Digital Selective Calling: Class–D

by James W. Hebert

For recreational vessels in the United States, Class-D Digital Selective Calling features are the current minimum recommended for VHF Marine Radios. The features available in Class-D DSC radios are a subset of those required by the International Marine Organization (IMO) for vessels meeting the International Convention on the Safety of Life at Sea (SOLAS). Class-D radios are intended to provide the minimum facilities for VHF DSC distress, urgency, and safety radio traffic, as well as routine calling and reception, but not necessarily in accordance with all the requirements of VHF DSC installations contained in ITU-R Recommendation M.493-10.

Class-D DSC Radio

DSC radios in general have been described as "a computer with a radio wrapped around it." They contain considerable complexity and automatic control features. The principal distinction between Class-D DSC radios and radios rated for lower levels of qualification is the ability to maintain a listening watch on VHF Channel 16 simultaneously with a watch on DSC Channel 70. This is typically accomplished by use of dual receivers in the radio, with one dedicated to monitoring Channel 70 for DSC calls. I have collected information about DSC Class-D radios from various sources. Every effort has been made to provide accurate information, but you should purchase the official publications from the various private agencies who publish them for more accurate and detailed information.

FEATURES OF CLASS-D RADIOS

• Simultaneous radio watch on VHF Channel 16 and DSC Channel 70;
• Automatic selection of radio channel;
• Distress calls only made via protected dedicated button; Channel 16 to be automatically selected after transmission of a distress or urgency call;
• Vessel position to be included in distress call, either through manual entry or by internal or external GPS; transmission of DSC expansion sentence after distress alert to enhance position resolution to better than one nautical mile;
• Routine calls to require only input of the called MMSI and a channel number; provision for temporary group MMSI; provision for easy configuration of automatic or manual channel number selection in response to incoming calls;
• Simple operation requiring less than ten minutes to learn; radio to have clear and simple menus with most frequently used functions at the top;
• Inhibit DSC operation until MMSI has been entered into radio (to prevent unidentified distress calls from being made);
• Radio to have 50-ohm external antenna and operate in the 156 to 174 MHz band allocated to maritime mobile service.

COMPONENTS OF CLASS-D RADIOS

• VHF radiotelephone transmitter;
• VHF radiotelephone receiver;
• Channel 70 watch keeping facility; this must keep continuous watch on DSC Channel 70, but need not operate when the transmitter is in use;
• DSC encoder;
• DSC decoder.

CALL FUNCTIONS OF CLASS-D RADIOS

• INDIVIDUAL: for making a call to a specific MMSI; either a MANUAL call or a DIRECTORY call can be made; the directory shall have provisions for at least ten entries;
• GROUP: for making a call to a specific GROUP MMSI;
• ALL SHIPS URGENCY/SAFETY: for making all-ships calls;
• RECEIVED CALLS: for retrieval of stored incoming DSC call;
• OTHER: for housekeeping functions.

Manual Calls

For making MANUAL calls, a means for easy entry of the MMSI of the called station must be provided. If the called station is a coastal station, i.e., its MMSI begins with "00", no further entry by the operator will be required. If the called station is a ship station, the radio must require the operator to input a suggested channel for communication. The radio shall assist the operator by suggesting suitable inter-ship radio channels for the area in which the vessel is operating. (Typically Channels 6, 8, 72, and 77 are reserved solely for inter-ship communication. Channels 9, 10, 13, 15, 17, 67, 69, and 73 may also be used for inter-ship communication but are also available for port operations and ship movement traffic.)

Distress Calls

Transmission of a DSC DISTRESS call shall only be possible by means of a dedicated button used for no other purpose. The button must not be part of other input panels or keyboards. The button must be protected by a spring loaded lid or cover permanently attached to the radio. It must be RED in color and be marked "DISTRESS".

When the DISTRESS button has been operated there shall be flashing visible and audible indication. The button must be depressed for three seconds, after which the distress transmission will begin and the visible and audible indications shall become steady.

The DISTRESS call shall be transmitted five times in succession with no intervals between the individual calls. (This is intended to improve the bit synchronization between the transmitter and receiver of the call.) Following the DISTRESS call sequence, a DSC expansion message shall be transmitted which will give enhanced position resolution. (Without enhanced resolution the vessel position is only transmitted to one minute resolution, or about a nautical mile.)

Following the DISTRESS call transmission, including the DSC expansion sequence, the radio shall automatically re-tune to Channel 16 and set the transmitter power to maximum.

How Class-D Differs From American RTCM SC-101 Recommendations

Previously many radios were manufactured to another DSC specification, often cited as RTCM SC-101. Class-D radios differ from that specification in the following areas:

• continuous Channel 70 monitoring required;
• ALL SHIPS ROUTINE calling is not included; routine calling to all ships is viewed as a nuisance to SOLAS vessels; other calling sequence procedures irrelevant to VHF operation are also changed;
• changed requirements for alarm indicators; all alarms are required to terminate after two minutes to limit the nuisance from unattended vessels in moorings;
• automatic switching to indicated channel on incoming DSC calls in order to simplify operation;
• mandatory information display to improve the user interface;
• mandatory transmission of enhanced position information in DISTRESS calls;
• improved test methods, and more environmental and EMC requirements added.

Attention must be given to the change described above as "mandatory transmission of enhanced position information in DISTRESS calls." This deserves more attention. In earlier DSC radios conforming to RTCM SC-101 the position of the vessel is sent with a resolution to one-minute of latitude and longitude. In Class-D DSC radios, the position can be sent with a precision of 0.00001-minute. This is a substantial improvement in the transmitted position of a vessel and could affect the rescuers' efforts to locate the vessel in distress.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

Copyright © 2006 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared January 28, 2006.