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Using Hyperterminal on your laptop

If you have had to configure a unit such as a multiplexer you will have come across the part in the manual where it says use Hyperterminal. This is a free communications program that you can use to monitor the signals going in/out of the serial port on your laptop and to send configuration commands from the laptop to the multiplexer.

For example to set one of the ports on the multiplexer to a higher speed such as 38,400 baud suitable for connecting your AIS unit. Signals and data from the AIS would then be routed via the multiplexer to your chart plotter for example.

In my case I link a NASA AIS unit via my Brookhouse multiplexer to my Raymarine C120 chart plotter in this way. But to make it work I had to get my laptop, plug in a cable in the laptops serial port then put the bare wires at the other end into the Brookhouse multiplexer and tell the multiplexer to change the speed of one of its ports from 4,800 to 38,400 baud. Then unplug the laptop, connect the AIS and that was it – AIS data on my C120.

Even if you have done all this before did you know that that little program called Hyperterminal is no longer given away by Microsoft with Windows Vista? {read my post here to find out more…}

If you are having trouble with Hyperterminal read these tech notes from Microsoft.

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Which Multiplexer ?

Over a year ago I searched high and low for the “perfect” multiplexer and readers of this blog will know that I selected the Brookhouse from New Zealand in the end {more of that in this post from last year}.

Configuration for nearly all these units is by using Hyperterminal or similar software – so you had better find out how to do that!. {see my posting on this program here}

This is a new unit from on sale at Marine Electronic Service Ltd for £194 inc VAT.

Eissing SB2006– 1x input NMEA0183 / RS-422 isolated

– 3x input NMEA0183 / RS-422 isolated

– 1x output NMEA0183 / RS-422 isolated

– 1x COM port (RS-422 level)
– power supply for external equipment

You should note that RS422 is not RS232 or RS232C which is the “older” and slower PC comms port standard – but nevertheless make sure that the laptop you want to connect to supports RS422 as its serial connection…{see my post here on the subject}.

I cant vouch for its operation since I am very happy with my Brookhouse. But you may want to compare the two if you are about to take the step of trying to integrate AIS and other slower NMEA instruments into your chart plotter or laptop. {See my posting here on the Brookhouse}

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AIS Class A – What is SOTDMA?

The operation of AIS depends on the Self Organizing Time Division Multiple Access (SOTDMA) data communication technology, which was developed in the 1980’s.

It allows for large numbers of transmitters to share one single narrow band radio channel, by synchronizing their data transmission to an exact timing standard. Under SOTDMA each minute of time is divided into 2250 time slots or 26.67 ms each time slot. With a transmission speed of 9.6 kbps this translates into 256 Bits/time-slot, sufficient for one AIS report.

The exact timing signal of the GPS receiver is essential to synchronize the time slots of communicating AIS ships, as well as providing the position data for each ship. When a ship first “talks” to another ship, it takes up an unoccupied time slot, and it reserves a time slot for its next contact, depending on the status of the vessel according to the standards

For example a ship sailing at 23 knots updates its information every two seconds and therefore “reserves” a time slot 75 slots on from the first contact (2250/30=75) and so on).

The range of the system is the VHF horizon for each AIS-ship, with the ship in the centre of its own communication “cell”. The size of this cell will adjust to the traffic density, if slot capacity starts to run out, the system will automatically discard targets at a greater distance and assign those time slots to targets of greater importance.

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Collision avoidance – ARPA , CPA or …what?

If you have a Radar it will probably be set to use ARPA to assist you in deciding whether you are on a collision course with another vessel . Many yachts have now added AIS to their nav electronics and can overlay the AIS targets on to the Radar and chart plotter display to add more information. So far the logic is that you calculate the Closest Point of approach (CPA) and Time to Closest Point of approach (TCPA) and make a judgement on which way to turn according to the Collision Regs.

SafePass is an idea from Fred Pot of SeaCAS. In this scheme the system calculates various CPA/TCPA results and plots “safe” and “dangerous” zones on your current heading to make it even easier to decide which new heading may be appropriate for your vessel. This depends on the other vessel not changing course and also on the circumstances of the impending collision with regard to the Collision Regs of course.

By the way one of the simplest, most practical and down to earth descriptions of how to use your radar can be found on Bill Dixon & Pat Watt’s personal web page

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What is the Automatic Identification System (AIS)

Courtesy of the excellent US Coasd Guard Navigation Site…

http://www.navcen.uscg.gov/enav/ais/default.htm

Picture a shipboard radar display, with overlaid electronic chart data, that includes a mark for every significant ship within radio range, each as desired with a velocity vector (indicating speed and heading). Each ship “mark” could reflect the actual size of the ship, with position to GPS or differential GPS accuracy. By “clicking” on a ship mark, you could learn the ship name, course and speed, classification, call sign, registration number, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available. Display information previously available only to modern Vessel Traffic Service operations centers could now be available to every AIS-equipped ship.

With this information, you could call any ship over VHF radiotelephone by name, rather than by “ship off my port bow” or some other imprecise means. Or you could dial it up directly using GMDSS equipment. Or you could send to the ship, or receive from it, short safety-related email messages.

The AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation.

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