There is no other frequency in the electro-magnetic spectrum assigned to a special service than 500 kHz / 600 meters for such a long period. Since the radio conference of Berlin in 1906 this frequency was allocated to maritime radio service. It was in use since that time and served as the international distress and calling frequency. It is given to the International Martime Organization (IMO) and manged by the International Telecommuncation Union (ITU).

It was used continously in maritime mobile service all over the world. Even in wartimes this frequency was used sometimes between the enemies to signal information. The medium wave bands were keyed in Morse/Gerke code only. This medium wave length frequency was for use for distances of up to approximate 500 miles or even more. This is the area ships are close enough to be of help for a ship in distress.