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Digital Selective Calling (DSC) Radios

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 torescue distressed aircraft, ships and boats. The GMDSS divides the globe intofour sea areas and specifies the carriage requirements of vessels that use theseareas. The vessel’s area of operation determines its carriage requirements. Forexample, a recreational vessel in sea area A1, which typically is about 20nautical miles from the shore, has no carriage requirements. Vessels in theremaining 3 sea areas, which collectively extend north and south to the polarregions, must variously carry combinations of VHF, HF and MF DSC radios,Category 1 or 2 Emergency Position Indicating Radio Beacons (EPIRB), Searchand Rescue Radar Transponders (SARTS) and NAVTEX receivers. Theremaining GMDSS components that make up the global system are ground

stations to detect VHF, HF and MF DSC distress calls, and satellite systems likethe CONPAS-SARSAT, which detects EPRIB distress signals, and INMARSAT,which is used for communications. The Great Lakes, including Lake Simcoe, arealso covered under the GMDSS. The Canadian Coast Guard installed DSCstations at shore based sites covering the Canadian sides of the Great Lakesand Lake Simcoe, in early 2006. The American side, if not already operational,apparently will be by the end of 2007. Most North American coastal stationsshould also be complete by 2007.

Introduct ion to DSC radiosMarine 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 islabeled ‘DISTRESS’ and covered with a transparent flap that has to be raisedbefore the button can be depressed. Depressing this button and holding it downfor 5 seconds will result in a distress message being sent The main technicaldifference between a standard marine VHF radio and one that has the DSCcapability lies in the way Channel 70 has been implemented. On a standardmarine radio CH 70 is implemented as a receive-only channel for a VHFFrequency Modulated (FM) signal. On a DSC radio this channel is now reservedfor DSC only and should never be used for voice communication. In other wordsall data transmitted and received on this channel should be digital and based onrecommendation ITU-R M.493-11, which uses a synchronous system based on

characters composed from a 10 bit error detecting code. Delving into thetechnical 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 atreceivers that have the proper key (MMSI number). The operational frequency ofCH 70 is 156.525 MHz and the operation is simplex (Transmission (TX) andReception (RX) use the same frequency). CH 70 is now the channel that theCoast 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

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monitoring CH 16 for the foreseeable future but the time will come when boaterswill have to rely on DSC rather than CH 16 as the distress, safety and callingchannel. Keep in mind though that DSC does not supplant or completely takeaway the need to use voice communications. If you send a distress signal usingDSC, 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 beensent. Voice communication can be carried out after any DSC message and, withsome radios, the radio will even switch to the channel of your choice. It worksmuch the same as a paging system where the recipient receives a text messageon the pager and then phones the sender.

Classes of DSC radiosClass A and Class B DSC radios are used on compulsory fitted vessels. Class Ais 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 beenwithdrawn. 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 usedon non-recreational vessels that do not require Class A or B equipment. There isanother specification out there called the SC-101. It is a US specification and isaimed 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 minimumcarriage requirements for non-recreational boats. They can therefore only beused 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 clearlyindicate it conforms to the SC-101 specification. The ideal DSC radio shouldhave two receivers, one of which is always tuned to CH 70. This is very importantbecause if you are not tuned to CH 70 when a DSC message has been sent, youwill miss it. On the other hand, if your radio is continuously tuned to CH 70 tomonitor DSC messages, you will have no voice communications at all. Asmentioned earlier CH 70 is for DSC only. Some of these cheaper radios jumpback to CH 70 at intervals to check for activity. This is much like when you selectTRI or DUAL-WATCH on your radio. On Wednesday nights for example, theprudent racer will tune to CH 72 and select TRI-WATCH. The radio will then atintervals check for activity on CH 9 and CH 16. If however you happen to betransmitting on CH 72 you will miss anything coming in on CH 16, or CH 9 forthat matter. This is the same problem that exists with the SC-101 specificationand the reliable monitoring of CH 70. A true Class D radio sells for around $400but 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 thatclaims to be a Class D radio, that is not a guarantee that it has two receivers.Sometimes specifications provided by manufacturers are muddy – perhapsingeniously so! Always ask the dealer to provide the manufacturers specificationthat matches the model number exactly. If it does not spell out two receivers,

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move on to the next model – or manufacturer. Mind you, if you stay withmanufacturers 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 becompared to a telephone number. Just as it possible to telephone someone bydialing a number, a digital transmission can be directed to a DSC radio that hasthe correct MMSI number. Industry Canada supplies MMSI numbers. There areMMSI numbers available for individual ship stations (for recreational boaters thiscategory covers VHF/DSC only for unlicensed radios). The first 3 digits identifythe county, so individual ship station MMSI numbers in Canada will always startwith 316. My MMSI number, for example, is 316010765. Anyone wishing tocontact me privately can send a message referencing that number and only myship station will receive it. There are also MMSI numbers available for shipstation 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 MMSInumber is 031600036. Whitby Yacht Club members with DSC radios can loadthis number into their radio’s appropriate directories and any message directed tothis group MMSI number will be received by all individual ship stations in thatgroup that have this number in their directories. There is another category ofMMSI numbers that is applicable to the CCG. These MMSI numbers add twozeros before the country identifier, for example 003169876. It is possible to get aship station MMSI number from Industry Canada before you even buy your DSCradio. When you get your radio one of the first things you should do afterinstallation is to install your MMSI number. Radios do come in different flavorsbut in general the DSC function will not operate without a ship station MMSInumber.

Operating a DSC radioDifferent manufacturers have different ideas about how to access the functions oftheir radios. In general though a good Class D radio will provide a menu thatallows for the following DSC options:

1. Individual2. Group3. All ships4. Position request5. Position send6. DSC standby7. Receive log8. 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 byentering the MMSI number. You can also receive individual calls. Responses

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to your calls will depend on whether the station you are contacting is staffedor unattended. If it is staffed a COMPLETED message will be received, and ifunattended, an UNATTENDED message will be received. Station name andMMSI may also be sent back, along with the COMPLETED or UNATTENDEDmessage.

2. Group calls allow the user to transmit DSC to a group ship station, forexample Whitby Yacht Club. Selecting the group MMSI number facilitatesthis.

3.  Al l ships cal ls , which cover urgency and safety messages, transmits DSC toall ships. The radio will allow you to select URGENCY or SAFETY beforetransmitting the DSC message. After the message is sent the radio willimmediately switch to CH 16 for voice communications for further informationon the nature of the URGENCY or SAFETY call.

4. Position request allows you to request the positional co-ordinates of anyship using DSC. This assumes the DSC radio on the ship being contacted isreceiving positional data from a GPS.

5. Position send allows you to send your present positional co-ordinates usingDSC 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 thisdirectory when making DSC calls. The number of vessels that can be storeddepends on the radio. One hundred is about an average amount.

The DSC distress callThis is not initiated from the menu. As mentioned earlier, lifting a flap anddepressing 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 bedone unless the vessel is in grave and imminent danger. Most DSC radios do notimmediately send the distress signal after 5 seconds but first present theoperator with a list of distress conditions. Once the operator has made theappropriate selection then the distress signal is sent. This distress signal willcontain the MMSI number of the vessel in distress. From this the CCG candetermine the name of the vessel, its gross tonnage, its length, the owner’s nameand address, its general classification (e.g. pleasure craft), the emergencycontact person ashore and telephone number, and the maximum number ofpeople that could be on board. Most important of all though is that if the radio isbeing updated with positional information from a GPS then the position of thevessel in distress is also relayed in the DSC distress signal. This most probablyis the most important component of any distress signal. Without it no one willknow where the vessel is; with it the CCG will have the vessel’s location to withina few meters, assuming data from a WAAS enabled GPS is being used for theupdates. The distress signal will be repeated approximately every 3 to 5 minutes

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unless the operator cancels it or the radio receives a digital acknowledgementsignal, indicating that the distress signal has been acknowledged by another shipstation. Please see further comments on this under the paragraph on theRestricted 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, likeany other VHF transceiver, and although it operates in the sophisticated GMDSSenvironment, its purpose is purely ship-to-ship and ship-to-shore DSCcommunications. It carries no sophisticated satellite technology and does notcommunicate with satellites at all. All Class D DSC radios do have NMEA 0183inputs however that allow them to receive GPS positional data, which of coursethe GPS derives from satellites in geo-stationary orbit. This is about as close as aDSC radio will ever come to using satellite data, even though it is passed on to itby a third party. The DSC functions that allow for individual and groupcommunications, all-ships calls, and position send and request, are all interesting

bells and whistles, but in reality, except for the selectivity issue, voicecommunication can be used for all of these. The only area where the DSC radiocan truly be worth its salt is when it comes to sending distress calls. But this isonly true if it is being updated with co-ordinates of latitude and longitude from aGPS. If a distressed vessel has a GPS on board that is not hooked up to theDSC radio, the radio will allow the operator to enter, using its keypad, the co-ordinates. Entering an alphanumeric sequence accurately on a small keypadwhile under duress may not be within everyone’s grasp – in particular if theowner’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 theCCG the boat’s position. After a distress signal is sent on CH 70 though, theradio will switch back to CH 16 for voice communications, so this channel can stillbe used to provide information to the CCG. If the CCG receive a DSC distresssignal it is certain they will monitor CH 16 for further information.

Restricted Operator’s Certificate (ROC) with DSC endorsementMarine VHF radios, including those with DSC capabilities, do not require a radiostation license. To operate one of these, though, requires the operator to obtain aRestricted Operator’s Certificate (ROC) with a DSC endorsement. The CanadianPower and Sail Squadrons put on an excellent training program. It is important tolearn to use your radio properly. Familiarizing yourself with the DSC features cannot only assist you in an emergency situation but you can also prevent yourselffrom being a hazard to others. Under the paragraph on how to send a distresscall it was mentioned that a distress call is terminated the moment it receives adigital acknowledgement signal. This can perhaps be regarded as a flaw in thesystem. A distress signal should only be terminated by the vessel sending it; bythe CCG; by a vessel authorized to do so by the CCG; or by a vessel that hasassisted the vessel in distress and the distress no longer exists.

Let’s take an example:

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 A boat Gambler  is in distress and sends a DSC distress signal. Another boatBandit is nearby and picks up the distress signal. The skipper of Bandit respondsby sending back a DSC signal acknowledging receipt of the distress signal. Thisimmediately terminates the distress signal sent by Gambler . If Bandit is not in aposition 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. WhatBandit should have done was to respond on CH 16 that she was aware ofGambler’s plight and what she could do to assist.

This is just one example why it is important to take an ROC course where issueslike the above are covered. The more you know about your radio, and theprocedures to use it, the better equipped you will be to assist yourself and othersshould the situation arise.

Have a safe and happy sailing season!