AIS surprise

[GILow]

So, how on earth did this work?

I was stuffing around with the display on the plotter, zoomed out to get a feel for my course and position and found an AIS target showing up from nearly 80 miles away. My understanding was that the maximum range should be around half that figure.

My AIS antenna is only 4 meters above the water.

I watched the boat for the next few hours and, to the best of my knowledge, I didn't lose contact with them at all in that time.

[Mike OReilly]

Maybe the sources has a really tall antenna. Could you see what it was?

[El Pinguino]

MMSI says she is 'CMA CGM Ural' https://www.marinetraffic.com/en/ais...l:CMA_CGM_URAL so a pretty high antenna.... counting the stacks maybe 120 feet above sea level.
Couple that with a big high over the eastern Bight at the time Latest Colour Mean Sea-Level Pressure Analysis ( click the 'animate' button....)

[Reefmagnet]

Originally Posted by GILow

So, how on earth did this work?

I was stuffing around with the display on the plotter, zoomed out to get a feel for my course and position and found an AIS target showing up from nearly 80 miles away. My understanding was that the maximum range should be around half that figure.

My AIS antenna is only 4 meters above the water.

I watched the boat for the next few hours and, to the best of my knowledge, I didn't lose contact with them at all in that time.
Attachment 231981Attachment 231982

Atmospheric conditions. At certain times I've picked up AIS signals spanning 700 nm along the coast. You'll find your VHF reception also starts to receive long distance signals when this happens.

[StuM]

A few forum threads have discussed similar VHF propogation issues at great length.



Quick summary:


VHF is essentially Line of Sight, but under certain conditions, you can pick up transmissiosn from dozens, hundreds and even thousands of miles away.


A wikipedia article to whet your appetite:
https://en.wikipedia.org/wiki/Tropos...pheric_ducting

[GILow]

Well I live and learn.

I wish it was something reliable.

I was also interested that it seemed I was only getting incomplete AIS data. I see this now and then, mostly from ships at anchor. Perhaps some of the encapsulated data was being lost in the bounce?

[Kelkara]

Originally Posted by GILow

Well I live and learn.

I wish it was something reliable.

I was also interested that it seemed I was only getting incomplete AIS data. I see this now and then, mostly from ships at anchor. Perhaps some of the encapsulated data was being lost in the bounce?

The AIS data gets broadcast as two separate messages. One message contains the dynamic data: position, speed etc i.e. all the info you need for navigation. The second message contains the static data such as the ship name, size etc. This second message is sent much less frequently. So when a ship first appears on your AIS, you will probably only get its position and it may be several minutes before the static data is broadcast, and until then you won't see the name of the ship etc. Also if the ship is on the limit of receivability you may only be receiving a fraction of the signals it is transmitting anyway, so you are quite likely to miss the static data.

[GILow]

Originally Posted by Kelkara

The AIS data gets broadcast as two separate messages. One message contains the dynamic data: position, speed etc i.e. all the info you need for navigation. The second message contains the static data such as the ship name, size etc. This second message is sent much less frequently. So when a ship first appears on your AIS, you will probably only get its position and it may be several minutes before the static data is broadcast, and until then you won't see the name of the ship etc. Also if the ship is on the limit of receivability you may only be receiving a fraction of the signals it is transmitting anyway, so you are quite likely to miss the static data.

Thank you, that explains what I have observed very well.

[chrisr]

Originally Posted by GILow

Thank you, that explains what I have observed very well.

assume that you have checked that you have not suppressed static targets ?

i do this when in a busy port and have on occasions forgotten to remove when heading out again...and wondered why all the anchored ships were not appearing on my AIS...

cheers,

[GILow]

Originally Posted by chrisr

assume that you have checked that you have not suppressed static targets ?



i do this when in a busy port and have on occasions forgotten to remove when heading out again...and wondered why all the anchored ships were not appearing on my AIS...



cheers,

Oh dear, good point, but thankfully there’s so little traffic around here that I’ve never had to use that function.

Looking at the waters around Hobart today though, I reckon that might have been a useful setting. [emoji846]

[JPA Cate]

You just wait till regatta day festivities are in full swing!

Ann

[GILow]

Originally Posted by JPA Cate

You just wait till regatta day festivities are in full swing!

Ann

You are just making me jealous. They'll be over before I get there at this rate.

[UFO]

Originally Posted by Kelkara

The AIS data gets broadcast as two separate messages. One message contains the dynamic data: position, speed etc i.e. all the info you need for navigation. The second message contains the static data such as the ship name, size etc. This second message is sent much less frequently. So when a ship first appears on your AIS, you will probably only get its position and it may be several minutes before the static data is broadcast, and until then you won't see the name of the ship etc. Also if the ship is on the limit of receivability you may only be receiving a fraction of the signals it is transmitting anyway, so you are quite likely to miss the static data.

That explains a lot - Thanks

[Dsanduril]

Australia has a pretty extensive network of shore repeater stations. These are intended to help “see around corners, over hills” and are used by governments for tracking. An AIS position message has a specific two-bit field that is incremented each time the message is forwarded by a repeater, thus a message can be forwarded three times, any repeater receiving a message with both bits sets knows it has already been forwarded three times and drops the message - it doesn’t get forwarded any further. If you can look at NMEA from the AIS unit or dig around on the target info you can see if the message was received directly (no forwarding bits set) or how many hops it has been forwarded. This greatly extends range and explains many long-distance contacts in places with AIS shore stations.

[GILow]

Originally Posted by Dsanduril

Australia has a pretty extensive network of shore repeater stations. These are intended to help “see around corners, over hills” and are used by governments for tracking. An AIS position message has a specific two-bit field that is incremented each time the message is forwarded by a repeater, thus a message can be forwarded three times, any repeater receiving a message with both bits sets knows it has already been forwarded three times and drops the message - it doesn’t get forwarded any further. If you can look at NMEA from the AIS unit or dig around on the target info you can see if the message was received directly (no forwarding bits set) or how many hops it has been forwarded. This greatly extends range and explains many long-distance contacts in places with AIS shore stations.

This is interesting, I’d not heard of AIS repeaters. Do you know of a map of the repeaters anywhere? I couldn’t find one with a bit of googling.