Digital Selective Calling (DSC) is for distress alerting. With the press of a button, users can send a pre-configured distress message to emergency personnel and other DSC-equipped boats in range. The digital message is sent over channel 70 and contains pertinent information about the boat, its Maritime Mobile Service Identity (MMSI) number, owner details, and emergency contact information. When a DSC radio is connected to a GPS, the Mayday includes the boat’s location. The transmission takes about one-third of a second and is automatically repeated until a rescue authority answers. Because the signal is digital, it has a better chance than a voice call of getting through in rough conditions.
A DSC transmission includes the priority of the call (distress, urgency, safety, routine), who the call is being sent to (all ships or a specific ship/station), and the transmitting boat’s identity, location, and nature of distress. DSC also allows sailors to use their VHF radios like a cell phone for ship-to-ship and ship-to-shore communications, without interfering with VHF hailing traffic and without the limitations and cost of cell coverage.
My DSC system consists of the Garmin 172C Sounder paired with Standard Horizon Intrepid VHF Radio. The VHF manual (now discontinued) couldn't include all the potential connections options but Standard Horizon was good enough to fill in the blanks.
Follow these steps to make the proper connections
- Standard Horizon Intrepid VHF Radio will need to receive GPS data on the Blue wire. This is an input to the radio. Connect the TX(+), Data Out wire of the Garmin. Usually Blue on Garmin.
- The Green and Brown wires should be attached to TX (-), RX (-) and or ground. Connect this to the black ground wire or (-) wires on the Garmin. Usually Black on Garmin
- The output of DSC data will occur on the Gray wire at 4800 speed. Output leaving the radio. Connect this to the RX(+), Data In Garmin wire. Usually Brown on Garmin
The Garmin GPS needs to be setup for NMEA 0183 (4800 baud rate) in this installation.
The INTREPID+ also displays time, date, Latitude/Longitude, Speed Over Ground (SOG), and Course Over Ground (COG). GPS repeatablility insures the accurate and safe transmission of your location in an emergency or when you are attempting to meet a fellow cruiser.
Obtaining a MMSI Number
Boat US has a simple MMSI request if you use them for your insurance needs. In the event of an emergency, Boat US will pass your information along to the Coast Guard, so there may be a slight delay.
I currently have 2 MMSI numbers:
- My US Boat issued MMSI# is 338087886.
- My FCC issued MMSI# is 368076210.
If traveling to Canada, Mexico, Bahama or other non-U.S. country, you are required to obtain a Ship's Station License from the FCC and they will provide an MMSI number that is valid for international cruising. To obtain a Ship’s Station License from the FCC, please visit fcc.gov/ship-radio-stations or call 877-480-3201
FCC Forms to request a MMSI#.
Testing your DSC
Do not test the red button as this will send out an illegal false alarm and cause an unnecessary rescue response! Instead, you may run a test call to the nationwide Coast Guard MMSI number, 003669999. Enter this number into your radio's calling memory and select the "Test Call" option from the radio's list of individual DSC calls. You can then press the "call" or "enter" key. Your radio will silently hail the Coast Guard with a digital signal on channel 70. If everything is working properly, your radio will almost immediately receive the Coast Guard's acknowledgement of your call, providing assurance that both your radio and the Rescue21 system are operating properly. You must be boating in an area served by Rescue21 to use this test.
Once you have an MMSI number programmed into your VHF, the DSC feature has other uses some boaters find convenient. The most common is to hail your friends without announcing on Channel 16. Instead you essential dial their MMSI number and your call is announced only on their radio, along with a notice of which channel on which you wish to communicate. Any voice exchange that follows will still be transmitted to all radios monitoring the designated channel, but at least you have not announced to all listeners on Channel 16 your intention to have that conversation. Using selective calling is easier on some radios than on others.
Practical Sailor Guide to DSC
Emergencies
If you have a life-threatening emergency and you have a DSC radio aboard, activating the Distress button is where to start. However, as there is no requirement for nearby vessels to have DSC capability, your DSC distress may go unanswered, in which case you will need to broadcast your distress on Channel 16. There is a "procedure" for sending out a distress call, but all you really need to know is to turn your VHF to Channel 16 and to high power, key the mike, and say "Mayday, Mayday, Mayday. This is the Tambourine. Our position is 24°33' north and 74°56' west and we are sinking." Try to speak slowly and clearly, and repeat this information three times. The essential information is Mayday, your position, and your emergency. If you have time, describe your boat and how many are aboard: "We are a 23' Mako, green hull, white decks, with two adults and two children aboard." If someone is injured, mention that.
If you don't get an immediate response keep periodically sending out a Mayday broadcast as long as the radio will function, taking care to give your position with every transmission. If time permits, scan through the other channels and interrupt any radio traffic you hear with your Mayday broadcast. If you don't hear traffic, try transmitting on Coast Guard Channel 22A.
If your emergency isn't immediately life threatening, activating the DSC distress button may not be appropriate. Try raising the Coast Guard and/or other vessels in the vicinity on Channel 16, saying Pan-Pan instead of Mayday. This is the urgency call--Pan-Pan, Pan-Pan, Pan-Pan (pronounced pahn-pahn). Make this call just like a distress call, except state exactly what assistance you want. For example, maybe you have a controllable leak, and you just want help standing by in case it gets worse.
Navigation and safety
Channel 13 is for navigational use between vessels. It is on this channel that large vessels in close proximity announce their intentions to one another. This is also the primary channel used at bridges and locks. Use this channel to announce your arrival to a bridge or lock tender or to communicate with a nearby ship or other large vessel. You do not need to call on Channel 16 first; Channel 13 serves both as a calling and a working channel. Transmission power on this channel is restricted to 1 watt, so be sure to switch your radio to low power.
Channels 6 and 22A are also important to pleasure boaters. Channel 6 is reserved for intership safety use, primarily during search and rescue operations. Channel 22A is reserved for communications with the Coast Guard. By the way, it is illegal to contact the Coast Guard for a radio check. Call another boat instead.
