Class B AIS pros and cons

Much of what the article was talking abuot was not what you may or may not see on your own AIS display but on sites like marine traffic, which are dependent on a limited number of shore stations for information. If there is a shore station linked to marine traffic near a marina, then you'll see lots if info: on the other hand there isn't a station nearby, then there will be no information from boats in that marina.
 
When we cross the N Sea to Ijmuiden we are usually out of range of shore stations for only a short time or not at all. The main thing is to regard the system as an additional form of location and not one that is absolutely to be depended on. Nothing is 100% but you will always be better off for having it. Although we have almost got to the point of being disappointed to see a yacht without AIS, I am all in favour of people being free to sail in stealth mode if they wish.
 
For sailing in open waters, as opposed to congested, sheltered waters, I believe that AIS transponders are a good aid. They'll give another piece of valuable information which when used by a prudent navigator in conjunction with all other available aids will help to sail safely within the colregs.

It's an added bonus that, when available, loved ones ashore can also keep track of your progress via the interweb.

AIS transmits on vhf. I did City & Guilds amateur radio theory more than 35 years ago so I stand to be corrected but iirc it is line-of-sight propagation. So assuming sufficient power, a yacht with mast head aerial at, say, 20m asl will have a horizon at 9.3M. If a commercial vessel has its aerial at 30m it's horizon is 11.4M (Almanac table 3(1)). So the max theoretical distance that the two vessels can "see" each other is 20M.

Atmospheric conditions don't generally have such a big impact on vhf propagation (compared to hf) except for Sporadic E, which tends to be an early summer phenomenon and this can significantly increase distance but not reliably. As I said above, it's been a long time since I studied this, I don't believe that the laws of physics have changed but I'm regularly known for talking bolox.
 
davidjackson said:
For sailing in open waters, as opposed to congested, sheltered waters, I believe that AIS transponders are a good aid. They'll give another piece of valuable information which when used by a prudent navigator in conjunction with all other available aids will help to sail safely within the colregs.

It's an added bonus that, when available, loved ones ashore can also keep track of your progress via the interweb.

AIS transmits on vhf. I did City & Guilds amateur radio theory more than 35 years ago so I stand to be corrected but iirc it is line-of-sight propagation. So assuming sufficient power, a yacht with mast head aerial at, say, 20m asl will have a horizon at 9.3M. If a commercial vessel has its aerial at 30m it's horizon is 11.4M (Almanac table 3(1)). So the max theoretical distance that the two vessels can "see" each other is 20M.

Atmospheric conditions don't generally have such a big impact on vhf propagation (compared to hf) except for Sporadic E, which tends to be an early summer phenomenon and this can significantly increase distance but not reliably. As I said above, it's been a long time since I studied this, I don't believe that the laws of physics have changed but I'm regularly known for talking bolox.
Click to expand...

The strange thing is that many forumites believe that "line of sight" applies to AIS, even though I keep posting screenshots, and could post hundreds more, of my AIS screen showing dozens of targets up to 50 miles in coastal waters, dozens of targets up to 100 miles in open waters, and occasional targets up to 500 miles when atmospheric conditions are favouring propagation.

Richard
 
I installed and used AIS for the first time this year.
On the Humber and North sea the traffic is almost entirely ships, commercial vessels associated with the wind farms, the survey and pilot boats which are in regular use . Not surprisingly everyone wants to be on the move with the tide. I found AIS provided useful added information.
On the upper Humber / lower River Trent there are ships and I did see one or two on the screen that were around the corner before making visual contact.
VTS could certainly see me as they called me as I entered the Humber estuary from the sea , which is reassuring.
Once in the marina the AIS can be switched off, assuming it has been wired to a switch .

I don't see any downside to AIS in my area but fully understand on a busy day somewhere like the Solent the number of targets on the screen becomes an issue.

By the way VHF signals may sometimes though not always reliably be sent and received where there is no line of sight .

Regarding the internet sites.
Surprisingly my boat in Nottinghamshire was located on marine ais by an AIS receiving station on Northern Ireland . Mind you that was very much a fluke and has not been repeated. That particular station has an average range of about 30 miles and maximum about 100mles but it is at 510metres elevation.
I looked at info on a land based station with 6m elevation and it had an average range of about 13 miles with a much more variable maximum but occasionally up to 100 miles.
 
VHF propagation is not exactly line of sight.
It's nearer the truth that losses mount up quickly beyond line of sight, and can be very variable.
So the range to be relied on is not much more than line of sight.
Beyond line of sight, some signal diffracts around the curvature of the earth. Reflects off things. Refracts sometimes.
It is the big vague area in radio as far as I'm concerned.
There's lots of stuff in the textbooks, but it seems to boil down to 'it depends'....
At sea one of the big variables is sea state.

How far beyond line of sight will depend on how much extra loss the link can tolerate.
So transmit power and receiver sensitivity will come into it.

There's also always a possibility that you get all the targets at up to 20 miles and some at 50 miles, but don't necessarily get everything between 20 and 50...
 
I pick up class B stuff at about 2 miles and class A at 5 which is nothing to boast about but I find fine for collision avoidance in the channel. On an ocean crossing I might want more and would investigate better siting of the aerials.

After being a bit dismissive of Class B for yachts at close quarters, I found their transmissions helpful coming down the Chanel Du Four in fog this year. Not as useful as the chart plotter which allowed me to hack off across country to avoid the turning marks but a great reassurance. One of the rare occasions where a fog signal was of great value.

I have had enough of poor viz in the western Channel and will be fitting a transponder thing of some sort this winter, I hope.
 
lw395 said:
VHF propagation is not exactly line of sight.
It's nearer the truth that losses mount up quickly beyond line of sight, and can be very variable.
So the range to be relied on is not much more than line of sight.
Beyond line of sight, some signal diffracts around the curvature of the earth. Reflects off things. Refracts sometimes.
Click to expand...

Also, AIS like DSC is a digital transmission mode...... it is also repetitive and cumulative. Your AIS target information on your screen could be made up from any number of transmissions in fringe locations.

Digital radio modes will always exceed any voice radio .... you can transmit completely around the globe with 10 watts using morse code but only about 1/10th of that using voice.
 
Alpha22 said:
Also, AIS like DSC is a digital transmission mode...... it is also repetitive and cumulative. Your AIS target information on your screen could be made up from any number of transmissions in fringe locations.

Digital radio modes will always exceed any voice radio .... you can transmit completely around the globe with 10 watts using morse code but only about 1/10th of that using voice.
Click to expand...

To be pedantic, digital will need less power than FM when the bit rate is low, like in morse. The bit rate in AIS is much higher than morse, but still fairly low. Unlike DAB which falls over before broadcast FM for example. Apparently error correction in AIS never got implemented so we have to receive one complete burst to get a target. But as you say not every burst. AIUI anyway.
 
RichardS said:
The strange thing is that many forumites believe that "line of sight" applies to AIS, even though I keep posting screenshots, and could post hundreds more, of my AIS screen showing dozens of targets up to 50 miles in coastal waters, dozens of targets up to 100 miles in open waters, and occasional targets up to 500 miles when atmospheric conditions are favouring propagation.

Richard
Click to expand...

Fully agree. I get the same results to the point that often, I have to expand the screen so I can see the coast line. Presently heading west to GIB... Line of sight ??? No more.
 
This thread has drifted somewhat but it's very informative.

I know nothing about digital transmissions so I'm reading the comments with great interest.

Is the analogy to morse valid? I understand that morse can "get through" when voice can't but iirc it's due to the relatively simple transmission of the carrier wave ( morse is often called cw). Morse is generally amplitude modulated with filtered out upper and lower sidebands. The receiving stations picks up the carrier and with a beat frequency oscillator adds the tone. Also, as 50% of an AM signal is the carrier, filtering out the sidebands effectively doubles the output. Even more thread drift, a sideband has 25% of the AM signal, hence x4 the output compared to AM.

So, transmitting on a marine vhf band, simplistically, how is the AIS digital information modulated within the wave? I'm trying to grasp the theory behind some of the long distance claims?
 
Pros:
I can take more control over collision avoidance because I can see more clearly where the ships are and more crucially which way they are headed which is a real pain by eye alone.
I think it may occasionally make me more "visible" to ships but not in crowded waters where they suppress class B signals
In emergencies its flipping useful, first one ever this year and it left the coastguard and lifeboat in no doubt as to exactly where we were. The position at mayday was irrelevant as the crew was high-tailing it back to port with the casualty, the transponder meant the lifeboat knew where to meet us without endless chatter and mistaken digits. Similarly if we were able to go to another boat's aid and they had a transponder I'd know where to go.
Off major ports I'm now finding that they've been tracking us for a mile a two, I've had VHF comments like "yes we've seen your approach" and it's vaguely reassuring that we're in the collision avoidance plan.

Cons:
My parents now know my every move :D actually it's quite sweet when I get a text message after passing Dover saying "you got a bit close to that dredger son"
 
davidjackson said:
This thread has drifted somewhat but it's very informative.

I know nothing about digital transmissions so I'm reading the comments with great interest.

Is the analogy to morse valid? I understand that morse can "get through" when voice can't but iirc it's due to the relatively simple transmission of the carrier wave ( morse is often called cw). Morse is generally amplitude modulated with filtered out upper and lower sidebands. The receiving stations picks up the carrier and with a beat frequency oscillator adds the tone. Also, as 50% of an AM signal is the carrier, filtering out the sidebands effectively doubles the output. Even more thread drift, a sideband has 25% of the AM signal, hence x4 the output compared to AM.

So, transmitting on a marine vhf band, simplistically, how is the AIS digital information modulated within the wave? I'm trying to grasp the theory behind some of the long distance claims?
Click to expand...

It's a modified form of FM (Frequency Modulation) called GMSK (Gaussian Minimum Shift Keying if you must ask!) as is also used in cell phones. It's primary advantage is that it minimises sidebands compared to simple FM reducing co-channel interference

Whlst the effect is similar, the reasons both CW morse and GM digital can achieve greater usable range from lower power compared to voice transmissions are different

In both cases the transmissions are much less affected by noise (in CW one could say unaffected) so at ranges where a 2W voice transmission would be unintelligible, the digital signal can still be received and decoded successfully as noise primarily affects amplitude rather than frequency
 
Bru said:
It's a modified form of FM (Frequency Modulation) called GMSK (Gaussian Minimum Shift Keying if you must ask!) as is also used in cell phones. It's primary advantage is that it minimises sidebands compared to simple FM reducing co-channel interference

Whlst the effect is similar, the reasons both CW morse and GM digital can achieve greater usable range from lower power compared to voice transmissions are different

In both cases the transmissions are much less affected by noise (in CW one could say unaffected) so at ranges where a 2W voice transmission would be unintelligible, the digital signal can still be received and decoded successfully as noise primarily affects amplitude rather than frequency
Click to expand...

Many thanks. I would be telling porkies if I said that I understood but what you've written makes sense. I'm pleased to read that basic radio CW theory and GM digital are different in terms greater usable range. 100M from a vhf signal remains a difficult concept for me to grasp (I'm not disbelieving the claims, I'm trying to understand the reason).
 
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