AIS target more than 2000km away

[conachair]

Isn't tropospheric ducting actually quite rare?

I reckon if anyone here really knows their stuff it's ka4wja...

http://www.cruisersforum.com/forums/...ge-149499.html

Also, marinetraffic stations are quite a good place to check if and when any enhancement was taking place >>



Also, if you stick an ais message into one of the online decoders and it will show if the message has been repeated or not.

http://www.maritec.co.za/tools/aisvdmvdodecoding/

[AllenRbrts]

Originally Posted by 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)."

This is referring to vhf tv signals not 1Mhz marine radio. The article you cite says the lower bound on ducting is 30Mhz.

Another possibility is ais signals were being created locally spoofing distant ships.

https://blog.trendmicro.com/trendlab...igence+Blog%29

[StuM]

Originally Posted by AllenRbrts

This is referring to vhf tv signals not 1Mhz marine radio. The article you cite says the lower bound on ducting is 30Mhz.

Another possibility is ais signals were being created locally spoofing distant ships.

https://blog.trendmicro.com/trendlab...igence+Blog%29

No one is talking about 1Mhz marine radio. We are talking about long range AIS reception:


AIS operates principally on two dedicated frequencies or VHF channels: AIS 1: Works on 161.975 MHz- Channel 87B (Simplex, for ship to ship) AIS 2: 162.025 MHz- Channel 88B (Duplex for ship to shore)Mar 27, 2017


The lower limit for ducting may well be 30MHz, (the link does not say so) but the upper limit is well into the UHF band (up to 3GHz)


161-162 Mhz is well within that range.


That is much more likely than several vessels spoofing at the same time for some reason if they could. There is nothing in your link which suggests that a vessel can spoof the location of a vessel.

[AllenRbrts]

Originally Posted by StuM

No one is talking about 1Mhz marine radio. We are talking about long range AIS reception:


AIS operates principally on two dedicated frequencies or VHF channels: AIS 1: Works on 161.975 MHz- Channel 87B (Simplex, for ship to ship) AIS 2: 162.025 MHz- Channel 88B (Duplex for ship to shore)Mar 27, 2017


The lower limit for ducting may well be 30MHz, but the upper limit is well in the UHF band (up to 3GHz)


161-162 Mhz is well within that range.

Sorry, my mistake, must not have been thinking clearly this morning.

[Captain Bill]

One night while anchored at Highbourne Cay in the Exumas I listened to a rescue involving the Coast Guard and a fishing vessel on Fryng Pan Shoals off the coast of North Carolina. Even the fishing vessel's transmissions we quite clear. Another time, while in Georgetown Exuma, one of the local radio Nazis started giving a couple of guys a hard time about talking on channel 68 (the local cruisers hailing channel). The "offenders" didn't seem to have a clue what she was talking about and after a bit of back and forth and confusion over Georgetown SC, it was determined that these guys were in Southport NC. These two incidents were about 10 years apart. Oddly enough Southport is very close to Frying Pan Shoals and both incidents occurred in late March. One has to wonder if conditions at that time of year make a tropospheric duct along that line fairly common.

[KP44]

Originally Posted by AllenRbrts

This is referring to vhf tv signals not 1Mhz marine radio. The article you cite says the lower bound on ducting is 30Mhz.

Another possibility is ais signals were being created locally spoofing distant ships.

Our Marine VHF frequencies are from 156.050 MHz to 162.025 MHz.

Our two AIS frequencies are:

AIS Channel 1 161.975 MHz Automatic Identification System #1
AIS Channel 2 162.025 MHz Automatic Identification System #2

The AIS channels are VHF and Tropospheric Ducting is likely the explanation of receiving far-away signals.

If we used High Frequency for AIS, our screens would be full of targets all over the world; even on the other side of continents. But Very High Frequency is appropriate for AIS because VHF usually is only line-of-sight; 40 miles max.

[transmitterdan]

Tropospheric ducting is well documented and signals from over 1,000 miles is possible. The waveguide created by the troposphere is very low loss and so signals can travel a long way. Also, AIS uses a robust digital modulation scheme making it reliable under conditions where voice comms would be unusable.

Here is a site that gives daily predictions of the tropospheric ducting “quality” world wide.

http://www.dxinfocentre.com/tropo_eur.html

[DefinitelyMe]

Originally Posted by Jim Cate

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

I don't know i'm afraid. My statement was based on a long conversation with the fisheries officer on Ascension Island, who explained what we were seeing in this way. He said it's quite common for fishing boats to falsify their AIS output either by manually inputting positions, offsetting the output (by thousands of miles - they watched a boat on radar a few years ago following the coastline of China....... in the South Atlantic) or pairing their AIS with another boat that it actually where it should be.

[StuM]

Spoofing? Occam's Razor! Which is more likely to explain the observed situation?

1. The not uncommon phenomenon of tropospheric ducting

2. 3 separate vessels in close proximity to the OP's vessel with the sophisticated electronics, hacking skills and a reason to report a false position, all of those positions being in the same approximate range and direction from the OP's position.

If you subscribe to the second alternative, I have a bridge you may like to buy

[Bill Streever]

Interesting discussion.

Off the coast of Colombia and around the ABCs, we were consistently seeing an AIS signal from well inland in Venezuela for weeks at a time. Atmospherics would not explain it (it was too consistent). Collision risk was zero, since our boat does not have wheels and it cannot fly, so we just ignored it.

[Nicholson58]

Originally Posted by sailormed

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

I go with this option. Check another day and also contact the CG and relate your observations.

[billdomb]

HOW are you spoofing the AIS signal, specifically?

[senormechanico]

OT.


I once experienced VHF tropospheric ducting while cruising in Mexico.
Perfectly intelligible conversation with a HAM friend of mine over 420 miles on 25 watts, and when we tried 1 watt just for the heck of it, he said he couldn't understand me, but could recognize my voice. The conversation was from San Jose del Cabo to Bahia Tenacatita.

[Fluenta]

In our seven years full time on the boat in the Pacific we have seen the atmospheric ducting several times. We have a dedicated AIS display which we jokingly call "AIS TV" as sometime there is not much else going on on passage so the kids are excited to see what ship if going by and where it is going as no other TV on Fluenta.


We have seen ducting off the Alaska Peninsula and in the Sea of Cortez but the best was sitting in New Caledonia and seeing the Class A contacts off Australia. Unlikely spoofing as it involved many ships and unlikely a repeater as we were seeing AIS contacts and hearing some VHF voice transmissions unless somebody - in a remote part of New Caledonia - went to a lot of trouble to set up repeater(s) for more than one frequency.


We have seen some suspicious AIS contacts in Micronesia from the Chinese fishing fleet emptying the oceans of fish there so that was likely spoofing.

[transmitterdan]

Originally Posted by billdomb

HOW are you spoofing the AIS signal, specifically?

Nobody here claimed to be spoofing. Only they maybe someone else is doing it.

Spoofing AIS is pretty trivially done. All you need is a software defined radio. Lots of Hams have them. You program it to send the AIS message which is not encrypted or otherwise distinguishable from and real AIS message. There is no secret key needed to transmit an AIS message.