Field Guide · technology

Also known as: marine VHF, VHF marine band

Marine VHF is the internationally standardised set of radio channels near 156–162 MHz used for communication at sea. It carries FM voice for ship-to-ship and ship-to-shore calling, distress and safety traffic, port and navigation coordination, and — on dedicated channels — digital data such as DSC calling and AIS position reporting.1 Its fixed, worldwide channel plan means a vessel can raise another ship, a marina, or a coast station almost anywhere using the same numbered channels.

Ch 16 Ch 70 156 MHz 162 MHz Ch 16 = distress & calling (FM voice) · Ch 70 = DSC (digital)
Marine VHF divides the band into standard numbered channels, with Channel 16 for FM distress and calling and Channel 70 reserved for DSC.

How it works

Marine VHF uses narrowband FM with channels spaced 25 kHz apart across roughly 156–162 MHz. Channels are numbered in an internationally agreed plan and split into simplex channels (transmit and receive on one frequency, for ship-to-ship) and duplex channels (separate ship and shore frequencies, for public correspondence and port operations). Two channels are effectively universal: Channel 16 (156.800 MHz) is the distress, safety, and calling channel that every vessel monitors, and Channel 70 (156.525 MHz) is reserved entirely for DSC digital calling rather than voice.

Sets typically offer a dual-watch feature that keeps Channel 16 monitored while the crew works another channel, and transmit power is switchable between about 25 W for range and 1 W for close-in or port use to limit congestion. Because VHF propagates roughly line-of-sight, practical range is set by antenna height — a handheld reaches a few miles, while a masthead antenna talking to a tall coast station can span tens of miles.

Relevance to SDR

The marine VHF band is an easy and rewarding target for a software-defined radio: FM voice channels demodulate with standard narrowband FM, while the same slice of spectrum carries the digital signals covered elsewhere in this guide — DSC bursts on Channel 70 and AIS near 162 MHz. GopherTrunk is built for land-mobile trunking rather than marine voice, but it does decode the marine data signals AIS and DSC that share this band; plain FM voice on the voice channels is outside its trunking focus and is better handled by a general scanner.

Sources

  1. Marine VHF radio — Wikipedia, for the marine VHF channel plan, FM voice operation, Channel 16 distress/calling and Channel 70 DSC assignments, and simplex/duplex usage. ↩

See also