters + array of AIS transponders on many commercial vessels that collect AIS signals from vessels in their vicinity and trasmits these signals to the a.m. shore stations.
So - your plotter shows that data as well, received from some shore station in your vicinity.
I would guess that once you will be out of range of the shore station/s you will see on the plotter only ships in your vicinity (+ some atmospheric miracles described in the other posts)
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Old 14-09-2019, 09:15   #12
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Re: How does AIS work?

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.

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Old 14-09-2019, 09:30   #13
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Re: How does AIS work?

For MarineTraffic, unless you pay you don't see satellite data for individual targets, although satellite data may be included in aggregate information. But they have thousands of cooperating land-based stations that forward information. If you look under the latest position report for a vessel you'll see a little note labeled "source". If you click on the link it will give you lat/lon and a little other information about the receiving station that provided the data.
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Old 14-09-2019, 09:34   #14
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Re: How does AIS work?

Sorry for second post, but one other thing, for long distance contacts, in some areas the government operates repeater stations that re-broadcast reports. This is particularly useful in seeing "around corners" where line of sight might be blind but knowing someone is coming is useful. Australia, for example, has an extensive network of repeaters along the Great Barrier Reef.
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Old 14-09-2019, 10:19   #15
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Re: How does AIS work?

Quote:
Originally Posted by StuM View Post
AIS signals are received by satellites and relayed to services such as MarineTraffic. So you can see world wide AIS traffic over the internet.

As for long range reception by an AIS receiver, this has been discussed at length in another thread over the last few days. Atmospheric conditions can result in "tropospheric ducting"

https://en.wikipedia.org/wiki/Tropospheric_propagation

"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 uncommon when the refractive index of the atmosphere is fairly high."
...
"
Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, Strait of Florida, and Bahrain and Pakistan, has produced VHF/UHF reception ranging from 1000 to 3,000 miles (1,600 – 4,800 km). A US listening post was built in Ethiopia to exploit a common ducting of signals from southern Russia."
Excellent description! The reception limitation for reliable continuous reception is the VHF line-of-sight distance, which is a radius slightly larger than visual line-of-sight because of atmospheric refraction. I presume my limit for my own reliable continuous AIS reception is 20 miles. But you can receive occasional AIS data packets from any distance.

I have received a few AIS data packets from literally the other side of the planet. This is with an ordinary AIS transceiver and an antenna 40 feet (12 meters) above the surface. There really isn't a limitation to the range, only a gradual reduction in the probability of reception. Since AIS data packets are very short (~20 milliseconds), any effect that can momentarily refract or reflect (such as meteor trails) RF for 20 milliseconds can pass an occasional distant packet transmission to your receiver.

APRS.FI logs my AIS receptions when I upload those packets to the aprs.fi service (similar to marinetraffic.com, which also receives my reception reports). My greatest reception distance for this month (at time of this writing) is: MAERSK ELBA at a distance of 8492.7 miles, located in this grid square (zoom out to view it) in the Malacca Strait off Malaysia: https://aprs.fi/#!addr=NJ94GU.

You can see the full log of my AIS receptions here: https://aprs.fi/info/i/N8QH. Look in the section: "Stations heard directly by N8QH."
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