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Old 02-11-2016, 15:39   #46
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Re: AIS shows many long distance targets

Quote:
Originally Posted by Paul Elliott View Post
For most message types the timestamp only gives the nearest second. This would probably not be sufficient to disambiguate repeated transmissions, and I am not aware of any receiving software that could use this timestamp for that purpose.
Repeated transmissions will have the same timestamp, it's not a case of disambiguating them, it's a case of disregarding a second one received within a short period with the same timestamp.

But as I pointed out above. It turns out that there is also a Repeat Indicator which serves the purpose.
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Old 02-11-2016, 15:58   #47
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Re: AIS shows many long distance targets

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Originally Posted by StuM View Post
Repeated transmissions will have the same timestamp, it's not a case of disambiguating them, it's a case of disregarding a second one received within a short period with the same timestamp.

But as I pointed out above. It turns out that there is also a Repeat Indicator which serves the purpose.
It's the same data, so there really isn't anything ambiguous about it.
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Old 02-11-2016, 18:39   #48
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Re: AIS shows many long distance targets

Stu
I just looked up the Repeat Indicator. I've just ignored in my non-commercial decoding code. Looks like it is an indicator to the receiver to retransmit the msg. It has a count associated with it so the msg will not be endlessly retransmitted.
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Old 09-11-2016, 13:22   #49
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Re: AIS shows many long distance targets

Wanderlust,
1) Sorry I'm a week late here....but Bill ("btrayfors") did give you the answer in brief, just a few hours after you posted your question....so, not too much more to say...
Quote:
Originally Posted by btrayfors View Post
Definitely VHF atmospheric ducting. This is a very common and well-known phenomenon. No repeaters involved.

The VHF signals, usually in paths over water, travel thru atmospheric layers ("ducts") over long distances, often 300-400 miles. The condition comes and goes; doesn't last long.

Bill
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But, then I saw a lot more discussion....and thought, hmmm....(and then I go busy with other stuff, and forgot...




2) So, if anyone is interested in detailed (and LONG) answer....have a look at this thread, where it all laid out in detail...
http://www.cruisersforum.com/forums/...ge-149499.html



3) But, what if you don't want to click on that link and read a lot of "stuff"??
Well, how about I summarize...

Although, most will call what you experienced "ducting" (and there is a slight possibility you were on the edge of a tropo duct), the more accurate description of this type of long-range VHF communications is:
Tropo-Enhancement (usually just called "Tropo", but scientifically named: "non-ducting tropospheric enhancement"....)
[now you know why almost everyone just calls it "tropo"!]

But, if anyone just wants to call it "ducting", I won't try to argue...
And, you can stop reading now....

This "tropo" can also co-exist along side a "duct", but usually the "duct" is short-lived and conditions return to more normal (or just "tropo-enhanced") after 3 - 12 hours....(sometimes days, but that's very rare!!)

"Tropo-enhancement" generally effects wide areas / large areas, a few hundred miles across....and sometimes as far 500 - 700 miles...but almost always large areas....and, these areas of tropo-enhancement can last for days...
"Ducts" are almost always long and narrow....usually only 50 - 100 miles wide, but almost always 500 - 1200 miles long...and except in some rare locations and times, they are usually short-lived...


Here are some details...
Quote:
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.

Alhough, "tropo-enhancement" can effect signals from great distances and allow for VHF communications farther than a few hundred miles, it is rare that these conditions extend more than 500 - 750nm, and when it does, it is using the highest part of the troposphere...and, with our 25 watt FM radios, it is unlikely that most will experience "tropo-enhancement" farther than that....
And, for us using Marine VHF, typical "tropo-enhancement" will show good signals from 100nm out to 300-400nm.....and rare instances out to 500nm - 750nm....



4) "Tropo-Ducting":
If you are communicating on VHF Marine radio farther than 500nm - 750nm, then you're likely using a "tropo-duct", but again except for some specific locales (such as Portugal / Morocco to Canaries, S. FL to Long Island, California to Hawaii, etc.) a "tropo-duct" is rare, and is also usually geographically fairly narrow....meaning that the distance covered might be quite long (1000 - 2000 miles), but the "duct" is typically fairly narrow (< 100 miles wide, and many times <50 miles wide...), and almost always over water, in areas of stable hi-pressure, but unlike "tropo-enhancement", a "tropo-duct" always uses the very lowest part of the troposphere (typically only 1000' - 3000' above sea level)....just like its name implies, it acts like a "duct", long and narrow, and close to the ground...

Typical "tropo-ducting" provides very strong distant signals, and many times these distant signals are as strong (or sometimes even stronger) than those in your local area...and a sure sign that you are working stations in a "tropo-duct" is that stations along the duct are all about the same signal strength, rather than the more distant stations being weaker....if you are working a station > 500nm away, and they are as strong (or stronger) than other stations along the same path that are only 100nm away, then you are working in a duct....
BUT...
But, as I write above, except for some specific geographical areas, "tropo-ducting" is rare!!!
So, most "beyond tropo-scatter" range VHF communications is via "tropo-enhancement", not tropo-ducting"...



4) Now Olddave, as for "sky waves" and "ground waves"???
Quote:
Originally Posted by Olddave View Post
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?
--- Except for a EXTREMELY RARE occurrences (once or twice, or a few times, every 10 - 11 years, when Es - Sporadic-E can happen at VHF hi-band, which I have worked) there is NO skywave communications at 156mhz VHF...

--- And, there is no "ground wave" on VHF frequencies at all...(not even much at the upper HF bands)....of course there is "direct wave" (line-of-sight) and "tropo-scatter"....but, no "ground wave"....

For details of VHF radiowave propagation, have a look here...
http://www.cruisersforum.com/forums/...ge-149499.html




Fair winds to all...

John
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Old 09-11-2016, 15:30   #50
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Re: AIS shows many long distance targets

4) Now Olddave, as for "sky waves" and "ground waves"???--- Except for a EXTREMELY RARE occurrences (once or twice, or a few times, every 10 - 11 years, when Es - Sporadic-E can happen at VHF hi-band, which I have worked) there is NO skywave communications at 156mhz VHF...

--- And, there is no "ground wave" on VHF frequencies at all...(not even much at the upper HF bands)....of course there is "direct wave" (line-of-sight) and "tropo-scatter"....but, no "ground wave"....

For details of VHF radiowave propagation, have a look here...
http://www.cruisersforum.com/forums/...ge-149499.html

Hi John,,

Thanks for responding to my questions from last week.

On Skywaves and VHF, I found this statement in the Australian GMDSS handbook.
"At VHF, UHF & SHF skywaves are not absorbed (by the ionosphere) to any great extent and travel through the ionosphere into space, thus enabling communications via satellite to be conducted.
My question for you is - are you aware of any actual use of this? I presumed that Marine Traffic and others are making use of AIS info received by satellite.

I googled VHF ground waves and of course there is a lot to read. Obviously the Australian GMDSS general operators course is kept simple. Direct waves as you say are not ground waves, however, they are often grouped with Ground Waves in texts because they travel within the area covered by Ground Waves.
Sun is shining here in Oz so better get on with the day.
Dave




Fair winds to all...

John[/QUOTE]
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Old 09-11-2016, 17:02   #51
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Re: AIS shows many long distance targets

Dave,
Yes, there are many services that use VHF, UHF, and SHF signals to/from the earth to satellites in orbit...and these are line-of-sight / direct waves...
(satellite TV, satellite paging, satellite phones, etc....satellite weather, imaging, etc....amateur radio, etc....there's a plethora of services that do this....)


Satellite-AIS is but one of these services...

(but, I don't know if Marine Traffic's free public-accessible service utilizes Satellite-AIS data....but if not, there are other services that do...)



FYI, I have used VHF, UHF, and SHF freqs to both communicate with satelittes and bounce my signals off the moon....but, that a whole 'nother discussion....
Have a look at this page, for the details of VHF, UHF, SHF propagation...
http://www.cruisersforum.com/forums/...ge-149499.html


Fair winds...

John
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Old 11-11-2016, 08:36   #52
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Re: AIS shows many long distance targets

In 2012 the World Radio Conference (WARC 12) resulted in changes to the VHF Marine Band frequency allocations to permit use of Channel 75 (156.775-MHz) and Channel 76 (156.825-MHz). Previously these two channels were not in use and served as guard channels to the adjacent Channel 16 (156.800-MHz). After WARC 12 the two channels are now available for use to send the LONG-RANGE AIS broadcast message, Message 27.

According to the relevant ITU recommendation, ITR-R M.1371-5, AIS Message 27 is:

Quote:
...primarily intended for long-range detection of AIS Class A and Class B SO equipped vessels (typically by satellite). This message has a similar content to Messages 1,2, and 3, but the total number of bits has been compressed to allow for increased propagation delays associated with long-range detection.
This provision appears to endorse and facilitate use of AIS transmission for global tracking of vessels by satellites in space. For more see the ITU document at

Recommendation ITU-R M.1371-5
Technical characteristics for an automatic
identification system using time division
multiple access in the VHF maritime
mobile frequency band

https://www.itu.int/dms_pubrec/itu-r...2-I!!PDF-E.pdf
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Old 01-12-2016, 04:26   #53
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Re: AIS shows many long distance targets

Those atmospheric conditions are at it again ... thanks to those who confirmed and explained about ducting.

Tonight I am anchored inside Curtis Island, about 20NM north of Gladstone, and am picking up ships as far north as PNG, 900NM away. There are 95 at least 100NM away.

However I suspect some may be a case of repeaters because when I was further north I would fequently see a few near Hydrographers Passage, where there is a 38 metre tower. No doubt the authorities would want to keep track of movements through the reef. There are towers on several of the outer reefs.

continuouswave, I doubt this is the new channel 75 and 76 system because I just have a basic Vesper XB8000 system which would not use those frequencies.
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Old 01-12-2016, 10:19   #54
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Re: AIS shows many long distance targets

From a post very early in this thread:

Quote:
Originally Posted by denverd0n View Post
...the AIS frequencies are very close to the amateur radio 2-meter band, and I know that there are a lot of 2-meter repeaters out there, so maybe some of those are forwarding the signal along.
It would be extraordinarily unlikely that an amateur radio repeater at 145-MHz would be able to perform a repeater function for AIS. First of all, the typical amateur radio repeater has an extremely narrow band receiver, listening only to a very specific and narrowly tuned input frequency. Second, the typical repeater is not a frequency translator that directly transforms the input to the output frequency. Instead the input signal is demodulated to baseband audio, and that audio is then modulated onto the output transmitter. On that basis, it would be impossible for an amateur radio repeater to retransmit an AIS signal.
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Old 28-10-2018, 19:04   #55
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Re: AIS shows many long distance targets

I found this thread as I was curious about AIS repeaters. For the last few days at Middle Percy island in Queensland I have been seeing ais targets at better than 20Nm. Never seen that here before. Targets are visible down at Island Head Creek and on the opposite side of the island which is hundeds of meteres high.
Does anyone know if an AIS repeater has been installed here? There is already a normal vhf repeater on the hill.
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Old 28-10-2018, 19:25   #56
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Re: AIS shows many long distance targets

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Originally Posted by swampbush View Post
I found this thread as I was curious about AIS repeaters. For the last few days at Middle Percy island in Queensland I have been seeing ais targets at better than 20Nm. Never seen that here before. Targets are visible down at Island Head Creek and on the opposite side of the island which is hundeds of meteres high.
Does anyone know if an AIS repeater has been installed here? There is already a normal vhf repeater on the hill.
I can answer my own question. It is on high peak island and is registered to AMSA.
https://web.acma.gov.au/rrl//site_se...SITE_ID=402952
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Old 28-10-2018, 19:35   #57
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Re: AIS shows many long distance targets

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I can answer my own question. It is on high peak island and is registered to AMSA.
https://web.acma.gov.au/rrl//site_se...SITE_ID=402952
Sorry, getting excited, that is an aid to navigation using ais. But I am sure there is a repeater here somewhere...
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Old 30-10-2018, 18:08   #58
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Re: AIS shows many long distance targets

AIS uses a reserved VHF channel, so it's entirely possible that the repeater is repeating it.
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Old 30-10-2018, 19:01   #59
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Re: AIS shows many long distance targets

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Originally Posted by swampbush View Post
Sorry, getting excited, that is an aid to navigation using ais. But I am sure there is a repeater here somewhere...

According to this site, there's a repeater:


https://www.e-navigation.nl/content/...eak-island-ais
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Old 31-10-2018, 14:46   #60
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Re: AIS shows many long distance targets

Thanks for the link.
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