AIS propgation

[GHA]

Having just rigged up the AIS antenna again in a boatyard, the receive distance is heartening Actually a dual band ham antenna sticking above the radar arch but does a very good job on marine VHF receive as well.

So first, I'm assuming these distances (180Nm one of them just now) are due to tropo scatter and not via a repeater - or do some ships have ais repeaters?

And is there any daily change? Cn't remember seeing much last night though think during the day there were some very distant targets.

Interesting though, trusty Raspbbery Pi is saving it all to a file as I type so will be able to have a look later

PS, just found this very informative thread over on cruisersforum, this guy seems to know his stuff on all things radio -.
http://www.cruisersforum.com/forums...f-and-ais-radio-range-149499.html#post1869342

 

[Hoolie]

A couple of weeks ago in the SoF we were picking up AIS via our masthead antenna from well over 150 miles away and tracked one that was 283 miles away. Tropospheric ducting was obviously the cause.

But it can also work at higher frequencies. Last year on the hard in Port Leucate I was picking up good signals from at least half a dozen Italian UHF TV and radio programmes. Could have come from Sardinia 400 miles away or the mainland at 600 miles. Rare at UHF apparently but the weather was strange at the time with a thick layer of low cloud.

 

[GHA]

A couple of weeks ago in the SoF we were picking up AIS via our masthead antenna from well over 150 miles away and tracked one that was 283 miles away. Tropospheric ducting was obviously the cause.

But it can also work at higher frequencies. Last year on the hard in Port Leucate I was picking up good signals from at least half a dozen Italian UHF TV and radio programmes. Could have come from Sardinia 400 miles away or the mainland at 600 miles. Rare at UHF apparently but the weather was strange at the time with a thick layer of low cloud.

Looks like the mechanism might actually be troposcatter for those distances rather than ducting..

Ducting (1.000+ km, e.g. Hawaii to California at 4000 km). Radio waves are sometimes trapped like in a wave guide between an inversion layer and the ground or between two inversion layers. Little attenuation, thus good signal strengths. Often signals are only heard at each end of the wave guide, and conditions are only good between relatively small geographical areas.


Troposcatter (100-700+ km). Scattering in the troposphere in the common volume of the radiation from the beams.

http://www.qsl.net/oz1rh/troposcatter99/troposcatter99.htm

 

[RichardS]

There appear to be land based AIS repeaters down the west coast of Italy but that should mean that you see a lot of ships all 100+ miles away. If you only see one or two, as I did in Greece which are anything up to 500 miles away, then it's almost certainly propagation.

Richard

 

[Hoolie]

Looks like the mechanism might actually be troposcatter for those distances rather than ducting..

Not sure. Troposcatter is always present, increases noise levels and (according to your reference) needs huge 16-20ft antennas to pick up the very low signal levels. Ducting is rare, very low loss and signals can be picked up with small antennas like our Glomex omni-directional on the mast. The Italian tv stations were only there for maybe 10 minutes at most then disappeared.

I'm sure the intermittent reception of distant AIS is due to ducting rather than scatter.

 

[MoodySabre]

How much distance do you need? The max radius shown on my SH AIS VHF is 10nm. Maybe that's a bit tight with lots of targets doing 20kn but SH obviously think it's enough.

Or is size everything?

 

[Hoolie]

Then you will miss all the fun and interest in wondering how/why you can see transmissions from hundreds of miles away ... ... ...

 

[GHA]

How much distance do you need? The max radius shown on my SH AIS VHF is 10nm. Maybe that's a bit tight with lots of targets doing 20kn but SH obviously think it's enough.

Or is size everything?

It's interesting, no mention of if needed or not.

 

[GHA]

Not sure. Troposcatter is always present, increases noise levels and (according to your reference) needs huge 16-20ft antennas to pick up the very low signal levels. Ducting is rare, very low loss and signals can be picked up with small antennas like our Glomex omni-directional on the mast. The Italian tv stations were only there for maybe 10 minutes at most then disappeared.

I'm sure the intermittent reception of distant AIS is due to ducting rather than scatter.

This guy is high up in hamland so dunno, apparently ducting is quite rare, this ain't

Maybe tropo-enhancement, ticks the boxes anyway, edge of a high , coastal, summer,


http://www.cruisersforum.com/forums...f-and-ais-radio-range-149499.html#post1869342

I will just attempt to inform you all of the main / typical means of VHF communications propagation...."line-of-sight" ("direct-wave") and "tropo-scatter", being the dominant VHF Marine propagation modes) and how those methods work...
As well as describe "tropo-enhancement" (fairly common along the edge of high pressure areas, and in summertime coastal areas, and many times misunderstood and erroneously referred to as "ducting"), and the much more rare "tropo-ducting" (which is actually very rare, except in certain geographic areas, during certain times of the year...)

3) "Tropo" / "Tropo-enhancement" (also sometimes referred to as "non-ducting-tropo")
This is the most common means of "enhanced" VHF (and UHF/SHF) communications....and is noticed by many mariners along coastal areas, especially in summertimes....
The technical reason for these fairly common, springtime and summertime coastal "tropo-enhancements" are due to daily temperature inversions in the troposphere.....where instead of the air temp descending as altitude increases, there is a slight rise in temperature at a higher altitude in the troposphere...
"Tropo-enhancement" (especially along the edge of hi-pressure areas, as well in coastal areas, or along island chains / archipelagos) typically covers rather broad areas, and can make signals from a few hundred miles away very strong....although there is typically also some fading, and while these signals are fairly strong, they do fall off in strength at typical inverse-square rates, meaning unlike in a "tropo-duct" these "tropo-enhanced" signals from 400 - 500 miles will be weaker than those 200-250 miles distant...
These are common in areas of high-pressure, especially in humid air, along the edge of a high-pressure area, etc.

 

[RIBW]

C'mon!
Real long distance radio interest stopped when 1010 in New York stopped braodcasting Rock & Roll in 1965 and those of us in the west of England had nothing like it but the pirates on the east and north coasts of England to listen to. Perhaps that is why I ended up getting an M.Sc. in Radiocommunications?
Cheers
Bob

 

[GHA]

C'mon!
Real long distance radio interest stopped when 1010 in New York stopped braodcasting Rock & Roll in 1965 and those of us in the west of England had nothing like it but the pirates on the east and north coasts of England to listen to. Perhaps that is why I ended up getting an M.Sc. in Radiocommunications?
Cheers
Bob

God is alive and well on HF, and he wants you to send some money to the big churches across the pond, plenty of that going on ....

 

[GHA]

Quite busy out there
Rings are 100Nm.
There were 2 not under command earlier..

Whatever is causing the propagation seems pretty stable, so far today anyway.

 

[GHA]

And this half eleven, the "disappear if nothing comes in" settings are very low, but even so all the local targets are gone as well. So is AIS reception worse at night? >

 

[cpns]

So first, I'm assuming these distances (180Nm one of them just now) are due to tropo scatter and not via a repeater - or do some ships have ais repeaters?

Repeated AIS messages have a non-zero repeat indicator. A repeater station increments this value up-to 3 - after which the message may not be repeated - a non-zero value indicates the message is repeated.

You can use Arundale AIS Decoder to process your live or logged data. If you filter on the MMSI of the distant target you can find the messages of interest and check the repeat indicator:

 

[Hoolie]

Nah - they've all gone home and are tucked up in bed

 

[RichardS]

And this half eleven, the "disappear if nothing comes in" settings are very low, but even so all the local targets are gone as well. So is AIS reception worse at night?

There should be loads of boats at anchor or in marinas that keep their AIS on, as well as the larger ships/cruise liner on passage at night. Reception is generally better at night unless you're tucked away in a bay surrounded by hills with no direct line of sight to the sea but even then there are usually moored boats with AIS in the same bay.

I've no idea what's going on but it's not usual.

Richard

 

[GHA]

Reception is generally better at night unless you're tucked away in a bay surrounded by hills with no direct line of sight to the sea but even then there are usually moored boats with AIS in the same bay.

I've no idea what's going on but it's not usual.

Richard

Why is reception better at night? Not heard that one before.

The boats anchored a few miles away have showed up again on AIS this morning.

 

[RichardS]

Why is reception better at night?

The boats anchored a few miles away have showed up again on AIS this morning.

It's better for more distant targets for atmospheric reasons. It might be better for nearer targets because of fewer VHF signals whizzing about but I've never tried to check that.

Do you have a line of sight out to open sea from where you are anchored? If you do not, and every boat in your enclosed anchorage turns off their AIS at night, then you would lose all signals but that's very unusual. If you can see the open sea, you could check on Marine Traffic to see if there are any signals either at sea or in the anchorage that you should be picking up.

Richard

 

[GHA]

It's better for more distant targets for atmospheric reasons. It might be better for nearer targets because of fewer VHF signals whizzing about but I've never tried to check that.

Do you have a line of sight out to open sea from where you are anchored? If you do not, and every boat in your enclosed anchorage turns off their AIS at night, then you would lose all signals but that's very unusual. If you can see the open sea, you could check on Marine Traffic to see if there are any signals either at sea or in the anchorage that you should be picking up.

Richard

Which propagation method makes 160Mhz better at night? , Sure your not thinking of the lower frequencies where skywave propagation is better at night?

Boat's on the hard , the anchorage boat have ais on all the time, some from about May to October
VHF is largely silent round here, anyway , would need to be a stunningly badly designed ais receiver not to filter out transmissions a few Mhz away...

 

[RichardS]

Which propagation method makes 160Mhz better at night? , Sure your not thinking of the lower frequencies where skywave propagation is better at night?

Boat's on the hard , the anchorage boat have ais on all the time, some from about May to October
VHF is largely silent round here, anyway , would need to be a stunningly badly designed ais receiver not to filter out transmissions a few Mhz away...

Are you trolling or something as this is getting silly?

You know perfectly well that I wouldn't be picking up AIS signals from ships 500 miles away unless it was due to atmospheric effects.

And if you know that these anchored boats are within sight and have their AIS on permanently but you are not seeing them on your system then you clearly have a technical fault or an operator error.

I suggest that you start this thread again once your head has cleared from last night's excess.

Richard