Digital selective calling (DSC) works by transmitting a hailing signal on channel 70, which is reserved for digital transmissions. Its advantages in emergencies are overwhelming. The complete Mayday call, including the identity of your boat and its precise position, is sent in about one-third of a second. Once initiated, your call will be automatically repeated until the Coast Guard answers. If you wish, the signal can include the specific nature of your distress and the number of people on board. Because it's digital, the message transmitted on channel 70 can get through to the Rescue 21 system when conditions make voice calls difficult. Once your Mayday call is received, it will be automatically recorded and will alert the crew on every boat with a VHF/DSC radio in range of your transmitter (up to 200 square miles).