What is AIS anyway? AIS, or more accurately, AIS Class A is a system required on virtually all commercial ships. Although it operates on VHF, it can be likened to a transponder used in commercial aircraft. Periodically, each ship transmits unique information on one of two VHF channels. This information, once received by an AIS receiver, can be used to overlay chartplotters and radar displays (assuming the display is AIS capable). For an in-depth discussion on AIS, read my article:
The first thing that is required for AIS reception is an AIS receiver. Single and dual channel receivers exist, but a single channel receiver will periodically switch between channels and should be considered if cost is more of an issue than performance. For this project, I am choosing Smart Radio's SR-161 AIS Receiver. It is probably the lowest cost receiver on the market, and works quite well.
The SR-161 receiver is fairly small, so it fits nicely on my wiring board under the helm area. This makes it easy to interface to the chartplotter as it is in the general area, so only short signal cables are required.
Three connections are required; the antenna, power, and RS-232. Prior to installation, you may wish to test the AIS, which can be done by connecting the receiver to your PC with a RS-232 cable along with the demo AIS software that came with the receiver. The manual provides for a few options which can be set by the PC interface; most noteable is the speed selection. The default is 38400 baud, but you can set the speed to 4800 baud if required to interface to your chartplotter. I left the speed at 38400 as I'll explain later.
The antenna connection on the AIS receiver is BNC, which may not be familiar to the typical boater. You either have to use a matching BNC connector on the VHF antenna (which is about impossible to find in a boating store or Radio Shack), or you can find a BNC-to-PL-259 adapter, which is easier to find. From there, you can use the standard PL-259 connector on the typical VHF Marine antenna.
The power connection goes to a dedicated switch so that I can turn the receiver on or off as needed. This is a low power device, so there is not a huge issue with voltage drop.
To connect the receiver to your chartplotter, you need to cut one end off the RS-232 connector; then connect it to a NMEA-0183 port. The instructions include what pins from the cable are neded. More on this later.
While you can purchase an antenna designed specifically for AIS, I have found that a standard 3" VHF Marine whip antenna works just fine. The AIS antenna is second from the left in this photo.
There is a little Voodoo science here, especially if there are multiple NMEA-0183 devices in the navigation system - which is typically the case. If there are none, or if there are multiple NMEA-0183 ports on your chartplotter, you are lucky, and you can simply connect the receiver to the chartplotter's NMEA-0183 port. All that is required is to match the speed of the AIS receiver to the chartplotter. The default speed of the AIS receiver is 38400 baud, while the default speed of the chartplotter is 4800 baud, so one of the two devices must be changed to match the other. Fortunately, the SR-161 AIS receiver can operate at either speed, which is a good thing as not all chartplotters may have the capability to operate at 38400. At any rate, consult your manuals for changing the port speed.
If, however, you need to support multiple devices with a single NMEA-0183 port, things become a bit more complicated. This was my situation, so in order to understand what I am doing, you may want to review my NMEA-0183 primer before continuing:
The SR-161 receiver contains a rudementary multiplexer, intended for multiplexing a GPS receiver with the AIS, but it should work with any NMEA-0183 device. Since, I have several devices, and more planned, I will be using an external 4 port NMEA-0183 multiplexer.
My muliplexer of choice is a Brookhouse Model AIS-C. It is made in New Zeeland, but they direct export to the US, and payment can be made with Credit Card. I chose this multiplexer because:
There is a method to this madness. The RayMarine C-80 chartplotter has the capability to operate at 38400 baud, which means it increases the data throughput from the multiplexer. This is important if there is a lot of signal activity as NMEA-0183's default speed of 4800 is on the slow side, and can quickly be overwelmed. Remember that NMEA-0183 was intended to be a simple point-to-point connection, and by multiplexing, we can quickly exceed the default port speed. The result of this would be dropped packets; essentially lost data.
If your chartplotter has this capability, I would highly recommend running it at 38400. However, there is an issue with doing so. For instance, the RayMarine C-80 can run at 38400, however both listener and talker run at the same speed. So, if I have any devices connected to the talker expecting 4800, such as the VHF DSC radio in the diagram, communication will be lost. Fortunately, the AIS-C multiplexer solves this problem with an internal buffer. The buffer allows the speed to be lowered on its output side, so that while the chartplotter sends data at 38400, the radio will receive data at 4800.
Of course, there is a risk of overfilling the buffer and losing data if there is too much output data, but this is rarely the case; and its better than not having one. I have only found this option on the Brookhouse AIS-C. However, products change all the time, so I would not be surprised if other manufacturers offer this as well.
Located at the back of the helm dash area, I mounted the Brookhouse AIS-C multiplexer in a waterproof enclosure. I also connected the multiplexer's power to a switch near the helm. As the current requirements of the multiplexer is very low, there is little issue with voltage drop.
You may notice that the waterproof enclosure is mounted on a hinged starboard panel. Normally, the panel is in the vertical position, however, this provides little access to the multiplexer. However, the hinge allows rotating of the panel 90 Deg (to the vertical), providing easy access. The panel is normally secured in the down position so that it cannot flop around.
I used a NEMA-4x style waterproof enclosure for the multiplexer. More information on these enclosures can be found here:
OK, so now you turn on the chartplotter and AIS receiver.... nothing.
The situation is that the AIS SR-161 receiver will not output anything if it is not receiving an active signal. Since ships transmit the signal once every 30 seconds, you might receive a signal shortly. Or, of course, if there are no ships in the area, no signal will be forthcoming. You may also have to program the chartplotter to show AIS data on screen.
However, when you do receive a signal, a target will be displayed on either the chartplotter or radar screen, depending on your display unit. As shown here, one boat object is in the middle of the round basin. That is actually my boat, in my slip. The other target, moving down the channel is an AIS reception. On my chartplotter, basic information, such as its current heading, speed, current distance from my boat, and closest point-of-approach are displayed. If I wish, I can bring up additional information on a second screen showing the ships name, next destination, length, and other data.
Our navigation project has now grown to the configuration shown above.
Milltech Marine (AIS Receive)