DSC is used for a number of reasons and these are:
- Automatic rather than manual radio watch-keeping is available
- Alerts using DSC are very quick (about 0.5 seconds on the dedicated frequency on marine VHF band) and do not occupy as much time as a manual voice call. This is very important particulary in areas where VHF channels are often occupied.
- Distress alerting can be enabled quickly with one press of the “Distress” push button.
- Various categories of alert are available, in the following order of priority: Distress, Urgency, Safety and Routine.
The following VHF DSC alerts are available (Whom are we calling?):
- ALL SHIPS – an alert to all ships received within the VHF range of the station sending the alert
- INDIVIDUAL – an alert addressed and received by only one radio station within the VHF range
- GROUP – an alert addressed and received by all those vessels having the group MMSI within the VHF range
- GEO – an alert to a specified geographical area received by all stations within that area
The types of DSC alerts are related to a particular category or priority.
Categories (priorities) of DSC Alerts (Why are we calling?):
DSC alerts are classified according to their priority and they are indicated below in decreasing order of priority:
- DISTRESS – indicates that a person or a ship is in grave and imminent danger and requires immediate assistance (the highest priority usually broadcast to all stations)
- URGENCY – indicates an urgent call concerning the safety of a person or vehicle (can be broadcast to all ships, a geographical area or to an individual station)
- SAFETY – indicates a call concerning MSI (Maritime Safety Information), for example, meteorological forecast/broadcast or navigational warning (can be broadcast to all ships, a geographical area or to an individual station)
- ROUTINE – indicates a call at the lowest priority concerning routine communication, for example, setting up of an RT link call via a CRS to a shore-based subscriber (normally broadcast to all ships or to an individual station)
DSC alerts are used to attract the attention automatically, of those stations addressed, normally so that voice communication is subsequently established.
The alerts are generated by the VHF DSC radio. Its function can be regarded as a cross between a normal telephone and a traditional marine VHF RT radio combined into one unit. The DSC functions via the DSC Controller or Modem, which simply sends a burst of digital code on VHF CH 70 that, will automatically “ring” another DSC radio.
Once the alert has been accepted or acknowledged by DSC, the equivalent of lifting a normal telephone receiver, you use voice on the radio in the normal way.
If for whatever reason the alert is not acknowledged, it remains unanswered, but the details will be stored in the memory or log of received alerts.
Practise on the VHF radio simulator
To use the DSC functions, the VHF radio should be in DSC mode. Press the uppermost softkey until the label Rad appears on the display.
There are several classes of DSC controller, each with differing capabilities for different applications. The VHF radio used in the simulator has the most common Class D DSC controller that doesn't support some capabilities that are supported by the most powerful Class A DSC controller (exceptions are indicated in the following chapters).
The following examples indicate the regulations for setting up different categories of DSC alert and the voice call and broadcast that would subsequently follow. Practical instructions in setting up the DSC for each type of alert is indicated in the following chapters.