DSC - Digital Selective Calling -

Nichita Radu-Andrei TA 31

Digital Selective Calling or DSC is a standard for sending pre-defined digital messages via the medium frequency (MF), high frequency (HF) and very high frequency (VHF) maritime radio systems. It is a core part of the Global Maritime Distress Safety System (GMDSS).

DSC was developed to replace a call in older procedures. Because a DSC signal uses a stable signal with a narrow bandwidth and the receiver has no squelch, it has a slightly longer range than analog signals,[1] with up 25 percent longer range and significantly faster.[2] DSC senders are programmed with the ship's Maritime Mobile Service Identity (MMSI) and may be connected to the ship's Global Positioning System (GPS), which allows the apparatus to know who it is, what time it is and where it is. This allows a distress signal to be sent very quickly

Often, ships use separate VHF DSC and MF/HF DSC controllers. For VHF, DSC has its own dedicated Receiver for monitoring Channel 70, but uses the main VHF transceiver for transmission. However, for the user, the controller is often a single unit.[1]

 MF/HF DSC devices monitor multiple distress, urgency and safety bands in the 2, 4, 6, 8, 12 and 16 MHz bands. At minimum, controllers will monitor 2187.5 kHz and 8414.5 kHz and one more band.[3]

When sending a distress signal, the DSC device will at minimum include the ship's MMSI number. It will also include the coordinates if available and, if necessary, the channel for the following radiotelephony or radiotelex messages.[3] The distress can be sent either as a single-frequency or multi-frequency attempt. In the former, a distress signal is sent on one band and the system will wait up to four minutes for a DSC acknowledgment from a coast station. If none is received, it will repeat the distress alert up to five times. In a multi-frequency attempt, the distress signal is sent on the MF and all the HF distress frequencies in turn. As this requires retuning the antenna for each sending, without waiting for an acknowledgment, a multi-frequency attempt should only be done if there are only a few minutes until the ship's batteries are under water. As the distress message can only be sent on one of the bands, many ships and coast stations may be listening to a band without the message, and will after five minutes relay the distress signal to a coast station.

Distress calls can be both non designated and designated. The latter allows one of ten pre-defined designations to be sent along with the distress signal. These are "abandoning ship", "fire or explosion", "flooding", "collision", "grounding", "listing", "sinking", "disabled and adrift", "piracy or attack" and "man overboard".[4] To avoid false distress alerts, distress buttons normally have protective covers, often with a spring-loaded cover so two hands need to be used simultaneously. Alternatively, some devices have two-button systems.[5]

 Operators are required to cancel falsely sent distress alerts with a transmission on the channel designated by the distress signal.[6]

A coast station which receives a DSC distress alert will immediately send an acknowledgment. The sending device will then both stop repeating the alert, and tune to the designated channel for the distress message to be sent. Ships receiving a distress alert who are outside of coast station range or do not receive an acknowledgment, are required to relay the distress alert by any means to land.[5]

Class A devices, used on commercial ships, have the ability to send distress, distress relay, all ships urgency, all ships safety, individual, group, geographic area and telephone alerts. Class D devices, used for most leisure vessels, can send distress, all ships urgency, all ships safety and individual on channels 06, 08, 72 and 77. The latter is only required to have one antenna and is thus not required to watch Channel 70 when in use. For routine alerts, which are used to establish communication with another station on a working channel, the receiver acknowledges to confirm that communication can be done on the appropriate channel.

The DSC is a synchronous system using characters composed from a ten-bit error detecting code. The bits are encoded using frequency shift keying at 100 Baud. For High Frequency and Medium Frequency two tones 170 Hz apart either side of the allocated frequency are used. For VHF the two frequencies used are 1300 and 2100 Hz. Each character is transmitted twice with a time delay. The detailed specification is published in the International Telecommunications Union recommendation ITU-R M.493-12[8]

Operations

DSC is used to establish initial contact between stations.

Following an alert by DSC, communications are normally carried out by radiotelephone or Narrow BandDirect 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; andthe nature of the distress.

DSC channels

The ITU has allocated a DSC distress and safety channel in the MF, each of the HF and the VHF marine radio bands.  These are:

MF/HF DSC

DISTRESS AND SAFETY CHANNELS

2187.5 4207.5 6312.0 8414.5 12577.0 16804.5 (kHz)

VHF DSC

DISTRESS AND SAFETY CHANNEL

VHF marine channel 70

Note that voice transmissions are PROHIBITED on the DSC channels.

The MF/HF channels are restricted to distress, urgency and safety traffic only because of the relatively low speeds of transmission of 100 baud. If too many calls were permitted on the MF/HF channels, the channels would quickly become overloaded to the point where a distress call may be blocked.

VHF DSC operates at 12 times the speed of MF/HF - accordingly, all priorities of call are allowed on the VHF channels.

The ITU has also allocated a suite of HF channels dedicated to DSC commercial operations.  These may be found in the ITU DSC operational specification

DSC call categories

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

DISTRESS

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.

Maritime Identification Digits (MID)

The first three digits of the MMSI are known as the Maritime Identification Digits (MID). The MID represents the country of registration of the vessel, or the country in which the DSC shore station is located. MIDs are allocated on an international basis by the ITU, in much the same way as a callsign prefix.

Formation of MMSI's

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.

Typical Australian merchant vessel MMSI- 503001000

where:

503 is the Australian MID;and01000 is the individual ship number

Typical Australian recreational vessel MMSI - 503000100

where:

503 is the Australian MID;and000100 is the individual ship number

Coast Station MMSI - 005030001

where:

503 is the Australian MID; and0001 is the individual Coast Station number

 

SAR aircraft MMSI - 111503123

where:

503 is the Australian MID; and123 is the individual aircraft number

Hand held radio with DSC MMSI - 850312345

where:

503 is the Australian MID; and12345 is the individual radio number

 

Typical DSC man overboard beacon - 972450001

where:

972 is the DSC MoB code;45 is the manufacturer codeand0001 is the individual beacon number

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".

OPERATIONAL PROCEDURES

The official guide to DSC procedures is contained in ITU recommendation ITU-R M.541-9.

IMO have issued a circular which contains further clarifications on DSC procedures: