Digital selective calling (DSC) forms a vital subsystem within the Global Maritime Distress and Safety System, being used as the primary alerting system of the MF, HF and VHF terrestrial services for distress, urgency and safety communications. Global Maritime Distress and Safety System (GMDSS) ships will need to monitor commercial DSC frequencies in order to receive incoming commercial traffic. However, there is a statutory requirement for GMDSS ships to monitor the DSC distress frequencies. A feature of DSC is the inclusion of telecommands that permit the operator to include additional information into a DSC call. DSC alerting equipment provides for automatic alerting without the need for aural watch, and can operate under conditions of poor signal and high noise interference. Acknowledgements using DSC are usually sent by a coast station and should be transmitted on the same frequency as that on which the alert was received.