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      Television on Board MMSI Issue Briefing on GMDSS

DSC VHF Radios
by Chuck Husick

While most bells and whistles are not of great importance in your choice of a VHF radio, there is one very important aspect of the radio to which you should pay careful attention. The newest radios are required by law to provide an operating mode called Digital Selective Calling, DSC. This new technique is a part of the Global Marine Distress Signaling System, GMDSS (a part of the international SOLAS treaty). DSC capability is available in two forms; Category A DSC and Category D DSC. Both category A and D radios can send and receive digitally encoded distress calls on channel 70, which is reserved exclusively for digital communications. Category A radios, however have two separate receivers, with one constantly tuned to channel 70. Category A radios are therefore more flexible and capable of delivering advanced communication capabilities.

With the press of a button, all DSC equipped VHF radios can send a distress call in digital form on channel 70. The call, sent in much less than one second, includes your unique identification number (MMSI) and if the radio is connected to a GPS or Loran C, the precise location of your boat. The distress call will continue to be sent until it is answered by another station. The station hearing the call will likely call your radio on channel 16 or, in the case of the Coast Guard on 22A and begin the process of bringing you the help you need. Having a highly automatic way of making your need for assistance known has obvious advantages.

Because class A VHF/DSC radios have added capability they are more costly than the Class D versions. The separate, dedicated channel 70 receiver in the Class A radio ensures that it is always ready to receive and respond to both emergency and routine DSC calls, even when it is monitoring any of the normal working channels or weather channels. In addition to the second receiver, most Class A DSC radios are equipped with extensive calling number memory and caller I.D. systems and telephone like keypads that greatly simplify entering the MMSIs and names of frequently called vessels and shore stations.

A Class A VHF/DSC radio makes use of the "Selective" in DSC by calling other DSC radios by their unique MMSI number. These radios usually provide an electronic notepad that stores names and MMSIs for other boats or shore stations. Another station can be called by selecting the name or number from the stored list, choosing the channel number on which you wish to communicate and pressing a button. A digital call containing the MMSI being called, the MMSI of your radio and the channel number on which you wish to communicate is sent on channel 70. The called radio recognizes its MMSI and alerts its operator to the incoming call by emitting a ringing tone. The radio automatically switches to the communication channel chosen by the caller, allowing voice communication to proceed with no further action required of either party. If your MMSI is stored in the called radio's memory the operator will be informed of your identity as in a land-line caller I.D. system. Depending on the particular capability of the radio, it may also provide call waiting alert and log calls received when the radio was unattended. Overall, a Class A DSC radio performs much like a cellular telephone.

With widespread use, the VHF/DSC system will gradually eliminate the need to monitor the often busy and noisy channel 16 for incoming calls or, as required by regulation, for distress calls. At some time in the future, monitoring of channel 16 will no longer be common practice for most boaters and will no longer be a legal requirement for vessels on the high seas.

For more information on DSC radios and MMSI registration click here.

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