Explanation: A marine DSC (Digital Selective Calling) VHF radio requires a GPS input to transmit your vessel's position during a distress call. When the radio is successfully receiving GPS data sentences (like NMEA 0183 GPGGA or GPRMC) from the GPS receiver, it processes this information and displays the current latitude and longitude on its screen. This direct visual display of the coordinates is the most reliable and immediate way for an installer or user to verify that the GPS unit is connected, powered, and sending valid position data to the radio. Pressing the distress button (A) sends an alert and does not confirm GPS input. While some radios may have a "GPS fix" indicator (C), seeing the actual latitude and longitude confirms the radio is interpreting the *sentence data* rather than just a signal presence. Asking for a VHF radio check position report (D) confirms radio operation with another station, not the internal GPS data integrity.