Explanation: The Digital Selective Calling (DSC) controller is a vital component of a vessel's Global Maritime Distress and Safety System (GMDSS) equipment. To ensure its immediate operational readiness for distress alerting and general communication, especially when a vessel is underway and potentially in a remote area, regulations mandate a functional check of the DSC controller **once a day**. This daily test verifies the equipment's ability to transmit and receive, often by initiating a test call to a shore station or another vessel, or by using an internal self-test function if it adequately confirms external functionality. This frequent testing is critical because an undetected malfunction could severely compromise a vessel's ability to send or receive distress alerts, thus jeopardizing safety of life at sea. Less frequent testing (weekly, twice a week, or monthly) would increase the risk of an unaddressed fault persisting for too long, diminishing the system's reliability in an emergency.