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Understanding VHF-DSC Marine Radios

Technical Information You Need to Know Before You Buy




Many marine VHF radios used by sailors and other boaters are equipped with DSC, or digital selective calling, functions. This article describes some of the technical aspects of DSC, different classes of equipment, and specific DSC functions. For a more general overview of what DSC adds to VHF radio for recreational boaters, start with this article, which covers these key points:

  • How a DSC distress radio call works and its benefits
  • Why DSC should be activated with a personal MMSI number
  • How to obtain an MMSI number free in the U.S.
  • Why the DSC-VHF radio should have built-in GPS or be connected to a GPS receiver

If you have questions related to those topics, please read that article first, and then return to this page to learn more about classes of DSC equipment (important if you will be buying a new radio) and other non-emergency DSC functions you can use.

Classes of DSC Equipment

DSC transmissions occur on the dedicated VHF channel 70 - this is an international standard and is used with all equipment classes. Therefore DSC communication is consistent among all DSC radios regardless of class. Regardless of the type of DSC-equipped VHF radio you now purchase and install, its signal can be picked up by any other DSC unit. Still, there are important differences among types of equipment.

Class A DSC radios meet higher standards and meet requirements for certain merchant vessels. Class A equipment, for example, includes a separate antenna and receiver that are tuned to and used exclusively by channel 70, so that DSC calls are received regardless of what the radio operator may be transmitting or receiving on another VHF channel. Class A radios have additional features useful for commercial ships but seldom used by recreational boaters - and cost substantially more. There is little reason for most recreational sailors to even consider a class A radio, and in fact you won't typically even see one at most marine stores.

Class D DSC radios meet the current standards for DSC for VHF radios for recreational boats. When activated and connected to a GPS receiver, a Class D DSC-VHF radio has all the functions listed below. Remember that to take advantage of these features, you must register your radio's MMSI and have GPS. Note that as of 2011, all new fixed-mount VHF radios in the U.S. must meet Class D standards - although some older units with SC101 may still be around. Be very careful when buying a handheld DSC VHF, however - see the following.

The SC101 standard is an older DSC standard that has more limited functions and has been replaced by Class D for recreational standards. In 2011 the sale of new SC101-type radios was banned by the FCC in the U.S., but it is important to note that for handheld DSC VHF radios, the ban starts in the year 2015. As of this writing, two of the three handheld DSC VHF radios being marketed widely are SC101 types. While these radios (if correctly registered and activated) will still broadcast the DSC distress call, they do not have all other DSC features. So if you are shopping now for a DSC handheld, it's worthwhile to consider what features you may want.

DSC Class D Functions

Again, these functions depend on the radio having been properly registered with the correct MMSI and having a GPS signal.

In an emergency, pressing a single button begins an automatic transmission of a distress call that will continue until you stop it. (This allows an untrained boat guest to make the call - or you to call quickly and then attend to the emergency rather than stay on the radio to manually relay your distress, your location, etc. The transmission is continually sent, reducing the risk of a garbled communication.) The distress call includes your personal ID (important for Coast Guard assistance) and your location. Importantly, all DSC-equipped radios functioning nearby will also receive your distress call (which sounds a radio alarm), so that nearby vessels may come to your aid.

If you have time and so choose, you can also specify the nature of your distress for the automatic call - if that information may assist rescuers.

Additional nonemergency functions of DSC VHF radios include:

  • Ability to store the MMSI numbers of friends, much like storing numbers in a cell phone
  • Ability to call another VHF directly (not by hailing on public channel) by its SSMI number
  • When receiving a private transmission to your personal SSMI, the radio "rings" like a phone and switches to the caller's frequency
  • Position polling is a function that allows one radio to request the position (GPS coordinates) of another radio with a known MMSI
  • In non-distress situations where urgency or safety is still an issue, a call can be directed to all stations nearby (such as to reach a ship a few miles away to discuss a potential collision course)

The exact steps for these DSC functions vary among different models and are described in the radio owners manual. (The distress call button is clearly labeled and its use generally self-evident.)

Finally, note that your unique SSMI number is assigned to the boat, not to you as a person - although it identifies you as well with personal data. Therefore you need to modify your data if you change boats or take the device to a different boat. Your same SSMI is also used for an EPIRB or PLB on the boat.

Related articles of interest:

Using AIS on Your Sailboat
Sailing Safety
Best Sailing and Boating Apps

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