AIS shows many long distance targets

[El Pinguino]

Originally Posted by Paul L

They work. We saw vessels hundred of miles away consistently while sailing around the Galapagos. Here's a write-up on their AIS repeater installation
Ais - Galapagos

Interesting.

I can see how they re-transmit the information to a control centre on a different frequency but I don't see anything there suggesting that the signals are re-transmitted on the same frequency in such a manner that other ships can receive the info on their AIS tx/rxs..

[El Pinguino]

This being the working AIS thread and all, this may be of interest to some.
My wee boat is currently appearing anonymousely ( unless you are paying for the full sat info) on Marinetraffic.com in 42*34 S, 73*28W, position current about 3 hours ago. I expect to see it updated in about 9 hours ( 0300Z ) to show me anchored in beautiful downtown Rilan.

[El Pinguino]

Originally Posted by Olddave

VHF has an amazingly strong sky wave that travels straight up for thousands of miles and is received by satellites. Sky waves are no use in ship to ship VHF radio calls because radios are picking up the ground waves that are only good for line of sight.
I'm no radio expert and do not have my GMDSS text books handy for reference, so I would like to hear from someone who really knows about this stuff. Can the sky waves sometimes bounce off the ionosphere and be received by ordinary VHF receivers?

Not an expert and making no claim to superior knowledge....

however.... VFH and above aren't reflected from the ionisphere but they are reflected by 'stuff' such as the moon Weak Signal -- (VHF-DX, Meteor Scatter, EME-Moonbounce) , meteorites, and even aircraft.

With the latter, once while in Port Phillip using UHF for bridge/focsle coms we heard the other company ship 200 miles away in Northern Tas... just a few sentences but 'clear as'.
Asked a chum who does know about this stuff... he said most likely reflected from an aircraft over central Bass Strait.

[StuM]

Originally Posted by El Pinguino

Interesting.

I can see how they re-transmit the information to a control centre on a different frequency but I don't see anything there suggesting that the signals are re-transmitted on the same frequency in such a manner that other ships can receive the info on their AIS tx/rxs..

From documentation of a typical AIS repeater:

"AtonisPro‐R is a unique, state‐of‐the‐art, AIS simplex repeater station. The AtonisPro‐R’s primary function is to extend the AIS coverage for AIS base station networks, or to overcome limitations in radio propagation due to topographical obstacles inhibiting line‐of‐sight communication.
The AIS repeater receives AIS message data from ships within its coverage area and retransmits the received information using RATDMA protocol.'"

Note the words I have highlighted. As I understand that, it means that signals from AIS transceivers are re-transmitted on the same frequency in a vacant time slot.

[MarkJ]

Well, AIS uses 2 frequencies.

So I guess you could pick k up the transmissions on one frequency and rettansmit them on the other.

AIS transponders and receivers use two VHF radio frequencies: 161.975 MHz (AIS1, or channel 87B) and 162.025 MHz (AIS2, or channel 88B).

[conachair]

Originally Posted by StuM

From documentation of a typical AIS repeater:

"AtonisPro‐R is a unique, state‐of‐the‐art, AIS simplex repeater station. The AtonisPro‐R’s primary function is to extend the AIS coverage for AIS base station networks, or to overcome limitations in radio propagation due to topographical obstacles inhibiting line‐of‐sight communication.
The AIS repeater receives AIS message data from ships within its coverage area and retransmits the received information using RATDMA protocol.'"

Note the words I have highlighted. As I understand that, it means that signals from AIS transceivers are re-transmitted on the same frequency in a vacant time slot.

Another one here...


The AIS Shore Station is also available as a Repeater Station, which is an ideal gap-filler at remote and isolated locations.

AIS Basic Repeater
The dual antenna configuration allows the Repeater Station to receive and transmit on directional antennas. This greatly improves coverage and distance in an AIS network.

[DeepFrz]

Very interesting.

I think we have two things happening. Tropospheric Ducting, for the long range stuff and repeaters for shorter re transmissions.

I didn't think they would re transmit on the same frequency. Learn something every day.

[MAC-A-TAC]

You wouldn't happen to have an Internet connection. The software supports targets captured by shore stations.

[buzzstar]

What is the concern? So what if you happen to be able to identify and see the position of a ship of other vessel hundred of miles away from your position? It is nice to find our about the possible reason(s), but I would be concerned only if vessels that should appear do not.

[Jim Cate]

We too have on occasion noted very long range AIS targets, once at over 1300 miles (near Hobart to Cape Leeuwin), and I believe this is due to ducting, not skip, not reflections from aircraft and not repeaters.

Reasoning: the conditions lasted for several hours, transcending sunset, and thus major changes in the ionosphere, which pretty well eliminates skip. Aircraft don't linger about in the same area nearly that long, thus not scatter off Qantas's 747 or such. And if it was from repeaters, it would be consistent, not so episodic.

Re repeaters: we often have well over 100 targets within normal range. If you start adding distant targets to this mess, even greater data overload will occur, and with no benefit to us, for we have no need to see distant targets. We normally see ship based class A transmissions at 30-40 miles, and this is already way more range than is useful for practical collision avoidance. For such as us, longer range is useless (although sometimes interesting!).

Jim

[Paul L]

Not saying that repeaters are necessarily a good thing, just that they do exist in some locations. The ones in the Galapagos are on high mountains and do rebroadcast. There is no need to use multiple frequencies when rebroadcasting as the info is saved digitally and each channel is broken up into time slots, so a msg can be received in one time slot and retransmitted in a latter timeslot.

[Jim Cate]

There is no need to use multiple frequencies when rebroadcasting as the info is saved digitally and each channel is broken up into time slots, so a msg can be received in one time slot and retransmitted in a latter timeslot.
__________________

So, what happens w hen a rx that is within range of both the ship and the repeater get two identical but time separated messages? Does that cause confusion in the rx data processing? I have little knowledge of how the rx "parses" all the incoming data, but that seems like it could cause trouble.

Jim

[Wanderlust]

This morning the number of long distance targets had reduced a lot, and now I see only 4 at over 100 miles, with one at 210 miles. Thay are all Class A offshore from Hydrographers Passage. Apart from them, it's looking normal.

As first noted by MarkJ, AIS transmits on 2 frequencies (alternately?), so I can imagine authorities probably do monitor ship movements through the reef passages using this feature. My charts shows there is a 38M tower at Hydrographers Passage, so that is within reason.

Responding to a couple of comments:
Buzzstar: "What is the concern?" No concern, just thought it was surprising.
"Targets is the wrong term". Perhaps. I just used the term from OpenCPN.

MAC-A-TAC: "Internet connection". I do have internet but there is no feed of such data to OpenCPN ... if I turn off my AIS they soon disappear. Didn't know OpenCPN could accept internet feed of AIS.

[Seeking Solace]

A simplex repeater doesn't require a frequency pair as it is not full duplex. It is a store and forward type of system.


However I don't believe that is what you experienced. It's Tropospheric Ducting.

Tropospheric propagation occurs when signals are reflected scattered or refracted in the troposphere. "Ducting" effects occur primarily because of temperature inversions at a height of between 500m and 1500m, and occasionally up to 3000m. Such inversions are not caused by local "weather" or terrain, but rather by climatic conditions such as frontal boundaries. Temperature inversions are usually layers rather than sharp lines separating regions of different temperature.
The thickness of the inversion layer affects the LUF (Lowest usable frequency) - thinner layers only propagate higher frequencies such as microwaves, thicker layers can be used into lower frequency regions of the spectrum.

This table shows inversion thickness and LUF data.
File:Vk4yeh tropoducting.jpg - Amateur-radio-wiki

This information is fromTropospheric ducting - Amateur-radio-wiki

[Paul L]

Originally Posted by Jim Cate

So, what happens w hen a rx that is within range of both the ship and the repeater get two identical but time separated messages? Does that cause confusion in the rx data processing? I have little knowledge of how the rx "parses" all the incoming data, but that seems like it could cause trouble.

Jim

I doubt there's an issue if a receiver receives two identical msgs 500 ms apart.

Similar to there not being an issue if a receiver misses a packet sent. It just picks it up on the next transmission.