Explanation: A Digital Selective Calling (DSC) Controller is the core component for initiating programmed transmissions of Distress alerts and calls to specific individual stations. A DSC controller is typically integrated into a marine VHF or HF radio. It allows users to digitally encode and transmit specific types of calls—like a distress alert (MAYDAY), urgency (PAN-PAN), safety (SECURITE), or routine calls to a vessel's unique Maritime Mobile Service Identity (MMSI) number. This digital signal is sent on a dedicated DSC frequency (e.g., VHF Channel 70). NAVTEX (A) is a receiver of maritime safety information, not a transmitter. GPS (B) provides position data that a DSC controller can utilize but does not initiate transmissions itself. A DSC Scanning Watch Receiver (D) monitors for incoming DSC calls, it does not initiate outgoing transmissions.