MF (Medium Frequency) and HF (High Frequency) radio equipment is primarily used for long range communications in the maritime industry. Medium frequency generally works well up to two hundred miles and high frequency is primarily for greater distances. Due to the nature of MF and HF radio signals, they can not be deemed totally reliable. Such factors as time of day, location of both sending and receiving stations, weather conditions and sun spot activity all contribute to making this service tenuous. With the arrival of GMDSS, a new feature of DSC (Digital Selective Calling) has been added to MF and HF communications.