Coast Guard Groups and Communications Stations (COMMSTAs) are responsible for maintaining radio distress watches. Presently, most radio contact is established by making a voice call and relying on an aural watch by the called party to hear the call. Digital Selective Calling (DSC) automatically monitors calling frequencies and signals when a communications link is established between marine radios (ship/shore ship/ship) for distress calls and routine operational communications, thus simplifying this process. The advantages of DSC include faster alerting capabilities and automatic transmission of information such as the nature of distress, situation, and the identity and location of the caller. Further, in a non-distress situation, DSC minimizes the time necessary to establish communications and increases spectrum efficiency.
DSC is a semi-automated method of establishing a radio call; it has been designated by the International Maritime Organization (IMO) as an international standard for establishing maritime MF, HF, and VHF radio calls. It has also been designated part of the Global Maritime Distress and Safety System (GMDSS). DSC will eventually replace aural watches on distress frequencies and will be used to announce routine and urgent maritime safety information broadcasts.
To understand DSC, let's look at the term
DSC uses a DIGITAL signal to send a specific set of information. The information that can be passed by a DSC call includes:
DSC calls are SELECTIVE because unlike traditional voice radio calls, DSC calls can be addressed to a certain user or set of users. For example, DSC calls can be addressed to:
When a DSC receiver "hears" a DSC call it looks at the address, geographic information, etc. It alerts an operator only if the call is intended for that unit. This eliminates the need for a watchstander to constantly monitor the receiver.
As stated before, DSC is primarily designed to establish a radio call. When a DSC call is received, the receiving station will use the DSC protocol to acknowledge the call. At this point the two parties will move to a working frequency and a different mode (voice, fax, RTTY) to complete the call. Generally a DSC-equipped radio will "ring" when called, as would a telephone.
The Coast Guard will use DSC to listen for distress calls, to initiate radiocommunications with other vessels, to announce urgent maritime information broadcasts, and to track vessels. Instead of a radio operator maintaining an aural watch on a voice channel, a computer built into the radio will listen for DSC calls on internationally-designated radio frequencies. Eventually, the Coast Guard will be able to discontinue the existing aural watch on 2182 kHz and other voice channels as mariners convert to DSC-equipped radios.
The Radio Systems Branch (TISCOMeng-3) has procured and is installing DSC systems at COMMSTAs, and will install DSC at Groups throughout the Coast Guard. The general design consists of a transmit/receive subsystem, DSC processor subsystem, and operator console (Coast Guard Standard Workstation II, using the CTOS operating system).
TISCOM will install a strip receiver specifically designed to receive DSC calls at the Group remote site. The same telephone lines used to receive the existing 2182 kHz audio signal will carry the DSC audio signal to the Group office. At the Group, the voice signal will follow its normal path to the existing 2182 kHz guard speaker and the DSC audio tones will go to the DSC processor. The DSC processor will then convert the tones to a format that is displayed on the operator console. The operator will acknowledge the DSC call through the console interface to the Group's existing HF transceiver.
Each COMMSTA will be equipped with six new receivers to handle DSC calls. The receivers are interfaced to the DSC processor that will decode the incoming DSC message and display it on the operator console. From the CGSW, the COMMSTA operator will develop a DSC outgoing message and (after selecting and tuning a transmitter to the appropriate frequency) acknowledge the DSC call.
HF and MF DSC-equipped stations are receiving a dozen or more relay calls for every distress call received, after the original distress call was acknowledged. About 99% of distress calls were sent unintentionally. Persons sending false distress calls often are not listening to the associated radiotelephone channel, do not respond to calls from the coast station receiving the call, and make little effort to cancel the inadvertent distress alert. Many distress calls have an improper or old position information. DSC is new, and many operators are not as familiar with its operation as they could be. We believe DSC problems can be resolved over time through improved standards, operator training and experience, and violations enforcement.
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Last Updated:
2002-05-07