Monday, July 19, 2010

Digital Mode Programs That Work #1: DSCdecoder

DSC stands for Digital Selective Calling. It's an automated mode used with the Global Maritime Distress and Safety System (GMDSS). DSCdecoder is a small, sleek little utility which does exactly what its name promises. It decodes DSC calls, on all three bands (MF, HF, and VHF). These are output to the screen with the MMSI (Maritime Mobile Service Identity) of the calling and called station, plus the message type and any optionally transmitted data such as the reply frequency and mode. You can have the program search over the Internet for the MMSI and substitute it with the station/vessel name, if found.

DSCdecoder is good to just leave going and see what accumulates. DSC is not a busy mode, but it's a fun one to DX. This program does a really nice job of grabbing the real weak stuff.

There's more, though. This program also decodes Navtex (Navigational Telex) and Differential Global Positioning System (DGPS). It does a similarly bang-up job on these. It's the best one of several ham-level programs that I use.

Navtex is a Sitor-B service on three primary frequencies. These are 518 kHz (main freq, nearly always in English), 490 kHz (specific nations and languages, not used in the US), and the more lightly used HF frequency of 4209.5 kHz. The Sitor-B waveform is very close to DSC's (on HF), though other parameters are quite different. Therefore, the decoder performs well on it too. Those who like to tweak things, and who can tune accurately, can set up the advanced decoder. The improvement is slight, but definite.

Navtex stations take turns broadcasting schedules of short, highly formatted, maritime safety and weather messages, from different coastal locations. These rotations typically repeat several times a day, usually every 4 hours. Professional GMDSS equipment keeps a list of headers and typically only prints new messages. Most large vessels will have a Navtex printer somewhere on the bridge.

Navtex is useful for weather forecasts, and it's also fun to DX. Navtex stations are designed for specific local coverage areas, but at night they can propagate amazingly well for this type of station.

The extra dividend is that any Sitor-B decoder that can handle Navtex will work on most other transmissions in this mode. These are all over HF, coming from coast guards and meteo services worldwide.

DGPS is an interesting mode, using minimum-shift keying (MSK). It takes some tuning around in its low band to find strong stations. Fortunately, these are being implemented pretty much worldwide.

Stations use two different baud rates, identifiable from their bandwidth. DGPS decoders for our use turn its packetized data streams into lines of rather cryptic text data. I mostly use this for DXing, and I have checked off an amazing number of DGPS stations, considering that it is designed as anything but a DX mode.

Not many PC programs do DGPS. (Mac users are lucky. They have MultiMode, an excellent program.) Two that do are this one and the no longer supported SkySweeper. I've run them side by side, and most of the time DSCdecoder has successfully recovered more packets.

DSCDecoder is available from The first 21 days are free, so you can try it out, get over a bit of a learning curve, and see if it's for you. After that, it's 25 Euros (a little over 32 bucks at today's exchange).