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Digital Selective Calling (DSC)

  ICOM IC-M802 - What is Digital Selective Calling (DSC)


By Chuck Husick 
Published: December, 2002 



ICOM's new IC—M802 is one of the first examples of a new generation of single sideband radio using Digital Selective Calling technology. It combines a DSC controller and a dedicated scanning DSC distress watch receiver with the company's well-regarded transceiver technology in a package designed for yachts and small commercial vessels. The superior range of a DSC SSB radio over a VHF to summon aid in an emergency makes it a must-have on any well-equipped ocean-going yacht.

DSC is an integral part of the Global Maritime Distress and Safety System. It automates the emergency calling process and ensures that all DSC-equipped radios within range will announce the receipt of an emergency or urgency call. DSC calling also facilitates routine communication with other vessels.

During an emergency, depressing the radio's Distress button for five seconds sends a call for assistance. This transmission contains the vessel's unique maritime mobile service identification number, the nature of the distress (undesignated, fire/explosion, flooding, collision, grounding, capsizing, sinking, disabled/adrift, abandoning ship, piracy attack, man overboard or epirb emission), the vessel's position (from a GPS interface) and the time of transmission.

Under the provisions of the GMDSS, coast stations, large ships and most commercial vessels on the high seas are equipped with DSC-capable VHF and SSB radios. Your chances of communicating with another vessel in an emergency or for routine business will be greatly enhanced if your vessel can send a DSC call that will "ring their bell by triggering the ship's DSC-watch receiver.

The basic technical specifications of the IC-M802 are typical for a radio of this class. The digitally tuned receiver covers the frequency range from 0.5 to 29.999 MHz for AM and SSB signals. The transmitter operates on all of the marine bands from 1.6 to 27.5 MHz at your choice of 150, 60 or 20 watts peak envelope power. The radio's extensive memory greatly simplifies the process of selecting frequencies/channels, e-mail operations and direct calls to stations identified by their MMSI numbers or names. The memory stores 242 SSB duplex channels, 72 SSB simplex, 662 FSK duplex, plus 160 user-programmable channel memory locations. In the set we tested, 134 of the 160 channels had been previously programmed for public correspondence, ship-to-ship frequencies, a number of ham nets and other services.

In addition to storing the main frequencies, the radio stores a maximum of 100 vessel/station names, MMSIs, plus transmit and receive frequencies. It can store as Call Frequency, Traffic Frequency or Scan Frequency a maximum of 50 frequency pairs. You may select for automatic continuous scanning up to six of the stored Scan frequencies. The radio's intermediate frequency amplifier passband and the FSK mark and shift frequencies and FSK polarity are easily set from the display screen.

A modem (compatible with the user's e-mail service) and a computer plug into the main receive/transmit unit. The user selects e-mail frequencies stored in the 160-capacity user memory by pressing the front panel e-mail button followed by use of the radio's group and channel selector knobs. The IC-M802 offers a narrow-band direct printing or fax system as an alternative to e-mail.

Unlike a VHF with DSC, which employs a single receiver for channel 70 DSC watch and regular communication channels, an SSB with DSC must have two separate receivers. One receiver is used for normal communication. The other, connected to its own antenna, is dedicated to monitoring the DSC distress frequencies. The receive-only antenna can be a relatively short vertical whip and does not require an antenna coupler or tuner.

Although yachts and other voluntarily equipped vessels are not legally required to maintain a constant listening watch on their SSB radios, doing so is part of good seamanship. Maintaining a watch with a DSC-equipped radio does not require listening to the radio's speaker. Any DSC emergency, urgency, all-ships call or call addressed to your vessel's MMSI will be announced by an alarm or alert tone. Information about the incoming call will appear on the radio's display screen, and all of the information contained in the distress call will be logged. Receipt of a distress call will automatically tune the communication receiver and the transmitter to the international voice-distress communication frequency, 2182.0 kHz.

Digital encoding for all distress calls provides advantages beyond eliminating the need to monitor the sound from the radio's speaker. When signal conditions are poor, the digital message is more likely to be received than a voice call. Incoming call information is placed in memory, simplifying the process of establishing voice contact with the calling station. Call categories, in addition to distress, include urgency calls ("Pan Pan), safety calls ("Securite), calls to stations within a geographic area you designate, calls to any station in listening range (all-ships calls), and routine calls to individual ships or shore stations addressed by their MMSI number.

Routine DSC calls to other vessels are sent using simplex frequencies agreed to beforehand by the vessels involved. Vessels traveling together can use group calling to exchange information throughout the flotilla. Send a position request call to a cooperating vessel, and its DSC radio will automatically and silently send you its position information.

You may transmit on any one of six DSC distress frequencies or in sequence on all of them, and the call automatically repeats at intervals of 31/2 to 41/2 minutes until another vessel answers or the vessel in distress cancels. An easily accessed on-screen menu is used to program the content of the distress call.

Operating an SSB DSC radio transceiver to the full extent of its capabilities can be challenging. Vessels required to have such equipment must carry crew who have undergone special training. In the face of the IC-M802's necessary complexity, ICOM has done a commendable job of making it easy to use, especially in an emergency when a person unfamiliar with the equipment may have to send a distress message.

The radio is controlled with three rotary controls: volume, frequency group and frequency channel selection; a 15-button keypad; and eight push buttons. Making optimum use of the radio's many functions requires considerable study of the instruction manual, followed by some hours of practice.

All DSC transceivers, including the IC-M802, must always be connected to a GPS receiver to ensure that your vessel's position information is sent as a part of any distress call. (Position information can be entered manually in the event the GPS fails.) Position information is also used in routine communications, including position reporting and when making or responding to a geographic call. 

Connecting a headset to the jack on the control unit's front panel cuts off the speaker. Anyone accustomed to using a combination headset/boom mike when piloting an aircraft will quickly figure out how to make one work with this radio. Keeping both hands free while communicating can be a real plus when taking notes or when the sea gets up.

Every yachtsman doesn't need an SSB with or without DSC, but anyone who ventures offshore would be wise to consider the IC-M802 as a supplement to a DSC VHF—for safety's sake. Price: $3,200, radio only.

[END ARTICLE]

What Antenna should I use for my M802 DSC receiver???

In order to receive DSC signals with the M-802, you must have the DSC-receive antenna connected. This is the only way that the radio will be able to receive DSC signals since it is a class D DSC radio. 

As the DSC antenna is only used for receive its performance / set-up is nowhere near as critical as your primary HF transmitting  antenna (ie a tuner or coupler is not required).

You can use a small HF whip antenna similar to what we sell on our website here. You cannot use your VHF receiving DSC antenna 

Without the DSC receiving antenna, you will still be able to transmit a distress call (this is transmitted via your primary backstay or whip antenna), however, the radio will never hear an acknowledgement nor would you be able to hear someone else in distress and come to their aid.

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