Marine VHF, DSC, and MMSI basics

Every marine VHF sold since 1999 has DSC (Digital Selective Calling) capability, but many boats never enable it. To use DSC you need an MMSI (Maritime Mobile Service Identity), a 9-digit ID. In the US, coastal recreational boaters get an MMSI free from BoatUS or Sea Tow (registered in the USCG database). Boats leaving US waters need an MMSI from the FCC ($225), which registers to the international database and requires a call sign. Once programmed (each radio only accepts an MMSI programming ONCE, a locked field for anti-theft reasons), pushing the red DISTRESS button broadcasts a Mayday with your MMSI, GPS position (if the radio has NMEA input), and boat name to every VHF and Coast Guard station in range. Without DSC, a Mayday is a voice call only, no automatic ID or position. Fixed vs handheld: fixed VHF must be 25W (75W for commercial); handheld is 6W max. Range: line-of-sight only, so a 25W fixed at the masthead reaches 20 nm, handheld at deck level reaches 5 nm. AIS transponders share the VHF channel plan, never install a VHF and AIS on the same antenna without a proper AIS splitter (a passive splitter kills your VHF sensitivity). Channel plan: 16 is distress/hailing, 22A is USCG working, 68/69/71/72/78A are recreational chat, 09 is alternate hailing in some areas.