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Date: Thu, 29 Jan 2009 14:48:23 +0000
From: John P-G <gm4slv@sighthound.demon.co.uk>
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Subject: Re: LF: Freq. question.
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J. B. Weazle McCreath wrote:
> Hi John, LFers,
> 
> What is the 2187.5 kHz. MF-DSC that you mentioned in your
> reply to the query about the grabbers?


Hello JB,

Not really on topic for LF, but....

DSC is "Digital Selective Calling" - a part of the maritime safety
system GMDSS. It allows individual stations/ships etc to be called for
routine traffic and for distress/mayday calls to be sent.

There are (in general) 2 forms - MF and VHF (although the "MF" variety
is also used on the HF bands).

There is software available to decode the transmissions, the most
popular of which is "DSC Decoder" available at :

http://www.coaa.co.uk/dscdecoder.htm

2187.5kHz is the international DSC distress watch channel. To receive
traffic it's necessary to tune in USB mode to 1.7kHz below this freq -
so the resolved tones are centred on 1700Hz (ie tune to 2185.8kHz) - and
also necessary to (if using DSC Decoder) to select "Invert tones" in the
setup for the MF/HF modem. I find the "Use alternative HF modem" option
also increases the sensitivity of the demod/decoder.

It's interesting to observe where in the world one receives signals on
2MHz - I often see Shanghai coast station, as well as several other
remote Coast Guard places, in the US and Canada.

Hope this at least points you in the right direction. There's plenty of
info on the web about GMDSS and DSC - one thing to bear in mind is that
all vessels and Coast Statios have an identifier called an MMSI - and
the messages decoded always bear the MMSI of the caller and called
station. The software will convert Coast Station MMSIs to names
automatically, and if you right click on a ship's MMSI it will perform a
lookup at the ITU website and translate the MMSI to the vessel's
name/callsign (if it's listed in the ITU database). Over time this
builds up a local file (shipid.txt) that is used by the software to
translate MMSI - Name automatically.

That's probably enough to get you started if you are intersted, but
again, it's not really an amateur LF/MF topic. The NDB hunters are
probably the ones most au fait with listening and hunting for this type
of traffic.

Just shows how multifaceted this hobby is... too many topics, too little
time (and equipment)

John
GM4SLV