Digital Selective Calling (DSC) Radios

Transcription

1 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 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 The American side, if not already operational, apparently will be by the end of Most North American coastal stations should also be complete by 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 , 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 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

2 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 nonpleasure 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,

3 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 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: Whitby Yacht Club is a ship station group and our ship station group MMSI number is 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 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 number. 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

4 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 message. 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 this. 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

5 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 coordinates. 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:

6 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!