Digital Selective Calling

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VHF radio with attached Digital Selective Calling Controller

Digital Selective Calling ( DSC ; Digital Selective Calling ) is a call process in the maritime radio , which is alternatively used for voice transmission call. A call is used to establish a radio connection with a certain radio station , all accessible radio stations ( all ships / stations call ) or all radio stations in a certain geographical area ( area call ), whereby the calling radio station informs the radio stations on which radio channel or on which frequency ( working channel ) she wishes to connect. DSC is an important element in the Global Maritime Distress Safety System .

With DSC, the call is made by transmitting a narrowband telex transmission, which is why both sending and receiving radio station (s) must be equipped with additional devices (so-called DSC controllers) on their marine radio systems (these days often integrated). The radio operator, d. H. The calling radio station provides the Maritime Mobile Service Identity (MMSI) of the radio stations called, the type of call (emergency, urgency, safety or routine traffic) and, if applicable, the channel (for VHF marine radio devices) or the frequency ( in the case of border wave / short wave marine radios) for the subsequent radio communication in the DSC controller. The DSC controller of the calling radio station automatically adds the exact time and position (if a GPS device is connected) or the last manually entered position and time as well as the MMSI of the calling radio station. All other information - such as B. the type of assistance required, the type of obstacle for shipping or the content of a routine radio call - are transmitted via voice radio after the DSC call. DSC is therefore not a substitute for radio communication, but only for radio calls.

Particular advantages of the DSC include: a .:

  • An acoustic and optical signal is used to indicate the incoming call to the radio station (s) called, so that constant monitoring of the entire radio traffic for relevant calls is not necessary
  • the data transmitted via DSC appear in plain text on the DSC controller of the radio station called, so that errors due to writing down the call or due to incomprehensible voice transmission do not occur
  • the data transmitted via DSC are stored in a memory of the DSC controller and can therefore be called up at any time.

The digital selective call is transmitted on special, reserved channels or frequencies. With VHF this is channel 70. With border and short wave there are separate frequencies for emergency, urgency and safety calls (on border wave the frequency is 2187.5 kHz ( MF ) and with short wave 4207.5 kHz, 6312 kHz, 8414.5 kHz , 12,577 kHz and 16,804.5 kHz ( HF )) and for ordinary calls, see the list in Maritime Mobile Service . Radiotelephony is prohibited on these channels or frequencies.

The data is transmitted on VHF channel 70 (156.525 MHz) at a data rate of 1200  baud , so that the transmission of a DSC call takes about half a second. In the case of borderline waves and shortwave, the data rate is 100 baud, so that the transmission of a DSC call takes about seven seconds. The frequencies used on UKW are 1300 and 2100 Hertz, on Grenzwelle, Kurzwelle the center frequency 1700 with deviations of + - 85 Hertz.

If the MMSI of a marine radio station is also known, its position can be queried via DSC and thus the ship's journey can be monitored. This usually happens without the ship's knowledge, but can lead to annoying acoustic error messages if the GPS receiver is switched off.

One should not confuse DSC with AIS ( Automatic Identification System ), which continuously transmits the position, course, destination and other information on two VHF channels in order to avoid collisions with other ships. Both systems can be listened to with standard radio receivers (scanners) and the data is often disseminated on the Internet (see Vesseltracker ).

Web links

  2. RECOMMENDATION ITU-R M.493-11 * Digital selective-calling system for use in the maritime mobile service