A recreational boat will typically have a CLASS-B AIS transceiver, and a large commercial vessel will have a CLASS-A AIS transceiver. In assessing the radio range (distance) between these two categories of vessels, consideration has to be given to the transmitter power. CLASS-A transmitters are 12.5-Watts, and CLASS-B transmitters are 2-Watts. This gives the CLASS-A signals an advantage of about 8-dB. The implication of this is as follows:
--assume everything else were the same, that is, the antenna height, the antenna gain, the transmission line loss, the receiver sensitivity;
--the CLASS-A vessel has an 8-dB stronger signal, which generally translates into an increase in radio range, that is, the CLASS-A signal can be heard at a greater distance than the CLASS-B signal.
The amount the radio range will increase by adding 8-dB to the signal power will depend on the nature of the path between the stations. For propagation over open water one can estimate the path loss will vary with distance (d) by
- dB = 40 x log(d)
Solving this for d when db=8 gives 1.58
This means the path distance can be 1.58-times longer for a signal with 8-dB more power. In simple terms, a CLASS-B vessel is going to hear a CLASS-A vessel's AIS signal for a long time before the CLASS-A vessel hears the CLASS-B, when the two vessels are far enough apart to be at the very limit of the radio communication path. That last part is the important part. If the vessels are only a few miles apart, the path loss is not going to put the signals at the threshold of being received. While the extra gain helps when the vessels are far apart, it won't have as much effect when they are closer.
Another factor is how often the vessels transmit. A CLASS-B vessel underway at less than 14-knots only transmits every 30-seconds, while a CLASS-A vessel at the same speed range transmits every 3.3-seconds if changing course and every 10-seconds if on steady course. This means the CLASS-B vessel is transmitting something like three-times to almost ten-times more often than the CLASS-B. The more often you transmit data the more likely it is to be received, so, again, the basic methods of AIS tend to favor the CLASS-A vessel being received and detected by the CLASS-B vessel before the other way around.
A further problem with AIS signals is the actual reception and accurate decoding of the signals. An AIS transmission is a broadcast transmission, that is, the transmitter is just sending out the signal, and it has no idea who might be receiving it. Many people misunderstand the nature of a digital transmission of data, and they anticipate that because the data is sent in a digital coding there can never be any error. This is far from the truth. Some digital transmission systems are bi-directional, and the sender and receiver switch roles frequently, with the receiver being able to interrupt and ask for a repeat transmission if there was some data loss. None of this happens with AIS. The transmitter just broadcasts a signal, out it goes, and who know what happens to it. Maybe another vessel hears it, but you can't really tell.
Even in a one-way broadcast transmission of digital data it is possible to incorporate a means of error detection. Indeed, in AIS signals the cyclical redundancy check (CRC) error checking method is employed. But this method generally only allows for the detection of errors. A number of very sophisticated methods have been proposed to allow error correction to occur by sophisticated signal analysis mathematics and use of the CRC data, but it seems very unlikely these complex computational methods would be available in an inexpensive ($200-class) AIS receiver. A typical recreational grade AIS receiver is just going to discard any data it received by AIS transmission if it detects an error in the data. On that basis, the more often the data is transmitted, as occurs with CLASS-A vessel, means a greater chance it will be received without error by other vessels.
If into this analysis we add the notion that the CLASS-B vessel has a much lower antenna height, probably a lower gain antenna, and is likely to have much more motion on the boat (causing the antenna radiation pattern to be skewed away from the horizon), we can find other influences that will tend to reduce the visibility of the CLASS-B vessel to the CLASS-A vessel.
There is a newer category of CLASS-B AIS transceivers called Self-Organizing, which will have increased power (5-Watts). This decreases the power difference to 4-dB, making the range factor 1.26-times, and helping to put CLASS-B SO transceiver on a more even footing with CLASS-A devices in terms of signal power. A closer look at the reporting intervals follows below.