Digital Selective Calling

Digital Selective Calling
(DSC) and the Mariner

"Digital Selective Calling" or "DSC" is part of the internationally adopted Global Maritime Distress Safety System (GMDSS). By law, DSC functions are required in all newly manufactured VHF radios sold in the United States. For Medium Frequency / High Frequency - Single Sideband (MF / HF-SSB), DSC is mandated for compulsory equipped vessels and is available for non-compulsory vessels.

DSC - What Does It Do?

The benefits of Digital Selective Calling are many due to its use of digital data rather than voice modulation; increased range, accuracy of data sent, and probably the most important it enhances safety by allowing distress alerts to be initiated rapidly when needed. DSC allows preformatted automated distress alerts to be sent much like today's EPIRB's (without the satellites.) Once activated, DSC radios will continue to send this distress alert until acknowledged or you cancel the alert yourself.

Routine traffic and public correspondence also benefits from DSC. Think of it as you do your home phone or cell phone where you can direct communications to specific vessels or shore stations and only they will receive the call alert.

How It Works. . .

In the U.S., it all starts with your request for a FCC Ship Station license or when needed, a Maritime Mobile Service Identity (MMSI) number. You can get your MMSI number from various organizations here in the U.S. including: Boat US, the Power Squadron, Sea Tow, Shine Micro, and the Federal Communications Commission. Keep in mind that MMSI numbers offered by any of the above organizations other than the FCC are only registered in a national database and provide no beneifit once outside of U.S. waters.

In Australia you would apply through the AMSA, in Bermuda MMSI numbers are provided by the Department of Telecommunications, in Canada you would apply through Industry Canada, New Zealand contact Radio Spectrum Management, and in the U. K. contact Ofcom.

Once you have received your MMSI number, you must enter it into your DSC equipped radio. Each radio's operations manual will have instructions how to enter the number. Once this is done your radio will have full Digital Selective Calling capabilities. If you elect not to apply for and/or enter a MMSI number into your marine radio, only the DSC features will not be functional. However, your radio will be completely functional in all other respects.

Distress, Urgency, & Safety

DSC "distress alerts", are a simple "push of a button" transmissions when your marine radio is properly set up.

When you push the "Distress Button" on your DSC equipped radio, a preformatted distress alert is transmitted. Your MMSI number is encoded in the message allowing rescue agencies to obtain valuable information about you and your vessel. If your radio is properly installed and coupled to a GPS unit, your vessel's current position and the current UTC (Zulu) time is also automatically sent.

When a DSC distress alert is sent, it will repeat itself every 3½ to 4½ minutes until it has been acknowledged or you cancel the transmission yourself.

Once a DSC alert acknowledgement is received or in the unlikely event that no acknowledgement is received within a few minutes, you should shift to the appropriate distress frequency. For VHF, you would switch to channel 16 (156.800 MHz); in the case of MF-SSB you would switch to 2182.0 kHz, for HF-SSB you would want to change to the voice distress frequency in the band that you transmitted the alert on. Some radios will automatically make this frequency change for you.

Responding to a DSC Distress Alert

If you happen to receive a DSC Distress Alert, you should take the following actions:

  1. Secure the radio alarm. This is typically done by pressing any button on your radio.
  2. Immediately switch to the appropriate voice frequency. In the case of VHF, Channel 16 (156.800 MHz); for MF distress alerts switch to 2182.00 kHz; and for HF alerts you would switch to the appropriate voice frequency of the band that the Distress Alert was received on. Many radios today will do this automatically.
  3. Write down the MMSI and position information being shown on your radio’s display.
  4. Now here is the hard part! Don’t do anything, Just Listen!
  5. Wait for 5 minutes for either the Coast Guard or a Coast Station to respond.
  6. If you do not hear an acknowledgement by any other station, you should attempt to contact the Coast Guard or other authority having jurisdiction and relay the MMSI and position information to them.
  7. If no one responds to the vessel in distress, you should contact the vessel and proceed to their location to assist if you are able to do so.

On most marine radios, the DSC Distress Alarm Signal consists of the continuous alternate transmission of two audio tones of different pitch for a period of at least thirty seconds but not to exceed one minute. The sound of this tone is similar to that used by some ambulances.

Below are flow diagrams outlining the action that should be taken by vessels receiving a DSC Distress Alert.

VHF & MF-SSB DSC Distress Actions

VHF/MF-SSB Distress Flow Diagram

HF-SSB DSC Distress Actions

HF-SSB Distress Flow Diagram

The above flow diagrams are available for download for either VHF/MF DSC or HF DSC.

VHF/MF DSC Distress Alert Flow Diagram
HF DSC Distress Alert Flow Diagram

On a personal note, when I hear a DSC Alert or any other type of Distress call, during the 5 minutes that I am waiting, I will plot the position of the vessel in distress. If the vessel is within (2) hours of my position I will immediately lay in a course to their position. If I hear someone closer is responding, I will resume my original course unless it is determined additional assistance is needed.

How Important Is It?

When the excrement has hit the fan aboard your boat, having your Digital Selective Calling radio set up properly is VITAL!

The USCG reports that of all VHF DSC distress alerts received by them:

The USCG cannot effectively respond to distress alerts in the above cases.

Because of this and (I am Guessing Here) the extraordinary cost of launching SAR efforts, that these efforts may normally be suspended in these cases when:

  1. No communications with the distressed vessel can be established.
  2. No further information or means of contacting the vessel can be obtained.
  3. No position information is known.

I mean think about it. The cost of a USCG rescue helicopter is in the neighborhood of $8500.00 per hour and that is for their smallest one, the MH-65 Dolphin, to operate.

For more information concerning this Safety Alert, see USCG Alert 04-11.

DSC Distress, Urgency, & Safety Alert Frequencies

The following frequencies are used for Digital Selective Calling - Distress, Urgency, and Safety alerts:

DSC Distress Alert & Associated Voice Distress Frequencies

DSC Distress, Urgency, & Safety Frequency Associated Voice Frequency
Channel 70 (156.525 MHz) Channel 16 (156.800 MHz)
2187.5 kHz 2182.0 kHz
4207.5 kHz 4125.0 kHz
6312.0 kHz 6215.0 kHz
8414.5 kHz 8291.0 kHz
12577.0 kHz 12290.0 kHz
16804.5 kHz 16420.0 kHz

DSC Distress Alert Frequency Table - Download

IMPORTANT!: Effective 01 August 2013 the USCG discontinued its radio guard of the international distress frequency 2182.0 kHz (voice) and 2187.5 kHz (DSC.)

Urgency and Safety alerts are generally menu driven selections from the front panel of the DSC radio. Do Not Use The Distress Alert Button! Once an urgency or safety alert has been transmitted they are handled in much the same manner as distress alerts. You would switch to the appropriate voice frequency and send your traffic.

Routine Traffic - Ship to Shore

Digital Selective Calling is also encouraged for routine communications use. Like the urgency and safety alerts, routine traffic alerts are menu driven and sent from the front panel. For VHF use, Channel 70 for all DSC alerts including those for routine traffic.

For MF / HF, the following table describes the calling frequencies available for authorized ship and coast stations for routine traffic alerts. There are three series of paired frequencies. One series is for worldwide or international use; the other two series are for regional use.

Stations should initiate routine traffic alerts on the appropriate regional frequency depending upon the location of the "station called" and propagation conditions. Acknowledgement is made on the paired frequency.

DSC Routine Ship to Shore Alert and Acknowledgements
¹2177.0 kHz is also available to ship stations as a simplex frequency for intership calling and acknowledgement of such calls only.
International Atlantic Ocean / Gulf of Mexico / Caribbean Sea All Other Areas
Ship TX Ship RX Ship TX Ship RX Ship TX Ship RX
458.5 kHz 455.5 kHz ********* ********* ********* *********
2189.5 kHz 2177.0¹ kHz ********* ********* ********* *********
4208.0 kHz 4219.5 kHz 4208.5 kHz 4220.0 kHz 4209.0 kHz 4220.5 kHz
6312.5 kHz 6331.0 kHz 6313.0 kHz 6331.5 kHz 6313.5 kHz 6332.0 kHz
8415.0 kHz 8436.5 kHz 8415.5 kHz 8437.0 kHz 8416.0 kHz 8437.5 kHz
12577.5 kHz 12657.0 kHz 12578.0 kHz 12657.5 kHz 12578.5 kHz 12658.0 kHz
16805.0 kHz 16903.0 kHz 16805.5 kHz 16903.5 kHz 16806.0 kHz 16904.0 kHz
18898.5 kHz 19703.5 kHz 18899.0 kHz 19704.0 kHz 18899.5 kHz 19704.5 kHz
22374.5 kHz 22444.0 kHz 22375.0 kHz 22444.5 kHz 22375.5 kHz 22445.0 kHz
25208.5 kHz 26121.0 kHz 25209.0 kHz 26121.5 kHz 25209.5 kHz 26122.0 kHz
156.525 MHz 156.525 MHz ********* ********* ********* *********

DSC Routine Traffic Frequency Table - Download

Because the frequencies shown in the table above are duplex their use is for ship to coast or coast to ship routine DSC calling.

During normal working hours, all public coast stations capable of DSC operations must monitor the worldwide and regional frequencies appropriate for their location. The specific frequencies to be monitored will vary with propagation conditions.

Routine Traffic - Ship to Ship

For ship to ship routine calling, 2177.01 kHz Simplex, is the only frequency specified for intership calling and acknowledgement. This does not mean this is the only frequency available to you for routine DSC calls, it simply means that no other standard set of frequencies have been set aside for routine ship to ship DSC calls.

The International Telecommunications Union (ITU) however, suggests that the International Ship Transmit frequencies listed below can be used on a simplex basis for ship to ship routine alerts and acknowledgements. Table Below.

DSC Routine Ship to Ship Traffic Alert and Acknowledgements
¹156.800 MHz When using VHF Channel 16 for routine DSC Alerts and Acknowledgements, DO NOT interfer with any ongoing Distress Traffic.
Ship TX Ship RX
156.800 MHz (Channel 16)¹ 156.800 MHz (Channel 16)¹
Medium Frequency
2177.0 kHz 2177.0
High Frequency
4208.0 kHz 4208.0
6312.5 kHz 6312.5 kHz
8415.0 kHz 8415.0 kHz
12577.5 kHz 12577.5 kHz
16805.0 kHz 16805.0 kHz
18898.5 kHz 18898.5 kHz
22374.5 kHz 22374.5 kHz
25208.5 kHz 25208.5 kHz

Notes and Cautions. . .

• MMSI numbers issued by Boat US, the United States Power Squadron, Sea Tow, and Shine Micro are valid only for ship stations that do not have FCC-issued ship station licenses. These MMSI numbers are maintained in the U.S. National Database only and are not available internationally.

• If you are sailing foreign or you decide to license your onboard radio station, you should apply to the FCC for your MMSI, it will then be placed into the International database and the information is then made available to search and rescue organizations worldwide.

• Remember, consider your future plans and whether they include sailing foreign. Don’t forget that our cousins to the north may be our friends and allies, but in the eyes of the FCC, Canada is still a different sovereign country.

• Dangers associated with entering incomplete MMSI information. See the US Coast Guard Safety Alert.

• Until you have entered your MMSI number into your radio, all of the Digital Selective Calling features will be inoperable.

• Even without a MMSI number entered, your DSC radio will remain fully functional for voice communications.

• Many radios only allow a limited number attempts to enter a MMSI number. Once you have exceeded that allowance the radio typically has to be sent back to the manufacturer or to a manufacturer’s certified repair facility to have the MMSI memory slot cleared.

• If you are considering buying a used DSC radio keep in mind what additional costs may be involved if the previous owner has entered their MMSI into the radio.

• Voice transmissions are prohibited on Digital Selective Calling alert frequencies.

• IF your DSC radio does not have a GPS built in, it is strongly recommended that they be connected to the vessel's Global Positioning System (GPS).

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