The Global Maritime Distress Safety System (GMDSS)

DSC radios are an integral part of the Global Maritime Distress Safety System

(GMDSS), which is an internationally recognized and agreed upon set of

procedures, communications protocols and equipment that can be used to

rescue distressed aircraft, ships and boats. The GMDSS divides the globe into

four sea areas and specifies the carriage requirements of vessels that use these

areas. The vessel’s area of operation determines its carriage requirements. For

example, a recreational vessel in sea area A1, which typically is about 20

nautical miles from the shore, has no carriage requirements. Vessels in the

remaining 3 sea areas, which collectively extend north and south to the polar

regions, must variously carry combinations of VHF, HF and MF DSC radios,

Category 1 or 2 Emergency Position Indicating Radio Beacons (EPIRB), Search

and Rescue Radar Transponders (SARTS) and NAVTEX receivers. The

remaining GMDSS components that make up the global system are ground

stations to detect VHF, HF and MF DSC distress calls, and satellite systems like

the CONPAS-SARSAT, which detects EPRIB distress signals, and INMARSAT,

which is used for communications. The Great Lakes, including Lake Simcoe, are

also covered under the GMDSS.  The Canadian Coast Guard installed DSC

stations at shore based sites covering the Canadian sides of the Great Lakes

and Lake Simcoe, in early 2006. The American side, if not already operational,

apparently will be by the end of 2007. Most North American coastal stations

should also be complete by 2007.


Introduction to DSC radios

Marine VHF radios that have Digital Selective Calling (DSC) capabilities are

easily recognized by the large red button located on the radio’s front panel. It is

labeled ‘DISTRESS’ and covered with a transparent flap that has to be raised

before the button can be depressed. Depressing this button and holding it down

for 5 seconds will result in a distress message being sent The main technical

difference between a standard marine VHF radio and one that has the DSC

capability lies in the way Channel 70 has been implemented. On a standard

marine radio CH 70 is implemented as a receive-only channel for a VHF

Frequency Modulated (FM) signal. On a DSC radio this channel is now reserved

for DSC only and should never be used for voice communication. In other words

all data transmitted and received on this channel should be digital and based on

recommendation ITU-R M.493-11, which uses a synchronous system based on

characters composed from a 10 bit error detecting code. Delving into the

technical details of this code is well beyond the scope of this short discussion.

Suffice to say the algorithms used allow for digital signals to be unlocked at

receivers that have the proper key (MMSI number). The operational frequency of

CH 70 is 156.525 MHz and the operation is simplex (Transmission (TX) and

Reception (RX) use the same frequency). CH 70 is now the channel that the

Coast Guard monitors. This is the channel that carries all the DSC transmissions.

In effect, this has become the new CH 16. The CCG will apparently be

monitoring CH 16 for the foreseeable future but the time will come when boaters

will have to rely on DSC rather than CH 16 as the distress, safety and calling

channel. Keep in mind though that DSC does not supplant or completely take

away the need to use voice communications. If you send a distress signal using

DSC, your radio will switch back to CH 16 to allow for further communications,

should this be necessary, or even practical, once the digital message has been

sent. Voice communication can be carried out after any DSC message and, with

some radios, the radio will even switch to the channel of your choice. It works

much the same as a paging system where the recipient receives a text message

on the pager and then phones the sender.


Classes of DSC radios

Class A and Class B DSC radios are used on compulsory fitted vessels. Class A

is MF/HF and is for vessels over 300 GRT. Class B is HF/VHF and used on non-

pleasure craft that do not require Class A equipment. Class C has been

withdrawn. Class D covers VHF DSC radios and therefore is the one of interest

to the recreational sailor. It should be noted that Class D radios can also be used

on non-recreational vessels that do not require Class A or B equipment. There is

another specification out there called the SC-101. It is a US specification and is

aimed at the recreational boat market only.


Differences between Class D and SC-101.

DSC radios manufactured to the SC-101 specification do not meet the minimum

carriage requirements for non-recreational boats. They can therefore only be

used on recreational vessels, which of course do not have carriage requirements.

It is better however to stay clear of this type of radio. Its price is a dead giveaway,

usually around $200 or cheaper. The box the radio is packed in should clearly

indicate it conforms to the SC-101 specification. The ideal DSC radio should

have two receivers, one of which is always tuned to CH 70. This is very important

because if you are not tuned to CH 70 when a DSC message has been sent, you

will miss it. On the other hand, if your radio is continuously tuned to CH 70 to

monitor DSC messages, you will have no voice communications at all. As

mentioned earlier CH 70 is for DSC only. Some of these cheaper radios jump

back to CH 70 at intervals to check for activity. This is much like when you select

TRI or DUAL-WATCH on your radio. On Wednesday nights for example, the

prudent racer will tune to CH 72 and select TRI-WATCH. The radio will then at

intervals check for activity on CH 9 and CH 16. If however you happen to be

transmitting on CH 72 you will miss anything coming in on CH 16, or CH 9 for

that matter. This is the same problem that exists with the SC-101 specification

and the reliable monitoring of CH 70. A true Class D radio sells for around $400

but is worth the extra money. One day your life could depend on it. Regarding

‘true’ Class D radios there is also a word of caution: because you buy a radio that

claims to be a Class D radio, that is not a guarantee that it has two receivers.

Sometimes specifications provided by manufacturers are muddy – perhaps

ingeniously so! Always ask the dealer to provide the manufacturers specification

that matches the model number exactly. If it does not spell out two receivers,

move on to the next model – or manufacturer. Mind you, if you stay with

manufacturers like ICOM for example ($400 +) you shouldn’t have any problems.


Maritime Mobile Service Identity (MMSI) numbers

As the acronym DSC implies, selective calling using digital transmissions is

possible. The key to DSC is the MMSI number. It is a 9-digit number that can be

compared to a telephone number. Just as it possible to telephone someone by

dialing a number, a digital transmission can be directed to a DSC radio that has

the correct MMSI number. Industry Canada supplies MMSI numbers. There are

MMSI numbers available for individual ship stations (for recreational boaters this

category covers VHF/DSC only for unlicensed radios). The first 3 digits identify

the county, so individual ship station MMSI numbers in Canada will always start

with 316. My MMSI number, for example, is 316010765. Anyone wishing to

contact me privately can send a message referencing that number and only my

ship station will receive it. There are also MMSI numbers available for ship

station groups. These numbers will have a zero preceding the country identifier:

0316. Whitby Yacht Club is a ship station group and our ship station group MMSI

number is 031600036. Whitby Yacht Club members with DSC radios can load

this number into their radio’s appropriate directories and any message directed to

this group MMSI number will be received by all individual ship stations in that

group that have this number in their directories. There is another category of

MMSI numbers that is applicable to the CCG. These MMSI numbers add two

zeros before the country identifier, for example 003169876. It is possible to get a

ship station MMSI number from Industry Canada before you even buy your DSC

radio. When you get your radio one of the first things you should do after

installation is to install your MMSI number. Radios do come in different flavors

but in general the DSC function will not operate without a ship station MMSI



Operating a DSC radio

Different manufacturers have different ideas about how to access the functions of

their radios. In general though a good Class D radio will provide a menu that

allows for the following DSC options:


1. Individual

2. Group

3. All ships

4. Position request

5. Position send

6. DSC standby

7. Receive log

8. Directory


1. Individual calls allow the user to transmit DSC to an individual ship station.

This can be done by selecting the ship’s name from the menu or manually by

entering the MMSI number. You can also receive individual calls. Responses

to your calls will depend on whether the station you are contacting is staffed

or unattended. If it is staffed a COMPLETED message will be received, and if

unattended, an UNATTENDED message will be received. Station name and

MMSI may also be sent back, along with the COMPLETED or UNATTENDED


2. Group calls allow the user to transmit DSC to a group ship station, for

example Whitby Yacht Club. Selecting the group MMSI number facilitates


3. All ships calls, which cover urgency and safety messages, transmits DSC to

all ships. The radio will allow you to select URGENCY or SAFETY before

transmitting the DSC message. After the message is sent the radio will

immediately switch to CH 16 for voice communications for further information

on the nature of the URGENCY or SAFETY call.

4. Position request allows you to request the positional co-ordinates of any

ship using DSC. This assumes the DSC radio on the ship being contacted is

receiving positional data from a GPS.

5. Position send allows you to send your present positional co-ordinates using

DSC to any ship station.

6. DSC standby call will alert any ship’s station trying to contact you that the

radio is unattended by sending back an UNATTENDED message.

7. DSC receive log will log received calls and distress calls. The number will

depend on the radio. Most will store 100 received calls, and 20 distress calls.

8. DSC call directory allows you to enter the name and MMSI number of

vessels you wish to communicate with. Selections are made from this

directory when making DSC calls. The number of vessels that can be stored

depends on the radio. One hundred is about an average amount.

The DSC distress call

This is not initiated from the menu. As mentioned earlier, lifting a flap and

depressing the front-panel red distress button for 5 seconds activates this call.

This is equivalent to sending a Mayday message on CH 16 and should never be

done unless the vessel is in grave and imminent danger. Most DSC radios do not

immediately send the distress signal after 5 seconds but first present the

operator with a list of distress conditions. Once the operator has made the

appropriate selection then the distress signal is sent. This distress signal will

contain the MMSI number of the vessel in distress. From this the CCG can

determine the name of the vessel, its gross tonnage, its length, the owner’s name

and address, its general classification (e.g. pleasure craft), the emergency

contact person ashore and telephone number, and the maximum number of

people that could be on board. Most important of all though is that if the radio is

being updated with positional information from a GPS then the position of the

vessel in distress is also relayed in the DSC distress signal. This most probably

is the most important component of any distress signal. Without it no one will

know where the vessel is; with it the CCG will have the vessel’s location to within

a few meters, assuming data from a WAAS enabled GPS is being used for the

updates. The distress signal will be repeated approximately every 3 to 5 minutes

unless the operator cancels it or the radio receives a digital acknowledgement

signal, indicating that the distress signal has been acknowledged by another ship

station. Please see further comments on this under the paragraph on the

Restricted Operator’s Certificate and DSC endorsement.


Limitations of DSC radios

A DSC radio is certainly no wunderkind. Its range is limited to line of sight, like

any other VHF transceiver, and although it operates in the sophisticated GMDSS

environment, its purpose is purely ship-to-ship and ship-to-shore DSC

communications. It carries no sophisticated satellite technology and does not

communicate with satellites at all. All Class D DSC radios do have NMEA 0183

inputs however that allow them to receive GPS positional data, which of course

the GPS derives from satellites in geo-stationary orbit. This is about as close as a

DSC radio will ever come to using satellite data, even though it is passed on to it

by a third party. The DSC functions that allow for individual and group

communications, all-ships calls, and position send and request, are all interesting

bells and whistles, but in reality, except for the selectivity issue, voice

communication can be used for all of these. The only area where the DSC radio

can truly be worth its salt is when it comes to sending distress calls. But this is

only true if it is being updated with co-ordinates of latitude and longitude from a

GPS. If a distressed vessel has a GPS on board that is not hooked up to the

DSC radio, the radio will allow the operator to enter, using its keypad, the co-

ordinates. Entering an alphanumeric sequence accurately on a small keypad

while under duress may not be within everyone’s grasp – in particular if the

owner’s manual is not handy. It may be easier to send a Mayday on CH 16

(assuming it is still being monitored) and use voice communication to tell the

CCG the boat’s position. After a distress signal is sent on CH 70 though, the

radio will switch back to CH 16 for voice communications, so this channel can still

be used to provide information to the CCG. If the CCG receive a DSC distress

signal it is certain they will monitor CH 16 for further information.


Restricted Operator’s Certificate (ROC) with DSC endorsement

Marine VHF radios, including those with DSC capabilities, do not require a radio

station license. To operate one of these, though, requires the operator to obtain a

Restricted Operator’s Certificate (ROC) with a DSC endorsement. The Canadian

Power and Sail Squadrons put on an excellent training program. It is important to

learn to use your radio properly. Familiarizing yourself with the DSC features can

not only assist you in an emergency situation but you can also prevent yourself

from being a hazard to others. Under the paragraph on how to send a distress

call it was mentioned that a distress call is terminated the moment it receives a

digital acknowledgement signal. This can perhaps be regarded as a flaw in the

system. A distress signal should only be terminated by the vessel sending it; by

the CCG; by a vessel authorized to do so by the CCG; or by a vessel that has

assisted the vessel in distress and the distress no longer exists.


Let’s take an example:

A boat Gambler is in distress and sends a DSC distress signal. Another boat

Bandit is nearby and picks up the distress signal. The skipper of Bandit responds

by sending back a DSC signal acknowledging receipt of the distress signal. This

immediately terminates the distress signal sent by Gambler. If Bandit is not in a

position to assist Gambler, and if Gambler is unaware her radio is no longer

automatically sending out a distress signal, then Gambler is in big trouble. What

Bandit should have done was to respond on CH 16 that she was aware of

Gambler’s plight and what she could do to assist.


This is just one example why it is important to take an ROC course where issues

like the above are covered. The more you know about your radio, and the

procedures to use it, the better equipped you will be to assist yourself and others

should the situation arise.


Have a safe and happy sailing season!




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