Digital selective calling
Digital selective calling (DSC) is an extension to maritime radio, especially on VHF but also MF, which gives "telephone numbers" called maritime mobile service identifiers (MMSI) to vessels. It allows a call to be made to a specific vessel or to a group of vessels. 
On VHF, the signaling is done on Channel 70. Implementing GPS does away with the need to continuously monitor distress Channel 16.
DSC and Safety of Life at Sea
It is also part of the Global Maritime Distress and Safety System(GMDSS), created by the Safety of Life at Sea (SOLAS) convention, under the International Maritime Organization. All marine radios currently manufactured are required to have a distress button or switch, which will send a MAYDAY distress signal on the appropriate emergency channel. There is no need to speak; the switch will contain the MMSI. Even more important, if the radio is connected to a GPS receiver, it will put the exact location of the vessel into the message, which is immensely beneficial to search and rescue operations.
Once the distress control is activated, the radio will distress message will be automatically repeated every 4 minutes until it is acknowledged either by a Coastguard station or ship within radio range. If it is activated by accident, while starting the transmission, immediately power off the transmitter. Otherwise, the call must be cancelled immediately, by switching to Channel 16 and make an "all ships" broadcast ship's name, call sign and DSC number, and cancel the false distress alert.
The basic DSC capability is built into all modern marine radios, and most will have a GPS connector; some radios already contain a GPS receiver. Some countries, such as Canada, require the DSC-GPS interface.
Some DSC-VHF radios do require two antennas, one for the DSC signaling on Channel 70, and the other for the actual conversation.
Obtaining and setting the MMSI can be an administrative and operational challenge. Each nation has its own procedure for assigning the MMSI unique to the vessel. In the United States, a commercial vessel that must obtain a Ship Radio License from the Federal Communications Commission can obtain the MMSI with the license.
Recreational boaters, in the U.S., can get an MMSI assigned, at no cost, by several boating organizations and service firms. Unfortunately, there is a gap in the system for small commercial fishing boats and other vessels that are not recreational, but are not legally required to spend $160 for a 10-year Ship Radio License. If you are a commercial fisherman or other small non-recreational boat operator, some research may be needed in finding a way to get the MMSI assigned.
Setting the MMSI
The DSC system assumes that radios stay with the vessel, so once set, the MMSI cannot be changed other than by the factory. It is wise to have two people checking the keying of the MMSI into the radio. While there is usually a method of starting over if the MMSI is not fully entered, depending on the radio, it may give only one or two tries to set an MMSI.
The NMEA 0183 standard for interconnection of marine electronics supports DSC and GPS messages, so, as long as the radio and the GPS receivers have NMEA 0183 interfaces, the technical requirement is connecting two wires.
Unfortunately, NMEA 0183 does not specify a physical connector, so it may require technical assistance to find the the output connector. on the GPS and the input connector on the DSC radio. In a few lucky cases, usually when both devices are made by the same manufacturer, there may be a premade cable.
More commonly, it will be necessary to make an adapter cable. This does not require a high degree of electronic skill, but, if one is not experienced in making reliable cable connections, it may be best to leave this to a competent technician. Your life may depend on that connection.
Relation to other GMDSS systems
In its emergency role, DSC complements, but does not replace, the emergency position indicating radio beacon (EPIRB). EPIRBs are designed to be triggered by water, floating free when a vessel sinks. While some have the ability to be triggered manually as well as floating, they provide no capability to talk to rescuers to give additional information.
For emergencies not involving sinking, such as fire, piracy, life-threatening medical problems, etc., DSC gives the ability to request help immediately. All vessels and aircraft with DSC receivers will sound an alarm on receiving a distress signal, and are required to relay it until search and rescue agencies take control. Since a receiving vessel will have the signaled position, and is nearby, it may be able to be of immediate assistance.
Maritime Safety Information (MSI) broadcasts from marine safety authorities automatically will ring DSC radios and alert mariners to listen for important safety information.
DSC also complements the automatic identification system (AIS). AIS will provide the MMSI of a nearby vessel, so it can be called directly, rather than the obscure "vessel steering course 340 in Boston Harbor"; while not widely implemented, DSC and AIS provide the ability to send short text messages to other vessels.
DSC is useful for routine situations. While it does not give the privacy of a cellular telephone, it does have the ability to "ring" a specific vessel or authorized shore station. For example, boats working together can, with a suitable chartplotter, display the location of a boat calling on VHF. This is useful, for example, if one boat finds a good fishing spot but doesn't want to broadcast its location in clear voice.
The call is set up digitally on Channel 70, and the vessels then switch to another channel for their conversation.
Depending on national policies, it may be possible to obtain a special MMSI for a group of vessels, such as participants in a boat club or regular race.