The little project presented here involved designing and building a simple, very low power alarm system to keep an AIS watch at sea. Avoiding undesirable encounters with ships in mid-ocean has always been a concern, especially for single-handers. One can sometimes go for weeks without seeing anything, but merchant vessels usually travel at some 15 knots or more and can appear out of nowhere at little notice. In the early s, a device nicknamed CARD for Collision Avoidance Radar Detector appeared on the market and quickly gained popularity on board offshore cruising yachts, first in North America.
Four directional receivers enclosed underneath a little dome picked up the beams of pulse radars and delivered an audible alarm and an indication of signal strength in the quadrant the signal came from. CARD devices were quite expensive, protected by a patent and handcrafted by a small company in Norfolk, Virginia. The last units seem to have shipped around A CARD device, with a very modest power consumption, was one hell of a lot better than nothing and I wish I had had one on board the Yarra.
Regardless, whether a ship would keep its radar scanning offshore was always an open question. I think we were all missing the odd ship going past now and then.
I just missed some of the ones with radar scanners in operation, and they missed the others. Together we probably achieved pretty good coverage. In the end, I managed to find a defective one, which I repaired and never installed so far, but this is a different story. The most interesting part was reverse-engineering it.
Due to some of the component choices made in the dome, they are all bound to fail from aging after some 15 years or so. My view is that the advent of the purpose-designed AIS system has now made radar detectors redundant in most situations. It came down to power, but also to the sensitivity range of the microwave detector diodes used. The development of the low-power broadband radar for small vessels is the other nail in the coffin for CARD systems.
The absence of high-energy pulse means no detection possible using this technology. The information is used for identifying nearby traffic, collision prevention and routing.
A Guide to the Collision Avoidance Rules
Carrying AIS transmitters was made mandatory for all cargo ships greater than gross tons and all passenger ships by end of While this leaves out navy ships, fishing ships and some others, it provides for detection of maritime traffic offshore that is undoubtedly far more extensive and reliable than passive radar emissions monitoring. A number of AIS monitors, receivers and later transmitters appeared on the market for small craft. Rather than running energy hungry equipment to receive, decode and display AIS data, I just wanted a simple audible alarm whenever a message is received.
The range of VHF radio transmissions is essentially limited to line of sight, with antenna height being a determining factor. Reception of an AIS message reveals the existence of another vessel within the reception range and at that point, having a look around for it is more important than the content of the message.
Appropriate elevation of the antenna can filter out traffic that is too distant to be of interest, keeping some peace and quiet on board. One common feature of a lot of AIS receivers is a serial data port for sending out the decoded messages. Designing an alarm circuit connecting to the data port seemed to be a reasonable path forward. This should make the alarm circuit compatible with any AIS receiver equipped with a RS serial port.
The circuit operates as a charge pump, stealing and accumulating a tiny amount of energy from the AIS data signal as it rapidly changes from low to high while spelling out the message. This accumulated charge is then amplified and a visual and audible alarm is activated until the stored energy has decayed.
In the case of a ship transmitting every few seconds, the circuit delivers a continuous, or near-continuous alarm. Intermittent signals coming from the limit of the reception range trigger intermittent alarms.
A short intermittent flash from a LED indicates that the system is active. Eliminating this functionality would reduce the component count by six. The prototype shown was installed in a small plastic enclosure with the board simply held in place by the threaded shanks of the switches.
Completed AIS alarm device installed in a small plastic housing. A small multicore cable coming out of the housing leads to a terminal block where the connections to the AIS receiver are made.
It includes scale layouts for the circuit board, a bill of materials and the schematic. If you want to get the PCB manufactured by a commercial service, Gerber files are available here , but check that the buzzer and switches you source actually fit the board! The small circuit board can be home-made quite easily.
The circuit was not tested with other AIS receivers than the SR, but as long as the signal voltage is sufficient, there are no reasons for issues to arise.
Maritime minutes – How well do you know the Collision Regulations?
It should usually read around 12VDC in the absence of data being received. Drop me a line by e-mail if you build one and let me know what AIS receiver you have used.
If the demand is sufficient, I may arrange a production run at some point. It is senseless.
Such an antenna is not suitable for transmitting however. AIS messages are broadcast on two separate channels, in order to provide some robustness in terms of interferences. Some receivers include dual reception circuits and listen on both frequencies simultaneously. Their performance is demonstrably superior in high traffic and they do receive and process more messages. For the offshore sailor, it is a moot point.
If there is any traffic, his or her place is outside dealing with it, not facing a little screen down below, and one ship is enough to prompt attention.
Collision avoidance at sea pdf reader
Single channel receivers instead switch frequencies every few seconds while listening, and presumably switch if they experience interference. One advantage they offer is lower power consumption and, in the worst case, they may just take a few seconds longer before receiving a well-formed message and triggering an alarm.
A peek inside the SR is an irresistible proposition. The CPU board is mounted above the radio board, which is missing the components for the second receiver available in the dual-channel model SR I have had in mind to write an article about repairing them, because they are all bound to fail as I explained, but it has remained an intent.
Finding the time…. After completely reverse-engineering mine, I occasionally repair them to help people out, using much more durable components than what is originally in them. They are great and it is a shame they are no longer manufactured. If you e-mail directly with some details and your location, we can talk about it.
Mine is getting power but will not turn on.
I do not know if it is the electronic box or the dome on the back rail. We are on Lake Erie. I can do some rudimentary testing if I would know what to check. The power supply is contained within the display unit and then there is quite a bit more electronics in the dome.
Bridge over the switch to test if needed. Getting the green centre LED to come on should be very easy, because it has nothing to do with the more complex circuitry inside the dome. I have close up photographs of both circuit boards in my files, I will have a look for you tonight.
Eric, I have a C.
Questions: — Is there anything else equivalent on the market? It turns on, one of the direction arrow does not light up. It does not receive any more it seems as it does not see my own radar. Since you have reverse engineered it it seems better use of my time to get your help then doing it myself. There has been a French product called Mer-Veille on the market that is similar to the C. The microwave detection diodes can require a bit of shopping around and different models were used over the years.
When this is the case, replacing the detection diodes in the dome is frequently necessary. As I have found a number of different failed components in C. It pays to start with the simplest things like checking for broken wires in the plug and broken solder joints on the display PCB.
Next, check for the power supply to the dome. The device operates with four identical channels each coming back to the display unit through their own wire, so you can normally swap the channels and determine whether a channel fault is in the dome or in the display module.
I have sometimes thought about writing an article dedicated to repairing these devices, but it would represent quite a job. If you can narrow the issues down somehow, e-mail and we can discuss them. Your Comment. Name required.
E-mail required. The well-known CARD radar detector display. AIS alarm system block diagram. The AIS alarm board measures 70x40mm only. Eric Bretscher.
Nordkyn Design. Gary peterson says:. Eric Bretscher says:. Jean-Francois Somerville says:.