NASA AIS - any good?

[EugeneR]

It's probably very unlikely that this would happen, but you will definitely miss some transmissions

I agree with your explanations; however, based on some rough calculations using yellowboat's numbers earlier, I do not see any realistic benefit to having a dual channel receiver, even in the worst case scenario.

I note the use of "very unlikely" and "will definitely" in the same sentence

If a vessel made a single, sudden change of course it would transmit that data within seconds, if not immediately. If you missed the transmission and the vessel then held it's course and speed it wouldn't be transmitted again for another three minutes, hopefully you don't miss this one.

Using yellowboat's numbers, a cargo ship doing 25 knots will transmit every 2 seconds i.e. every few metres or 0.0139 nautical miles.

If that cargo ship was that distance away (pretty near alongside), and now aiming for you, then your single channel receiver would have a 50% chance of picking up the sudden change compared to a dual channel receiver.

As you go further, the probability of your single channel receiver picking up the change increases exponentially i.e. if the boat was n multiples of 0.0139 miles away, the relative probability of getting the message would be (2^n)-1 / (2^n).

For example, if the cargo ship is 0.11 nautical miles away (8 times, too close even without a sudden change), the chance of your single channel receiver receiving the message would be 99.61% (255/256) compared to that of a dual channel receiver.

In that example, the other ship could be up to 0.11 nautical miles closer by the time you are 99.6% certain to get the message on your single channel receiver. Otherwise put, you get the message up to 16 seconds earlier on your dual channel receiver.

Now, before you tell me that 16 seconds is enough time to get your boat out of the hole, have you ever seen a cargo ship at 25 knots and following a safe course 0.11NM away, changing onto a collision course within 16 seconds?

Probably very unlikely that this would happen, but it will definately happen? ;-)

 

[YELLOW BOAT]

I totally agree that there is little advantage of a two channel reciever.
The only advantages are that , after recieving for the first time or after switching on it can take twice as long to display the static figures of a ship .
That also accounts for the name/callsign of the ship and other , less important figures.
But apart from this there is indeed not much advantage of a 2 channel reciever.

By the way, from another forum I got the name of a supplier in the USA of a cheap AIS transponder which si in fact identicalo to the "AIS-CTRX True heading" but much cheaper. It also hat a CE marking.
Have a look at: http://www.westmarine.com/webapp/wc...toreNum=10103&subdeptNum=10268&classNum=10275

Hardly more expensive than a two channel reciever!
There is at least one German sailer that installed one with approval.