Base Stations only supply timing info if the mobiles lose their UTC
signal from the GPS, otherwise the AIS system is self-organizing (hence
the term SOTDMA). One of their tasks is to manage the AIS system in
times of overload, to interrogate specific ships for data, or to
broadcast safety messages. Up till very recently, most BS AIS units have
been modified Class A's.
If its a real SAR aircraft, ie: one of the yellow ones, then I think
their AIS units come on with the rest of their radio kit - assuming
they've been fitted with them. This is so the CG can monitor their
progress to the incident(s).
The Nuclear subs do have AIS, but they also have a switch for putting it
into Rx only.....
> My understanding is that the base stations are there to transmit
> date/time information for synchronisation and also syhchronise
> the mobile stations so that they don't all use the same transmission
> slots. There is much more detail in the AIS specification which can
> be found by pointing your browser to www.uais.org/downloadlibrary.htm
> and then selecting item 1 which is a ZIP file of a Word document. Do
> a search on base station in this document and you should get a 'feel'
> for what the base station does. I also believe in the case of very
> high traffic they can 'control' the network to avoid data collisions.
> Ver 1.4.0 of Aismon (released Apr 23rd) when used with SeaClear will
> display these base stations (at least for me!) while Ver 1.3.0 did
> Here on the West coast of Scotland I can receive 2 base stations
> operated by the Coastguard. They are situated on hill-top radio
> sites quite a distance from the sea and so you would end up on the
> beach if you used these as the position of a hazard. These will also
> be the receive sites used by the Coastguard so they can 'watch' the
> ship traffic.(I've been in the local Coastguard Center here and have
> seen their display)
> Looking forward here to the rescue helicopters being fitted with AIS
> as one flies past my house every day although I don't know if it will
> be switched on for non-emergency flights. Also it's a pity the
> nuclear subs that pass in front of my house don't switch it on..
> Neil Robertson
> --- In email@example.com <mailto:aismon%40yahoogroups.com>, "Gary
> Hahn" <garyhahn@...> wrote:
> > Oh maybe I do recall that. Basically it's to show fixed points for
> > collision avoidance. So in effect it's a mobile that never
> moves... In
> > other words, aside from never moving, it has no unique qualities
> that make