AIS target more than 2000km away

[Karanga]

Recently I was anchored off Hinchinbrook Island on East Coast of Australia, and checking out AIS targets.
I was surprised to be picking up yachts (class B) targets as far away as 750km away.
I scrolled out further, and found targets (class A) at Noumea in New Caledonia, which is more than 2100km away!!
This is VHF only - no internet access.

Is this the effect of sunspot activity?
Or bouncing off satellites?
Or...?

Tim

[sailormed]

...... or a repeater installed by the Coast Guard or an passionate amateur .... ?

[StuM]

Atmospheric conditions.

https://en.wikipedia.org/wiki/Tropos...pheric_ducting


"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)."

[ohgary]

Most likely troposphere ducting.

[gcrothers]

You usually get this in High Pressure conditions. I've picked up American FM radio station mid Atlantic on occasions.

[Redline452]

it could also be a mis-configured or rogue transponder - the position we see is whatever they report, regardless of where they actually are.

[barnakiel]

You are VHF only but the shore station in range can forward AIS messages from anywhere over the net - also data derived from the www component.


AIS is so much more than we initially assume it is. Just think of virtual AIS channel markers - there is none on the water and yet they all pop up on our screens.



Or think of AIS generated mayday message - these can be actually collected by the satellite network.



b.

[KP44]

Originally Posted by ohgary

Most likely troposphere ducting.

Tropospheric ducting is almost always how a VHF will sometimes receive from far away. Last winter a friend was having conversations with another boat across the Sea of Cortez; over 80 miles.

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

[smac999]

I could pull a boat up beside yours (or 10 mikes away) . And feed fake nmea183 data to a class A ais. And make it show up 2000km away on your screen. Because it’s transmitting within your range you will pick it up and see it’s (fake) data.

But I doubt that is happening if you have multiple boats in big range.

[RaymondR]

It's not particularly unusual to pick up Japanese VHF traffic in Perth in Western Australia.

[DefinitelyMe]

Some AIS transponders allow the user to manually input or offset the reported position. I've seen fishing boats do this when they want to make it look like they are not where they shouldn't be!

[Jim Cate]

These conditions are not all that rare. We often see them here in Oz, and I have been in Noumea and listened to the Tin Can Bay VMR talking on channel 16. Ducting is real and not uncommon... no spoofing of AIS needed to explain the observation.

Jim

[Jim Cate]

Originally Posted by DefinitelyMe

Some AIS transponders allow the user to manually input or offset the reported position. I've seen fishing boats do this when they want to make it look like they are not where they shouldn't be!

Could you please tell us which ones do allow this? Transponders, IIRC, require an internal GPS receiver to be coupled to the system specifically to eliminate false info from being broadcast.

Jim

[Diamond Life]

Agree with tropospheric ducting, I have picked up ships several hundred miles south of me on my AIS as they travel around the bottom of Australia. I confirmed their positions later when I was able to access data on MarineTraffic.

[clownfishsydney]

When sailing the GBR in 2015 and 2017 I regularly saw ships on AIS which were thousands of miles away. Not uncommon. Sometimes you will get one that is 8,000 miles away, but when you check you will find that it is 0 degrees lat and long, meaning its GPS has outputted nil location. The real boat is probably close by.