Class B AIS

[colemj]

Originally Posted by El Pinguino

Here are the results of my little bit of research...
The northern most boat, Laissez-faire, is about 14 miles from St Heliers - the monitoring station for the three Class B boats to the north and east ....
The most westerly one, Equilibrium , is being monitored by Torbay.
St Heliers is at the bottom of the shot.... Rangitoto island is between the yachts and the shore station.

Sort of scotches the idea that Class B can only be received out to 5 miles.

Not sure I understand this - you have no data on the height of the receiving or transmitting stations. Without that, there isn't much conclusion to be drawn.

I know I have visually seen class B transmitting boats before I saw them on AIS.

I have also seen boats disappear half way up to the spreaders in waves - wonder what that does to AIS transmission from deck level? I guess it isn't a problem if one times the transmissions to the wave peaks…

What about heeled over?

I don't understand why having navigation lights and VHF antenna up high is considered good, safe operation, particularly on a small sailboat, while having the AIS antenna high is not.

Mark

[Dockhead]

Originally Posted by colemj

But you did have a valid point about the digital aspect of AIS - no doubt there is signal processing and filtering going on in the receiving end that increases the S/N more so than is done on the voice end.

Mark

Yes, and in general digital signals are more robust than analogue ones -- just think about the guys with CW -- morse code -- or PSK31 -- using 1 watt or less , and talking halfway around the world. And FM -- the modulation scheme used in marine VHF radio -- is the least robust of all (best quality, but least robust), because of the "capture effect".

[J Clark H356]

I have the DigitalYacht IAIS Receiver only. My antenna is on the top of my mast and that puts it at the top at 58.4 feet above the water, and the top of my masthead at about 55 feet. I use a splitter, but don't remember which model I used. It seems to be working just fine for both AIS and VHF. I got Class A signals last February, while anchored in Indian Key in SW Florida at 155NM. There were two freighters near each other south of the Dry Tortugas and heading west. This was a fluke, but I regularly pick up 35 to 40 miles in open water. I also get 35 miles from my home marina at Kentucky Dam (Lighthouse Landing) and connect with tows at Metropolis, Ohio on the Ohio River west of Paducah.

I get my display by USB output to a laptop running RayTech 6.0, but it only displays A signals. I get it on my IPhone or IPad (only one at a time) on INavX, where I see both A and B signals. I haven't known that Class B would have less range, but have not noted my distance from B's, so it could be less. I've been more worried by the big guys than Class B boats and have not had collision courses with any B boats to date. I was disappointed while crossing the Northern Gulf near Cape San Blas that the 8 shrimp boats I was tracking did not have their AIS on. With my radar only and MARPHA, I could miss them, but it was a confusing 30 minutes the way they were zig zagging across the area. Somebody told me that fishing boats aren't required to run AIS. They should.

When I was transiting the Tenn-Tom coming back up last spring I used a Shakespear antenna tuned for AIS and mounted it on a 10 foot piece of pvc pipe u bolted to a stanchion. This put the antenna at around 14 feet above the water. This is a different antenna than my mast mounted one. I was getting hits at 8 to 10 miles and attributed that to the lower antenna height and trees, etc along the river bank. That distance was still plenty good on the river as the closure rate on the river would give me at least 30 to 45 minutes to plan where I wanted to pass, or I would know I had a wait when I got to the next lock.


Sent from my iPad using Cruisers Sailing Forum

[El Pinguino]

Originally Posted by colemj

Not sure I understand this - you have no data on the height of the receiving or transmitting stations. Without that, there isn't much conclusion to be drawn.

Well there is a conclusion to be drawn..... the power output of a Class B will go an adequate distance... the limiting factor is in the height of the transmit and receive antennas.
The info re the receiving stations is on marinetraffic.com
St Heliers is at 25 metres, Torbay is at 50 metres. Not all that high really.

[colemj]

Originally Posted by El Pinguino

Well there is a conclusion to be drawn..... the power output of a Class B will go an adequate distance... the limiting factor is in the height of the transmit and receive antennas.

Yes, I agree - but that is a well-established principle. I am more interested in the difference between transmission range from a class-B antenna mounted within a meter or two from the water and one mounted 15 meters up - and how they are received by antennas mounted at similar heights.

Reception of class A will be adequate no matter what height the receiving antenna is. Noting how many miles away one can receive a class A ship is not very relevant. More meaningful is how far away your signal is received.

I agree with those that say they will make course corrections from large ships well before those ships see them on AIS - that is our strategy also (I view a ship needing to make a course correction to avoid us as a failure on our part).

However, I want that small boat single-hander who is asleep to have their CPA alarm ring as early as possible so that they have sufficient time to shake the cobwebs, get oriented and be in the moment for a potential crossing. I also want to see them with as large amount of TCPA as possible so I can maybe make it so their CPA alarm doesn't ring at all…

Mark

[El Pinguino]

I must admit my research was rather un-scientific having been completed over morning tea with a sample of 3.
I was responding to post #2 where it was stated 'ClassB xmit is designed for 5 mile range.'.
That might be the class requirement but the reality is a bit different... a bit like the 'use by' dates on tins of Spam
El Ping

[colemj]

Originally Posted by El Pinguino

I must admit my research was rather un-scientific having been completed over morning tea with a sample of 3.
I was responding to post #2 where it was stated 'ClassB xmit is designed for 5 mile range.'.
That might be the class requirement but the reality is a bit different... a bit like the 'use by' dates on tins of Spam
El Ping

Sorry, I didn't mean to argue while you were uncaffeinated. I agree that "designed for 5nm range" is silly - it is a 2W signal, period. The range depends on other factors - height, gain, atmospheric conditions, etc.

If it was stated "designed for 5nm line of sight using 3dB RX and TX antennas at 3 meter heights", then it would be a bit more meaningful.

Mark

[Ziggy]

Originally Posted by continuouswave

AIS transmissions are one-way broadcast transmissions. Please explain how there is error correction.

I don't know if the AIS standard includes error correction, but error correction is quite possible with one-way broadcast transmission. Look up Hamming code, for example.

[continuouswave]

Originally Posted by Dockhead

You are right, and I stand corrected. There is no error correction in our AIS systems. Thanks for clearing this up.

I'm not sure where I got that idea, except that it is often discussed in connection with space-borne AIS systems.

It is not strictly true, however, that AIS transmissions are "one-way broadcast transmission" only. Various aspects of the system involve two-way communication. AIS stations can interrogate one another, and base stations can order mobile stations to change frequencies or timing.

It is possible to incorporate error correction in a one-way transmission, and this is called forward error correction. An interesting paper by FIDUS corporation, "AIS: Guide to System Development," says forward error correction is not used in AIS. (See section 3.1)

http://www.fidus.com/downloads/hardw...March_2009.pdf

I believe some sophisticated algorithms have been developed that are able to process received AIS data in such a way as to provide some error correction, but I doubt that a typical AIS receiver that sells for $200 incorporates these methods.

When I said that AIS transmission are broadcasts, I think they really are. Each AIS transmitter sends its message, and does not expect a reply. Someone receiving an AIS message can't reply to the sender and ask for repeats or fills for the message they received. Base stations can send commands, but I think they just send them, and they don't wait for acknowledgement.

Re the AIS Satellite receivers, they have a special problem because they may be receiving transmissions from more than one self-organized cell. The transmissions might interfere with each other. I think there are methods of processing the signals to overcome that, but I think it is a proprietary method.

[roetter]

I have done some testing recently with my em-trak AIS B100.

The boat is located in a marina deep in Tsehum Harbor. With the antenna on top of the bimini the Marine Traffic station in Roche Habor does not see my boat. Switching to the mast top VHF antenna Marine Traffic shows the boat.

Roche Harbor antenna height 41m
Mast top antenna height 20m
Bimini top antenna height 2m



I love the Vesper XB8000 on my other boat. It is integrated with the Raymarine plotters via NMEA2000 to Raymarine adaptor cable. I use the WIFI connection to the iPad for setup and Watchmate App display. The best thing I found was the XB8000 with the B&W display, as the display is very low energy and I leave it on all the time. I use it for the anchor watch. Checking on the Vesper site today I do not see the stand alone B&W display any more, just the integrated systems. Too bad.

[continuouswave]

Regarding the range of an AIS Class B transponder:

The range of any communication circuit is based on the transmitter power, antenna gain, path loss, receiver antenna gain, transmission line loses, and receiver sensitivity.

For an AIS receiver, the sensitivity is typically specified as better than -107dBm for 20-percent packet error rate (PER).

See: http://digitalyacht.co.uk/files/AIS%...ON%20TABLE.pdf

A typical FM VHF voice receiver is rated for a sensitivity of 0.25-microVolt for 12dB-SINAD. Converting this voltage to dBm for a 50-Ohm antenna system results in -107dBm

See: continuousWave: Whaler: Reference: Marine VHF Radio Communications

We see that the sensitivity of a VHF Marine Band voice receiver and AIS receiver are rated the same.

The path loss will be the same for both systems. If the transmit and receive antennas and their feedlines have the same gains and losses, these variables will be the same for both VOICE and AIS transmission.

The remaining variable in the Path Loss equation is transmitter power. Typically the VOICE transmission will be at 20-Watts while an AIS Class B will be at 2-Watts. This gives a 10-dB advantage to VOICE compared to AIS.

The only other factor to consider is the readability of the signals. The VOICE signal is interpreted by human hearing. The AIS signal is demodulated by a combination of hardware and software. In both cases there is some variability. Not all humans can copy a weak signal on a radio receiver at the same readability, and I am also certain that some variation exists among AIS receivers.

I don't see a particular advantage to the AIS transmission compared to VOICE, as the VOICE transmission has a 10-dB greater signal while the receiver sensitivities are equal. An improvement in signal of 10-dB often represents a very marked improvement in readability, either with demodulation by human ear or by machine.

On this basis, I am not inclined to accept the notion put forth earlier in the discussion that AIS can go farther than VOICE, and particularly in the comparison of a 20-Watt VOICE signal to a 2-Watt AIS signal.

If comparing a 2-Watt VOICE signal to a 2-Watt AIS signal, the received signals are now equal. In this case there may be some advantage to the AIS signal because it will be demodulated by hardware and software, rather than by ear.

[MarkJ]

Originally Posted by continuouswave

On this basis, I am not inclined to accept the notion put forth earlier in the discussion that AIS can go farther than VOICE,

Then I put it to you that you do not have an AIS.


Mark

[Dockhead]

Originally Posted by continuouswave

Regarding the range of an AIS Class B transponder:

The range of any communication circuit is based on the transmitter power, antenna gain, path loss, receiver antenna gain, transmission line loses, and receiver sensitivity.

For an AIS receiver, the sensitivity is typically specified as better than -107dBm for 20-percent packet error rate (PER).

See: http://digitalyacht.co.uk/files/AIS%...ON%20TABLE.pdf

A typical FM VHF voice receiver is rated for a sensitivity of 0.25-microVolt for 12dB-SINAD. Converting this voltage to dBm for a 50-Ohm antenna system results in -107dBm

See: continuousWave: Whaler: Reference: Marine VHF Radio Communications

We see that the sensitivity of a VHF Marine Band voice receiver and AIS receiver are rated the same.

The path loss will be the same for both systems. If the transmit and receive antennas and their feedlines have the same gains and losses, these variables will be the same for both VOICE and AIS transmission.

The remaining variable in the Path Loss equation is transmitter power. Typically the VOICE transmission will be at 20-Watts while an AIS Class B will be at 2-Watts. This gives a 10-dB advantage to VOICE compared to AIS.

The only other factor to consider is the readability of the signals. The VOICE signal is interpreted by human hearing. The AIS signal is demodulated by a combination of hardware and software. In both cases there is some variability. Not all humans can copy a weak signal on a radio receiver at the same readability, and I am also certain that some variation exists among AIS receivers.

I don't see a particular advantage to the AIS transmission compared to VOICE, as the VOICE transmission has a 10-dB greater signal while the receiver sensitivities are equal. An improvement in signal of 10-dB often represents a very marked improvement in readability, either with demodulation by human ear or by machine.

On this basis, I am not inclined to accept the notion put forth earlier in the discussion that AIS can go farther than VOICE, and particularly in the comparison of a 20-Watt VOICE signal to a 2-Watt AIS signal.

If comparing a 2-Watt VOICE signal to a 2-Watt AIS signal, the received signals are now equal. In this case there may be some advantage to the AIS signal because it will be demodulated by hardware and software, rather than by ear.

You are leaving out the all-important issue of how the signal is read. The final strength of the signal at the end of the receiver's processes is by no means the whole story, or even the main part of it. This argument is only slightly more valid than saying that I've got a good AF amplifier in this radio, and can get this signal up to the same strength as that one by cranking up the gain, therefore they are the same.

Reading a digital signal is like discerning a white versus black flag. It's, well, binary -- no need to discern shapes, tones, etc. Hearing and understanding an audio signal is like reading a newspaper -- it's not just on or off, you have to discern various things in it and read it. The fact that the flag and the newspaper are the same size, and are at the same distance, in the same light, and the same atmospheric conditions, does not begin to tell the story. You can discern the binary nature of the flags from many times the distance that you can read the same sized newspaper.

Your username implies you are a fan of CW and you are obviously a knowledgeable ham. Surely you have experienced for yourself the enormous practical difference in readability of a binary CW signal compared to a phone emission all sent and received with the same power and using all the same equipment? Every ham knows about this.

Furthermore, we are comparing AIS not to AM or SSB, but to FM -- which is subject to capture effect and which in general is not conducive to readability of weak signals (the reason why AM modulation is still used in aviation VHF, instead of FM, I hear told).

[Wotname]

Without gong into the technical details, it is a lot lot easier to extract digital (binary) data from background noise than analogue data.

[Wotname]

Originally Posted by Dockhead

............
Furthermore, we are comparing AIS not to AM or SSB, but to FM -- which is subject to capture effect and which in general is not conducive to readability of weak signals (the reason why AM modulation is still used in aviation VHF, instead of FM, I hear told).

Off topic so I won't go into detail here but the are several different reasons why aviation is still using double sideband full carrier AM for VHF coms but as generalisation, FM is always better than AM. We can take this via PM if you wish .