MMSI's allocated to merchant vessels are normally allocated with three trailing
zero's. Those allocated to recreational craft have two or one trailing zero,
and Coast Station MMSI's are formed with two leading zero's.
For example:
Typical Australian merchant vessel MMSI- 503001000
where:
503 is the Australian MID;
and
01000 is the individual ship number
Typical Australian recreational vessel MMSI - 503000100
where:
503 is the Australian MID;
and
000100 is the individual ship number
Coast Station MMSI - 005030001
where:
503 is the Australian MID; and
0001 is the individual Coast Station number
The ITU operate an international web-based data base of MMSI's, known as the
Maritime Mobile Access and Retrieval System (MARS).
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.
DSC controller
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.
DSC system
The DSC modem is also connected to an associated MF/HF or VHF transceiver to
allow the transmission of DSC calls.
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".
DSC controller