Class B AIS

[continuouswave]

Originally Posted by transmitterdan

II would be interested in which NOAA call sign you can hear 100 miles away and what is your receive antenna height above average terrain.

ASIDE to Dan:

My antenna is about 20-feet above terrain; it is on a mast on the garage roof. It is a vertically polarized Yagi with a bit of gain; it is just three elements. The feedline is RG-213. For details on reception, see

Receiver Testing with NOAA Weather Radio Broadcasts - Moderated Discussion Areas

The antenna orientation is fixed; no rotator, so I can't turn it to point at NOAA stations. KHB97, Belleview, Ohio, at 80-miles is roughly on the main lobe, and comes in almost every day very readable. A couple of stations off the side of the main lobe at over 100-miles come in on the better days. My terrain is all urban terrain for at least the first 20-miles, and my location is not particularly advantageously sited for elevation. Many factors affect reception, including vegetation on the trees and particularly if rain and wet vegetation. But on a nice dry day in fall after the leaves are gone off the trees, reception is quite impressive.

On my boat I routinely copy NOAA stations at 70 to 80-miles. See

continuousWave: Whaler: Reference: Assesing Antennas Performance

For details about the house antenna, see

continuousWave: Whaler: Reference: AIS Antenna

Best AIS DX so far is 54-miles--and that vessel was right on the antenna main lobe.

Also, I have a large mono-band television 10-element Yagi so I can watch a station over the border in Canada, CBET-9 Windsor. It is a 29-mile path, very urban terrain, looking right through downtown area with fringing path interference from high-rise buildings, and looking right at a local station just a few miles away on CH-7 with enormous signal level; without a big notch filter to knock down the local station, reception would be very difficult if not impossible. That's there just to be able to get Hockey Night in Canada in HDTV with Dolby Digital 5.1 Surround Sound on Saturday nights in the winter. It typically gets about 20-dB carrier to noise ratio on the HDTV transmission, which is just enough to get a solid lock and error-free demodulation of the advanced television signal. Otherwise I'd have to watch on cable, with lower image quality due to re-encoding and reduced bandwidth on cable retransmission, and lousy audio, too.

[colemj]

Originally Posted by continuouswave

That's there just to be able to get Hockey Night in Canada in HDTV with Dolby Digital 5.1 Surround Sound on Saturday nights in the winter.

Finally, someone using their gear in anger and with proper perspective!

I would turn our whole boat into a giant Yagi if I thought there was a chance to get Hockey Night in Canada down here.

Do they still have Peter Puck?

Mark

[Dockhead]

Originally Posted by continuouswave

I must be on a different planet. I never make radio check calls to see what my radiotelephone range might be. I find little value it randomly getting reports from other stations. I would have no idea what sort of radio, antenna, mounting height, feedline loss, and so on, that other station might have. I find little value in getting a "radio check" with some unknown other station. I evaluate my transmitter by measuring the power output with a accurate directional wattmeter. I evaluate the antenna by its VSWR and by how well it receives. I can evaluate the transmitter modulation by listening with another radio. It is quite simple to determine if my radiotelephone is working: if I call another station that I can hear, he should hear me. If he doesn't hear me, I'll be alerted to a problem. Radiotelephone communication is a two-way communication.

AIS is completely different. AIS is a one-way transmission. AIS just sends a broadcast. There is no mechanism in the AIS system to know if anyone has received your signal. I don't think your proposal of calling other vessels on radiotelephone to check if they received your AIS transmission is really a workable method. Imagine if everyone did this all the time.

You can evaluate the range of your AIS system by what signals it receives. If you can receive Class-B AIS targets at a certain range, they should be able to see your Class-B AIS signal at that same range, assuming you don't have some problem with low transmitter signal output. The propagation loss, feed line losses, and antenna gains are reciprocal.

Concerning radio checks: I always do it with the same Coast Guard station, not random vessels, so it's a consistent antenna and equpiment setup, professional operators, etc. I initiate the call with DSC to check that system as well. How else can you tell that you have a problem developing? I think it's pretty basic procedure to do regular radio checks. I have also started doing QRP calls to them from across the Channel -- a real test of the antenna and cabling at 1 watt and 60-odd miles. Without doing procedures like this, I have no idea how you could catch a developing problem. It's certainly the way I was taught.

And why wouldn't you do the same thing with AIS? I usually ask the Coast Guard to have a look and see if they see me on AIS at the same time I do radio checks, but I also like to know whether ordinary ships are picking me up at the range I expect, or whether a problem is developing, or whether I just don't understand how the system performs. I get calls like that all the time, and am always very happy to respond. When I call ships with this request, I am always asked the same question back -- I think most people want to know. How would you even know whether your system works if you never ask anyone if they see you?

A little thread drift: But it is amazing what low penetration DSC has even to this day, after all these decades. I doubt that even 10% of ships respond to DSC calls, although they respond perfectly well to a regular hail on 16, when you know their name. The only station I ever call which always answers DSC calls is the Coast Guard, and they are always very happy to get a call on DSC rather than someone cluttering up 16.

[Dockhead]

Originally Posted by poiu

What have you found? Has your new antenna helped distant boats to see you?

Well, I have two new antennae. The VHF antenna at the masthead -- a Shakespeare Galaxy internal dipole fed by a continuous run of RG213 with no connectors except at the radio -- is brilliant. The difference to the old one is so dramatic that I have started using an attenuator (LOC switch) in order not to be overwhelmed with chatter from France, and I usually transmit at 1 watt. I never even dreamed the difference would be so great.

The AIS uses a Diamond antenna, also fed by RG213. I have nothing to compare it to, since this is my first AIS set. Ships see me usually from about 20 miles off; ground stations from much further than that. I see ships from much further away than that. Class "A" has more power and better equipment.

[continuouswave]

Originally Posted by Dockhead

Concerning radio checks: I always do it with the same Coast Guard station, not random vessels, so it's a consistent antenna and equpiment setup, professional operators, etc. I initiate the call with DSC to check that system as well. How else can you tell that you have a problem developing? I think it's pretty basic procedure to do regular radio checks. I have also started doing QRP calls to them from across the Channel -- a real test of the antenna and cabling at 1 watt and 60-odd miles. Without doing procedures like this, I have no idea how you could catch a developing problem. It's certainly the way I was taught.

And why wouldn't you do the same thing with AIS? I usually ask the Coast Guard to have a look and see if they see me on AIS at the same time I do radio checks, but I also like to know whether ordinary ships are picking me up at the range I expect, or whether a problem is developing, or whether I just don't understand how the system performs. I get calls like that all the time, and am always very happy to respond. When I call ships with this request, I am always asked the same question back -- I think most people want to know. How would you even know whether your system works if you never ask anyone if they see you?

A little thread drift: But it is amazing what low penetration DSC has even to this day, after all these decades. I doubt that even 10% of ships respond to DSC calls, although they respond perfectly well to a regular hail on 16, when you know their name. The only station I ever call which always answers DSC calls is the Coast Guard, and they are always very happy to get a call on DSC rather than someone cluttering up 16.

I see the major problem: you are in the U.K. where the Coast Guard is still a maritime organization.

I am in the USA. The USA Coast Guard is now a military wing of Homeland Security. They don't want to be bothered with assisting boaters--no towing, no radio checks, no AIS checks. If I were in the U.K. I might call the Coast Guard and ask them for a courtesy radio report, but in the USA I doubt the Coast Guard would respond with anything other than a command to stop asking for radio checks from them.

Yes, it is best to perform a radio check with a reliable station. Half the boats on the water don't have radio installations that work very well. Your chances with a random station for getting an accurate check are about 50-50.

[transmitterdan]

Originally Posted by Dockhead

A little thread drift: But it is amazing what low penetration DSC has even to this day, after all these decades. I doubt that even 10% of ships respond to DSC calls, although they respond perfectly well to a regular hail on 16, when you know their name. The only station I ever call which always answers DSC calls is the Coast Guard, and they are always very happy to get a call on DSC rather than someone cluttering up 16.

My guess is the way DSC radios implement the call alert is part of the problem. Our radio makes such an awful racket when a DSC call comes in that if I were on a commercial vessel standing a watch in a busy seaway I would probably set the radio to only accept DSC calls from "known" MMSI callers. When ours goes off my wife asks if she should prepare to abandon ship. It sounds pretty similar to the high water bilge alarm.

Here in the US there are automated VHF stations in many east coast locations operated by one of the towing services. These "usually" work except when they are offline. They are not DSC but just simple voice. Whatever you say on the assigned channel is recorded and played back so you can check your own radio without bothering other boaters. Channels are 24, 26, 27 or 28 depending on your location.

In most coastal areas of the US the USCG supports DSC "test" calls to MMSI 003669999. Unfortunately there are a lot of DSC radios sold before the test concept was standardized so a lot of DSC radios don't have that feature. The service will not respond to "normal" DSC calls.

[colemj]

Originally Posted by transmitterdan

Our radio makes such an awful racket when a DSC call comes in that if I were on a commercial vessel standing a watch in a busy seaway I would probably set the radio to only accept DSC calls from "known" MMSI callers. When ours goes off my wife asks if she should prepare to abandon ship. It sounds pretty similar to the high water bilge alarm.

Our Standard Horizon rings like a telephone when a DSC voice call comes in. It does screech an alarm when a DSC emergency call comes in.

Our Standard Horizon hand held does the same.

Since these are our only experience with DSC, I didn't know others were preparing to abandon ship when I call them!

We love the DSC function and can't figure out why others do not use it - except for the fact that some radios require a working knowledge of Sanskrit to understand how to operate it. Again, this is dead-simple on our two Standard Horizon radios - select the name of the boat/mmsi and push "call".

It seems like in every anchorage there are several boats who are calling each other every minute. Each exchange requires hailing on the local hailing channel, negotiating a working channel, coming back to renegotiate because one of them is set on US and the other INT and they picked a wrong frequency, coming back again to negotiate because one of them misheard the other, etc.

Every friggin' minute of the day.

These people should be REQUIRED to get DSC - actually, most of them have it, but don't use, or don't know how to use, it.

Rant off (I was just listening to two of those boats doing this for the third time THIS MORNING when this thread came up).

Mark

[ka4wja]

I've steered clear of this discussion as it wasn't adding much to the general understanding of things....but, I thought it might be best to correct something here that is often misunderstood, and that is radiowave propagation....

This is not only one of my fortes in communications, but also something that I have actually taught in classes / seminars (as well as written papers on it...)

Don't worry, I wont ramble on and on about esoteric stuff that nobody cares about...
Just hope to clarify a few points...


Although we don't have to go to ridiculous limits, Dan is correct that reliable VHF propagation significantly beyond "radio-line-of-sight", does require higher ERP's (high power / high gain antennas) which does NOT really apply to our marine VHF system....
{Please note, this has NOTHING at all to do with "tropo-enhancement"."tropo-inversion", tropo-ducting, etc....but this is reliable 24/7/365 propagation well beyond VHF-radio-line-of-sight....}
But, unfortunately Dan this is NOT "groundwave" at all....it is called "tropo-scatter", and is often misunderstood....

Originally Posted by transmitterdan

If you want to go to ridiculous limits then yes, more power will go farther. But that isn't practical for VHF marine ship-ship communications because it would require 50-100 times the legal power limit.

There is a small groundwave component to VHF that most can safely ignore but when the transmit power is large enough it matters.

"Tropo-scatter" is a regular / normal mode of VHF (and UHF and SHF) radiowave propagation and does NOT require / use any special atmospheric enhancements, nor temperature inversions, nor ducts, at all....but rather is a very reliable (24/7/365) mode of VHF propagation that allows communications well beyond "vhf-radio-line-of-sight", at all times!!
With relatively inexpensive radios, power outputs of 100 - 200 watts, and small / modest sized antennas, VHF troposcatter can provide reliable 24/7/365 comms out a few hundred miles on CW/SSB, and 100+ miles on FM....(although the occupied bandwidth of FM is a limiting factor, using higher power does work....)
The reason I'm mentioning this is....this is NOT groundwave (aside form the slight diffraction that allows the VHF radio waves to travel beyond the visual line-of-sight there is no actual "groundwave" on VHF), NOR is this "tropo-enhancement" / "tropo-inversion", NOR "tropo-ducting"....
Tropo-scatter is a regular mode of propagation, just like "driectwave" (line-of-sight).....

{My personal 144mhz ham station at home has quite a range for tropo-scatter....using my moonbounce array and 1kw+ PA, I typically have a range of ~ 800mi / 1200km on CW, and ~ 650 - 750mi on SSB....and ~ 300mi on FM.....but, this is with quite a LOT of ERP, ~ 200,000 watts EIRP...}



If anyone is interested in some on-line references that explain various VHF propagation modes, have a look at these....(sorry, I don't have any of my papers on-line, but some of these do a good job!!!)

Troposcatter inside and out:

Propagation Tutorial - Tropospheric scattering

Troposcatter Communications & Propagation :: Radio-Electronics.Com



I hope this helps clear up some confusions over VHF propagation....

Fair winds...

John
s/v Annie Laurie

[Paul Elliott]

Originally Posted by continuouswave

The ability of an AIS receiver to copy is set out in its receiver specification. At -107 dBm the AIS receiver has a packet error rate of 20-percent. This is a specified level of signal and of readability. I don't quite understand how you can insist that the AIS receiver is going to work a great deal better that it is specified.

I can think a few factors that should be considered:

1) Some AIS splitters include a good, low-noise receive preamp. This can improve the sensitivity over a similar non-amplified system. Not by a huge amount, but some.

2) The spec you are quoting is for a 20% packet loss. Once you reach a 20% loss level things starts to go downhill pretty fast, but an AIS system is still quite usable at a 50% loss, which will give you a few dB more sensitivity.

3) I see at least two AIS receivers that specify a 20% data loss at -115dBm. This is 8 dB more sensitive than the number you are using.

4) Regardless of the published specifications, it's just a fact that a receiver optimized to demodulate the 9600 bit/second GMSK (Gaussian Minimum Shift Keying) used for AIS should be able to decode at a much lower SNR (Signal to Noise Ratio) than a human listening to a voice transmission. To even attempt to listen to similarly-weak voice transmissions we would have to have the squelch turned off and nobody wants to listen to that level of hiss all day (except a crazy ham -- my callsign is wb6cxc). The GMSK decoder doesn't mind the hiss, so practically speaking the AIS decoder had a big advantage.

5) There is a lot more AIS being transmitted than VHF voice, and the AIS messages tell us how far away they are. We will notice when we receive an AIS signal from (say) 20 miles distance, but we obviously won't know when we fail to receive a signal ten miles off. Thus, our perception of AIS range may be biased. True, we do receive the distant ones, but perhaps not as reliably as we think.

For what it's worth, I have a land-based AIS receiver north of San Francisco, at about 1000 ft elevation. I regularly receive to 50 miles (my radio horizon), often receive 100 or 200 miles, occasionally beyond 1000 miles, and a few times to 2000 nautical miles. These long ranges are due to tropospheric ducting, and obviously not to be relied upon.

[Dockhead]

Originally Posted by continuouswave

I see the major problem: you are in the U.K. where the Coast Guard is still a maritime organization.

I am in the USA. The USA Coast Guard is now a military wing of Homeland Security. They don't want to be bothered with assisting boaters--no towing, no radio checks, no AIS checks. If I were in the U.K. I might call the Coast Guard and ask them for a courtesy radio report, but in the USA I doubt the Coast Guard would respond with anything other than a command to stop asking for radio checks from them.

Yes, it is best to perform a radio check with a reliable station. Half the boats on the water don't have radio installations that work very well. Your chances with a random station for getting an accurate check are about 50-50.

Indeed. I experienced severe culture shock with the Coast Guard when my center of cruising activity shifted from SW Florida to the UK. The UK Coasties have a completely different mission and attitude -- they are so extremely friendly and helpful -- as if they are there to help you -- what a concept!

They are also excellent radio operators (the radio training in the UK in general is vastly better).

[transmitterdan]

John,

Thanks for the detailed explanation and I apologize for using the wrong term. My point was that once you get past line of sight the path loss goes up fast and it is unpredictable and therefore unreliable. These stories we hear where people talk about coverage way past the line of site are just random data points that don't inform about "normal" VHF marine range. VHF voice radios are massively over powered at 25 watts for line of site coverage. The best thing you can do to increase coverage is get the antenna as high as practical thus increasing line of site range. If you give up a dB in transmit power with an AIS splitter that is no big loss. I get kinda frustrated with "experts" lecturing about how splitters are "worthless" and we should fret over tenths of dBs and ultra low loss coax when there is 20dB of margin in 25 watt line of site links. If you think you can increase reliable coverage with an extra dB (or even 3dB) of transmit power you are kidding yourself.

[ka4wja]

Dan,
I'm finding myself conflicted here, since I don't want to get bogged down in a long discussion, but I do wish to make sure everyone understands a VERY important point....confusion about reliable 24/7/365 vhf range/propagation beyond "radio-line-of-sight"....


And specifically, this point is that the VHF path loss (as well as uhf and shf), beyond "radio-line-of-sight", is VERY predictable and VERY reliable...

Originally Posted by transmitterdan

My point was that once you get past line of sight the path loss goes up fast and it is unpredictable and therefore unreliable.

Yes, the path loss does go up rapidly once past the "radio-line-of-sight" (past the diffraction/refraction zone), but this is extremely predictable and reliable, as are the range of communications!!!
These are NOT unknowns, and have been studied/published decades and decades ago (actually decades before I was born!)....

To reiterate, the vhf path losses and communications ranges (beyond "radio-line-of-sight") are extremely reliable, well known and understood, and many vhf (and uhf) communications paths (both data and voice) of hundreds of miles are still being used to this day, 24/7/365....
(if you read the above referenced links, you'll see...)


Forgive me for being so strident about this (some may think it a minor point, but it is not)....the facts are clear that this is an extremely reliable means of medium-range communications, and calculating a link budget for this is quite easy (and I've done it more than once!!)....



And, yes, the 25-watt Marine VHF radios on small boats / power boats, etc. have way more transmit power than needed....and yes, I'm one that won't obsess over a couple tenths of a db for terrestrial comms (and certainly not FM), but when you get out in the real world, you DO find that many VHF systems on sailboats are rather poor, and some are truly crappy (that's a technical radio term, for those of you scratching your head ....
So, with the 65' height over sea water (~ 10 wavelengths), it DOES make sense to reduce cable loss as best you can....obsessing over it, no....but giving it proper attention, yes!!!
(and please remember that there are / have been "AIS splitters" sold that have 4db - 5db losses....and many sailors buying vhf antennas with RG-58!!!)



I hope this resolves the confusion....if not, please read the above referenced links...
(as for experiences of enhanced / inversion tropo, and ducting, etc....I don't see others discussing those here....so, these are really off-topic / red herrings...)



Fair winds..

John
s/v Annie Laurie

[transmitterdan]

This is a pretty good lay person's writeup as well:

VHF Signal Propagation

When I said "unpredictable and therefore unreliable" I meant for typical cruising boats on the move trying to talk to other boats or shore stations of unknown location and configuration. Predictions of complex propagation paths beyond line of sight require substantial input data such as precise location of transmit/receive antenna, terrain, time of day, time of year, atmospheric pressure and temperature gradients, etc. Cruisers seldom have data enough to predict anything beyond line of site. Therefore, they cannot (or at least should not) depend on VHF coverage beyond that. If you occasionally can reach way beyond that then great. But it is poor seamanship to make decisions based on the assumption that you will always reliably do so.

[Wotname]

Originally Posted by El Pinguino

Please keep it here.... I'd like to know why they use AM....

Well just for you sir (and perhaps Dockhead ).

A little thread drift may be tolerable at this stage now the dust is settling over transmission paths and the digital / analogue divide ; however if you are only interested in AIS, please skip the rest of this post.

To be perfectly honest, I don't think there is one over riding reason why aviation VHF uses AM but the following issues do conspirecso that it will probably never be changed. Although one reason does stand head and shoulders above the rest and that is, IMO, there is simply no need to change. The aeronautical service works perfectly well using AM, double sideband, full carrier.

OK the other issues are:

• Historical - voice comms for aircraft were developed in the main, during WWII. During the 50's and 60's it was rapidly developed using the cheapest and best technology available at that time - AM.
• It was clearly seen that all aircraft should be operating the same technology.
• These days, aviation radios are built to a robust standard and thus cost accordingly. Any change is going to be very costly.
• Additionally, any change would have to be introduced worldwide simultaneously - and that isn't going to happen!!!
• Or the service would have to operate two platforms simultaneously, the old and the new one - and that isn't going to happen due to massive cost to both the aviator and the air traffic control. In particular, many aircraft would have significant problems in physically fitting multiple platforms.
• Finding spectrum space for another platform would also be an issue.
• For any change to be considered, there would have to be a cost / benefit analysis done and given the huge costs involved, the benefits would have to be huge as well. Otherwise, no one will be willing to undertake the cost.
• I fail to see what large benefit could be gained. The existing service works very well. Range isn't an issue, most antenna heights range from a thousand feet or so to 30 or 40 thousand feet. Transmit power varies from about 10 watts to 25 watts. All (or almost all?) operators are professionally trained and can cope with the perceived shortcomings of AM - noise and distortion being the main ones.
• The number of available channels is an issue but this is being solved by technological advances in design. Currently the US and most of the world outside Europe uses 25KHz channel spacing giving a 720 available channels inside the aviation use of the spectrum of 118 to 136 MHz. Europe uses 8.33KHz spacing giving 2,160 available channels. Adjacent channel interference is minimized by geographical spacing of where the the channel are used. Of course, every third channel matches the US channel spacing so that their radios are perfectly usuable in US airspace. All new radios can be set to 8.33 or 25 KHz spacing at the push of a button.
• I think that the "capture effect" of FM might play a small part in keeping it AM but IMO, this is sideline issue

Now back to AIS...

[carstenb]

Getting back to the OP. Why do you want to only receive but not transmit?

I have an Em-trak transponder and I absolutely love it (when sailing in our heavily trafficed waters it is a godsend) I have the receive range set to 20nm and I can see boats popping up when they enter that range.

Xmiting is a safety feature - others can now see you. If you call to agree starboard/starboard or for another reason - the ship you are calling can see you. Without Xmit - you're just a voice in the night.

I'm not sure of the range of my Xmit. I use a splitter and my VHF antenna is mounted at the top of my mast - 18 meters (approx 60 feet). I've talked with ships 10nm out and they see me just fine.