Class B AIS

[clownfishsydney]

I can pick up yachts about 25 nm from where my yacht is moored. This is totally across land, including some quite high hills (say well over 1000 feet). I can also pick up ships at a greater distance.

[continuouswave]

I made an error in my earlier article in converting the sensitivity of a voice receiver to dBm. A voice receiver with a sensitivity of

0.25-microVolt for 12-dB SINAD

means the signal level of 0.25-microVolt is -119dBM (not -107 as I incorrectly said).

This gives the voice receiver a 12-dB advantage in sensitivity. Coupled with a 10-dB advantage in transmitter power, the voice system is now about 22-dB better than AIS. This again suggests that the range of AIS Class-B is not going to be better than voice.

[transmitterdan]

CW,

The range of VHF voice is not a matter of link budget in my experience. It is the line of sight distance and/or local interference that dominates most of the time. So it is quite likely that in the real world AIS can equal or in some cases exceed usable voice distances. This is why I keep having friendly arguments with fellow cruisers over 1dB of loss in a coax or AIS splitter. As far as voice goes link budget just isn't the dominant factor in determining coverage. I think the regulators and SAR groups want it that way so that VHF voice is almost never limited by link budget. Antenna height is the determining factor in most cases.

[continuouswave]

Originally Posted by transmitterdan

The range of VHF voice is not a matter of link budget in my experience...[rest of very cogent reply not included]

Why would an AIS transmission not be affected in the same way?

[Dockhead]

Originally Posted by continuouswave

Mark--I am not sure how my ownership of an AIS Class-B transponder would affect the range of AIS Class-B transponders or my knowledge about that topic. First of all, let us assume I have an AIS transponder. How will I gain information about its range? My AIS transponder only transmits. I don't have any way to know how far its signal is being received by others. I could go about transmitting with my AIS transponder for years and have no idea how far its signal was being received. Thus, I don't see any sort of tie-in between knowledge of AIS range and ownership of an AIS Class-B transponder.

You check the range of your VHF phone comms with radio checks, don't you? Why would you "go around for years" never knowing whether anyone can hear you on the VHF or not?

Same with AIS -- highly recommended to call AIS targets (it's easy when AIS gives you the vessel name and even MMSI and callsign) at various ranges and ask them whether they see you or not. I do this actually more often than I do radio checks on VHF, and I recommend the practice to you, if you have an AIS set.

[Dockhead]

Originally Posted by continuouswave

Frequency shift keying is a form of frequency modulation, just like on-off telegraphy is a form of amplitude modulation.

You are mixing up two attributes: the nature of the modulation technique and the nature of the modulating signal. There is nothing about frequency modulation that limits its use to analogue or continuously varying signals. Indeed, frequency shift keying is a good example of frequency modulation by a digital signal.

Changing frequency = modulation -- I get it.

It's not what the books say, but I will defer to yours and Transmitter Dan's superior knowledge. I am actually an Extra Class ham, but do not pretend to be an expert in radio -- got my qualification by cramming because I needed it for reciprocity in CEPT.

[Dockhead]

Originally Posted by transmitterdan

CW,

The range of VHF voice is not a matter of link budget in my experience. It is the line of sight distance and/or local interference that dominates most of the time. So it is quite likely that in the real world AIS can equal or in some cases exceed usable voice distances. This is why I keep having friendly arguments with fellow cruisers over 1dB of loss in a coax or AIS splitter. As far as voice goes link budget just isn't the dominant factor in determining coverage. I think the regulators and SAR groups want it that way so that VHF voice is almost never limited by link budget. Antenna height is the determining factor in most cases.

I may be missing something, but I'm not getting the arguments that you can compare AIS digital signals and analogue FM phone signals purely by dB of attenuation. The readability of both types of signals is fundamentally different -- based on different principles. Same signal strength cannot give you any assurance that readability is the same. A digital signal can be read as soon as the signal can be distinguished consistently over background noise -- you only have to read an "on" or "off" state. An analogue signal will not be read yet, just because you can tell that it exists. You have to "read" the complex waveform which represents sound in an analogue way, and comprehend it, which depends not only on enough of that complex waveform getting through over noise, but on the subjective ability of the operator to have this comprehension -- which cannot possibly be measured in dB.

[transmitterdan]

Originally Posted by continuouswave

Why would an AIS transmission not be affected in the same way?

More power does not help increase range in line of sight limited systems such as VHF marine band radio.

[transmitterdan]

Originally Posted by Dockhead

I may be missing something, but I'm not getting the arguments that you can compare AIS digital signals and analogue FM phone signals purely by dB of attenuation. The readability of both types of signals is fundamentally different -- based on different principles. Same signal strength cannot give you any assurance that readability is the same. A digital signal can be read as soon as the signal can be distinguished consistently over background noise -- you only have to read an "on" or "off" state. An analogue signal will not be read yet, just because you can tell that it exists. You have to "read" the complex waveform which represents sound in an analogue way, and comprehend it, which depends not only on enough of that complex waveform getting through over noise, but on the subjective ability of the operator to have this comprehension -- which cannot possibly be measured in dB.

My point is that AIS and voice have the same line of site limitations assuming same antenna height. Your description of how AIS digital modulation works isn't quite right. FM has a "cliff" effect at the radio horizon that affects voice and data pretty much the same. The reason AIS can use less power is simply because voice is overpowered at 25W.

[continuouswave]

Originally Posted by transmitterdan

More power does not help increase range in line of sight limited systems such as VHF marine band radio.

Well, I disagree. Paths that are over the radio horizon are possible if there is more transmitter power. You overcome increased path loss with more signal.

I can copy many NOAA weather broadcasts from distances of more than 100-miles away fairly routinely. Their higher power overcomes the path loss increase.

[continuouswave]

Originally Posted by Dockhead

I may be missing something...Same signal strength cannot give you any assurance that readability is the same...

The basis for readability is contained in the receiver specification. The AIS receivers say they can get copy with a 20-percent packet error rate with a certain level of signal.

Yes, there are other modulation systems that can offer better performance than AIS. For example, a GNSS receiver typically can operate with signals that are extremely weak. For example, an inexpensive SiRF Star III chip works with signals that are -159dBm. There is no way you could copy FM voice at that signal level. Some more recent receivers are able to work with -164dBm signals. These are signals that, without their special modulation techniques, are so weak they could probably not even be detected.

[continuouswave]

If you consider the (original) purpose of AIS was collision avoidance (before it became a surveillance tool for Homeland Security, another topic), the concept of AIS was to let vessels receive data about the position, speed, course, and so on, from other vessels. If there were a risk of collision, the vessels would contact each other via radiotelephone to arrange some mutually agreed change in course or speed. If AIS signals carried far beyond the range of radiotelephone signals, AIS would not be much of a collision avoidance system. You'd see the other vessel but couldn't contact the vessel until it was much closer.

Also, modern radios tend to feature the ability to set up a radiotelephone call using digital selective calling from the AIS data. If AIS targets were showing up far beyond the range of radiotelephone, what would be the point of trying to call them?

[continuouswave]

Originally Posted by Dockhead

You check the range of your VHF phone comms with radio checks, don't you? Why would you "go around for years" never knowing whether anyone can hear you on the VHF or not?

Same with AIS -- highly recommended to call AIS targets (it's easy when AIS gives you the vessel name and even MMSI and callsign) at various ranges and ask them whether they see you or not. I do this actually more often than I do radio checks on VHF, and I recommend the practice to you, if you have an AIS set.

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.

[poiu]

Originally Posted by Dockhead

highly recommended to call AIS targets (it's easy when AIS gives you the vessel name and even MMSI and callsign) at various ranges and ask them whether they see you or not. I do this actually more often than I do radio checks on VHF, and I recommend the practice to you, if you have an AIS set.

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

[transmitterdan]

Originally Posted by continuouswave

Well, I disagree. Paths that are over the radio horizon are possible if there is more transmitter power. You overcome increased path loss with more signal.

I can copy many NOAA weather broadcasts from distances of more than 100-miles away fairly routinely. Their higher power overcomes the path loss increase.

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.

NOAA transmits with TX power of 300-1000 watts which is as much as 50 times that of a VHF marine radio. There is a small groundwave component to VHF that most can safely ignore but when the transmit power is large enough it matters.

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