DSC Class-D VHF and Position Reporting

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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DSC Class-D VHF and Position Reporting

Postby jimh » Thu Apr 29, 2021 11:38 am

Below I explain three epochs of ITU recommendations for the behavior of a VHF DSC radio rated Class-D with regard to transmitting a request for position to, or to receiving a request for position from another vessel, and also in regard to transmitting a position acknowledgement to or receiving a position acknowledgement from another vessel. These behaviors have changed over the last ten years in the last three recommendations, starting in 2009 and continuing to 2019.

In ITU recommendation M.493-13 that was approved in 2009, a DSC VHF Class-D radi was NOT required to handle either reception or transmission of POSITION REQUEST or POSITION ACKNOWLEDGEMENT calls. (This is the current FCC mandated compliance in the USA for DSC Class-D radios. See below.)

In ITU recommendation M.493-14 that was approved in 2015, a DSC VHF Class-D radio is require to be able to RECEIVE an INDIVIDUAL call of type SAFETY and of category SAFETY or URGENCY for a POSITION REQUEST.

A DSC VHF Class-D radio is required to be able to TRANSMIT an INDIVIDUAL call of type SAFETY and of category SAFETY or URGENCY as a POSITION ACKNOWLEDGEMENT

The logic behind this is for the radio to be able to send its position in reply to a request for position, and that this behavior should be set to AUTOMATIC following a DISTRESS ALERT CALL being transmitted so the rescuers can get an updated position. However, the operator can suppress the automatic POSITION ACKNOWLEDGEMENT transmission to maintain privacy if there has not been a DISTRESS ALERT sent.

classD-14.jpg
From R-REC-M.493-14 Table A1-4.7
classD-14.jpg (19.34 KiB) Viewed 1183 times


Cf.: https://www.itu.int/dms_pubrec/itu-r/re ... !PDF-E.pdf
at Table A1-4.7

In ITU recommendation M.493-15, the latest version and approved in January 2019 and in effect perhaps in other maritime countries, a DSC VHF Class-D radio is required to both RECEIVE and TRANSMIT an INDIVIDUAL call of type SAFETY of category SAFETY or URGENCY for POSITION REQUEST (from or to another individual vessel) and to both RECEIVE and TRANSMIT a POSITION ACKNOWLEDGEMENT (from or to another individual vessel).

classD-15.jpg
From R-REC-M.493-15 Table A1-4.7
classD-15.jpg (19.72 KiB) Viewed 1180 times


Cf.: https://www.itu.int/dms_pubrec/itu-r/re ... !PDF-E.pdf

The behavior that a Class-D SC radio can also TRANSMIT an INDIVIDUAL call of type SAFETY of category SAFETY or URGENCY for POSITION REQUEST (to another individual vessel) and RECEIVE a POSITION ACKNOWLEDGEMENT (from another individual vessel) was available in many VHF DSC Class-D radios prior to that behavior being mandatory in the ITU recommendations. In the USA at the moment, the official mandatory behavior is still stuck at the -13 revision, which requires no position reporting or position requesting.
USCG Navigation Center Website wrote:DIGITAL SELECTIVE CALLING STANDARDS
ITU-R Rec. M.493-13:
Digital selective-calling system for use in the maritime mobile service.

Source: https://www.navcen.uscg.gov/?pageName=DSCStandards

Most VHF DSC Class-D radios being sold in the USA provide the non-mandatory position request and position acknowledgement calling. I am fairly certain that radios from Standard-Horizon, for example, were providing DSC position request and position acknowledgement for both reception and transmission before any of that was made a mandatory behavior by the ITU recommendations.