"Peter Bennett" wrote in message
...
On Mon, 13 Mar 2006 16:36:32 GMT, Daniele Fua
wrote:


I am quite interested in this topic but I have a question: isn't the AIS
system meant to rely on transponders, i.e. on Transmitters/Responders?
If so, a plain receiver like the one you write about is bound to receive
the information only from the vessels that were "excited" by someone
else. Is it so?

Daniel

Vessels underway transmit their position, heading and speed every
second or so, depending on speed, and full data every six minutes.

They do not wait until interrogated.

This is correct. The transmitters are indeed transponders, though, since
they have the capability of responding to a specific interrogation, and they
participate in the coordination of the timeslotted transmissions from all
local transmitters.

The receive-only units are much simpler, as they only have to listen.

-Paul