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Author Topic:   DSC Recommendations
jimh posted 01-23-2006 07:50 PM ET (US)   Profile for jimh   Send Email to jimh  
There are a number of recommendations cited in Digital Selective Calling equipment. This thread will contain links to this information.

The most often cited is compliance with RTCM SC-101. I have been searching for an on-line link to this information. So far, about as close to it as I can come is a chance to purchase it for $10.

This publication of the RTCM, the Radio Technical Commission for Maritime Services, is incorporated by reference into many FCC regulations. It defines a particular class of DSC radio.

Some general reference to it include the following description:

RTCM Recommended Minimum Standard for DSC, Version 1.0, 10 Aug 1995, RTCM Paper 56-95/SC101-STD. Applies to VHF and to MF/HF. SC101 required capabilities include:

Distress call
All-ships call
Individual station call
Use of distress and routine priorities
Use of safety priority (MF/HF only)
Distress coordinates
Time for last (distress) position update
Acknowledgment or unable to comply response
Receive distress relay and distress acknowledgment calls
Receive Geographical area calls
Test call (MF/HF only)

I am seeking a source of the complete document in PDF version from a free on-line source. It would be greatly appreciated if leads to this could be posted to this article as a follow up.

This document describes a now out-of-date standard for DSC radios intended for use in the United States by recreational vessels. This standard is no longer recommended and the FCC have changed their rules (effective in August of 2005 I believe) to eliminate this level of DSC radio. However, the majority of DSC radios on the market today continue to cite this deprecated standard as part of the compliance with DSC regulations.

Executive Summary: a DSC radio which says "SC-101" is out of date.

jimh posted 01-23-2006 09:13 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is an excellent hyperlink which will lead you to technical papers from the ITU on Digital Selective Calling:

If you check Table-6 on Page 39 you will find the specifications for position reporting which seem to imply that the data only resolve to the units digit for minutes, i.e., about a nautical mile of resolution:

ITU Recommendation ITU-R M.493-11

jimh posted 01-24-2006 09:10 AM ET (US)     Profile for jimh  Send Email to jimh     
The ITU-R M.493-11 recommendation for DSC make reference to another recommendation, ITU-R M.541-9, Operational procedures for the use of digital selective-calling equipment in the maritime mobile service. This publication can also be found on-line:

jimh posted 01-24-2006 09:28 AM ET (US)     Profile for jimh  Send Email to jimh     
ITU-R Recommendation M.821, Optional expansion to DSC protocol.

Perhaps the most important M.821 expansion, now incorporated in most new DSC-equipped radios, improves the accuracy of distress position from 1 mile to the accuracy of your interconnected GPS receiver (about 13 meters).

I am looking for a source for this recommendation on-line.

jimh posted 01-26-2006 08:33 AM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the resolution of the position which should be transmitted via DSC, I do note that in ITU Recommendation ITU-R M.493-11 (see hyperlink above) the following recommendation is given:

11.4 Immediately following a distress alert a DSC expansion message giving enhanced position resolution according to Recommendation ITU-R M.821 should be transmitted in the following manner.

11.4.1 For a single frequency distress alert attempt the expansion message should be transmitted immediately after the last of five consecutive distress alerts.

11.4.2 For a multi-frequency distress alert attempt the expansion message should be transmitted immediately after each distress alert.

Note that ITU Recommendation ITU-R M.493-11 appears to be the most recent document on Digital Selective Calling recommendations.

The overall effect of this is to increase the precision of the position information which is transmitted. Previously, the recommendation was to transmit a position which was only given to a minute of latitude/longitude and enhanced with quadrant information about the vessel's location, thus a resolution of about one nautical mile. Now a more precise position can be transmitted via DSC radio protocol.

jimh posted 01-26-2006 08:35 AM ET (US)     Profile for jimh  Send Email to jimh     
After some research, it has become apparent that the often cited acronym RTCM SC-101 refers to:

Radio Technical Commission for Maritime Services Special Committee Number 101.

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