Tuesday, October 23, 2012

Digital Selective Calling (DSC)

DSC equipment is an ALERT PROGRAMER which can transmits and receives all kinds of DSC alerts. It works on digital mode (Telex mode Emission F1B). It is only a MODEM. It can not transmit and receive all kinds of DSC alerts. However a dedicated DSC scanning watch keeping receiver is also available to receive DSC alerts which is connected to modem.

The DSC system's digital processing techniques, combined with the relatively narrow receiver bandwidths used, provide a DSC signal with resistance to noise and fading over the radio path.
This results in increased range compared with radiotelephone transmissions.

Unfortunately, DSC remains one of the GMDSS' least understood sub-systems.  This lack of understanding is reflected in the very high DSC false alert rate.


DSC is used to establish initial contact between stations.

Following an alert by DSC, communications are normally carried out by radiotelephone or Narrow Band Direct Printing (NBDP - radio telex).

DSC can be considered as a replacement for the radiotelephone and radiotelegraph (Morse) alarm signals.

Rather than just indicate that the sending station is in distress, the DSC system allows a great deal more information to be transmitted, including:

- the priority of the call - DISTRESS, URGENCY, SAFETY or ROUTINE;
- the address - ie: all ships or a single ship/station;
- the identification of the ship in distress;
- the position of the ship in distress; and
- the nature of the distress.

DSC call categories

The DSC system supports a number of call categories. These categories mirror the standard maritime prioritisation of message traffic, ie:

URGENCY               SAFETY               ROUTINE

Distress alerts are automatically addressed to all stations.

Urgency, safety and routine calls can be addressed to all stations, an individual station, or a group of stations.

Maritime Mobile Service Identities (MMSI)

All DSC equipment is programmed with a unique nine digit identification number, known as a Maritime Mobile Service Identity (MMSI).

The MMSI is sent automatically with each and every DSC transmission made.

MMSIs allocated to merchant vessels are normally allocated with three trailing zero's.
Those allocated to recreational craft have two or one trailing zero, Coast Station MMSI's are formed with two leading zero's, those allocated to SAR aircraft use 111 as the first three digits, hand held radios have 8 as a leading digit and Man Overboard beacons have their own code structure, starting with 972.
 DSC equipment
GMDSS DSC equipment is normally comprised of a stand alone control unit, with an alpha-numeric display screen and a keyboard on which to compose messages.

The control unit controls the actions of the DSC modem (modulator demodulator).  The modem is interfaced to a DSC watchkeeping receiver - this receiver is fixed tuned to either the VHF DSC channel (ch. 70), the 2 MHz DSC channel, or the HF DSC channels.

HF DSC watchkeeping receivers are designed to scan the 6 MF/HF DSC channels in rapid sequence (2 seconds or less).

DSC watchkeeping receivers are fitted with their own dedicated antennas.

The DSC modem decodes all calls on the frequency to which the watchkeeping receiver is tuned.  If calls are received addressed to all ships, or to the particular ship on which the DSC system is fitted, the DSC controller sounds an alarm, and displays the decoded information on the alpha-numeric display.

To transmit a DSC call, a GMDSS operator enters the required commands to identify the station (or stations) with which communication is desired, the priority (DISTRESS, URGENT, SAFETY or ROUTINE) and the purpose of the call.

Once the call is composed, the CALL button is pressed on the DSC controller, and the information is sent to the associated transmitter for transmission.

All DSC systems provide complete remote control of the associated transmitter - the selected DSC frequency information is fed to the transmitter over a serial control link from the DSC controller.

The whole process is automated - the DSC system instructs the transmitter to change to the required DSC channel, the transmitter changes channel and (in the case of MF/HF systems) tunes its antenna system.  The transmitter then signals a ready command to the DSC controller, which sends the information for broadcasting.  The entire process takes only 3 to 5 seconds.

DSC controllers are also required to be interfaced to GPS receivers for automatic updating of position and time information.  This information is automatically included in distress calls.

DSC controllers are also required to be equipped with a DISTRESS button, which allows the transmission of a distress call with minimum delay. The button is required to be protected by a cover, and also can only be activated after "2 separate and independent actions".

If the false alert is sent on any DSC frequency. Switch off/Reset the equipment to stop auto repetition. On corresponding voice frequency, broadcast cancellation message to All stations giving ships particulars.