On HF you'll hear AMTOR/FEC (Amateur Teleprinting Over Radio/Forward Error Correction) and NAVTEX (Navigational Telex) transmissions scattered about the band. It is a digital mode, sounds somewhat like RTTY but with a more rhythmical sound. These transmissions often consist of navigational warnings, maritime weather, and alerts on ships missing at sea. Interesting reading. Below is one gotten last night, seems like some ships are missing. Description of the ship is included, for those at sea to "keep a sharp lookout" for. Used a FRG/7, SignaLink USB, and MultiPSK to get this one. You can also use an SDR dongle too, they are stable enough for these transmissions. But have to say there's nothing like a large tabletop shortwave radio aglow with lots of knobs and dials to spin.
