Tuesday, March 15, 2011

The AIS Radio Project

Boot Key Harbor, Marathon, FL
34 42.54 N 081 05.58

For several years we wanted to add AIS to Tarwathie's inventory. What's that? Automatic Identification System: It is a system where ships transmit critical information digitally over VHF radio. They send their name, position, speed, course, and lots of other info useful to other vessels in the area, such as us. Having AIS will make us feel much safer regarding collisions at sea.

The best feature of AIS, as far as I'm concerned is a CPA alarm. CPA means Closest Point Of Approach. The computer in the AIS looks at each target's speed and direction, plus my speed and direction and projects ahead how close we will come in the future. For example, I can set the CPA alarm for 2 miles. Then, if any ship is projected to come within 2 miles of me in the future, the alarm will go off.

A couple of years ago, I bought a SITEX AIS receiver and display. I rejected that and sold it later unused. It had two problems. 1) The display was not waterproof. It had to be mounted inside the cabin. That's not where the helmsman can see it at night. 2) It had no CPA alarm, just a distance alarm.\
I theorized that if I waited, the technology would improve and I could get what we want at an affordable price. I was right. Bob on Carpe Diem, showed me his new Standard Horizon VHF radio with AIS built in. Best of all, it had a remote microphone with a display screen. That means that the AIS information is shown on a display you can hold in your hand while in the cockpit in foul weather. Perfect. The radio plus remote mike cost about $400; that's cheaper than the SCITEX display I rejected.

The picture above show things as I finished installation of the new radio/AIS. I had to rearrange all the electronics boxes there. SSB radio, VHF radio, AM/FM Radio, and Pactor Modem (connects PC to SSB). It took me a whole day's work to do that installation, plus another half day to install the remote mike in the cockpit. Routing wires and making adequate and reliable 12 volt connections are very difficult in Tarwathie.   The remaining wire hanging down is the one that connects to the USB port of my laptop when I transmit blogs at sea via SSB.

The picture above shows the AIS display on the radio.  The circles show an area around us.  Little balls in the circle show other vessels transmitting AIS.  The one selected (black ball) shows its name, distance, bearing, CPA, and TCPA (time to the CPA) and speed.   Better still, since this AIS is integrated with the radio, there is a button there called CALL.  Push that and the radio on his bridhe will ring like a telephone thus establishing a conversation between us and them.

Of course all this is theoretical.   We have yet to test it while actually at sea.   We must also remember that there not all vessels out there transmit AIS data.  We still need to keep a sharp lookout.  Nevertheless, I predict that Libby and I will feel less anxiety when out there.  Money well spent.

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