3.2 DSC Distress Alerts & Automatic Acknowledgement Rules

3.2 DSC Distress Alerts & Automatic Acknowledgement Rules

Digital Selective Calling (DSC) forms the core automated alerting mechanism of the Global Maritime Distress and Safety System (GMDSS). Under modern RYA SRC syllabus requirements, maritime VHF radios are equipped with Class D DSC capabilities. DSC operates exclusively on VHF Channel 70 (156.525 MHz) for digital signaling. Voice communications are strictly prohibited on Channel 70.

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The Dedicated DSC Distress Button

Every DSC-equipped marine VHF radio features a dedicated red DISTRESS button protected by a spring-loaded protective cover to prevent accidental activation.

+------------------------------------------------------------------+
|                   DSC DISTRESS BUTTON OPERATION                  |
+------------------------------------------------------------------+
| 1. LIFT SPRING-LOADED RED COVER                                  |
| 2. PRESS & HOLD RED 'DISTRESS' BUTTON FOR 5 SECONDS              |
| 3. AUDIBLE BEEPING & COUNTDOWN DISPLAYED ON SCREEN               |
| 4. ALERT TRANSMITTED ON VHF CH 70 AT FULL POWER (25W)            |
+------------------------------------------------------------------+

To fire a DSC Distress Alert:

1. Lift the protective cover.
2. Press and hold the DISTRESS button continuously for 5 seconds.
3. The radio emits an accelerating audible countdown tone and displays a countdown timer. Releasing the button before 5 seconds elapses cancels the sequence.

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Designated vs. Undesignated DSC Alerts

DSC distress alerts are categorized into two types based on operator input prior to transmission:

1. Undesignated Distress Alert

• Execution: Lift cover and hold DISTRESS button for 5 seconds.
• Data Transmitted: MMSI number, current position (from connected GPS/GNSS), time of position, and distress nature recorded as Undesignated.
• Use Case: Used in extreme emergencies where immediate vessel evacuation or rapid sinking leaves no time to navigate radio sub-menus.

2. Designated Distress Alert

• Execution: Access the radio's Distress Menu, scroll and select a specific nature of distress, press to confirm, then hold DISTRESS button for 5 seconds.
• Standard Nature Categories:
  • Fire / Explosion
  • Flooding
  • Collision
  • Grounding
  • Listing / Danger of Capsizing
  • Sinking
  • Disabled and Adrift
  • Abandoning Ship
  • Piracy / Armed Robbery
  • Man Overboard (MOB)
• Data Transmitted: MMSI, GPS Position, UTC Time, plus the specific pre-selected distress nature.

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Automatic Repeat Sequence & Channel Switching

Once a DSC Distress Alert is transmitted on Channel 70:

1. Automatic Frequency Repeat: The radio enters an automated loop, re-transmitting the DSC distress alert at random intervals of 3.5 to 4.5 minutes.
2. Automatic Channel Switch: Transmitting or receiving a DSC distress alert automatically forces the radio (and all receiving DSC radios in range) to switch from Channel 70 to VHF Channel 16 for voice distress traffic.
3. Continuous Duty Watch: The dual-receiver architecture of Class D VHF radios ensures that the receiver continuously monitors Channel 70 on a dedicated secondary receiver while voice communications take place on Channel 16.

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Strict Acknowledgement Rules: Ship Stations vs. Coast Guard

One of the most heavily tested concepts on the RYA SRC exam is the rule governing who may acknowledge a DSC distress alert via DSC.

+------------------------------------------------------------------+
|                  DSC ACKNOWLEDGEMENT PROTOCOL                    |
+------------------------------------------------------------------+
| RECEIVE DSC DISTRESS ALERT ON CH 70                              |
|        |                                                         |
|        v                                                         |
| ARE YOU A SHORE COAST RADIO STATION (CRS)?                        |
|   YES --> Send DSC Acknowledgement on Ch 70                      |
|   NO  --> DO NOT SEND DSC ACKNOWLEDGEMENT!                        |
|           1. Mute alarm & record MMSI, Position, Nature          |
|           2. Switch to VHF Ch 16                                 |
|           3. Listen for Coastguard voice acknowledgement         |
+------------------------------------------------------------------+

Rule 1: Coast Radio Stations (CRS / MRCC)

Shore-based Coastguard Maritime Rescue Coordination Centres (MRCC) have sole primary responsibility for acknowledging DSC distress alerts via DSC. When a CRS transmits a DSC acknowledgement on Channel 70, it automatically stops all further DSC repeat transmissions from the distress vessel's radio.

Rule 2: Ship Stations MUST NOT Send DSC Acknowledgements

If a vessel receives a DSC distress alert from another ship, the ship station MUST NOT transmit a DSC acknowledgement.

Why Ship DSC Acknowledgement is Forbidden: If a ship station sends a DSC acknowledgement on Channel 70, it instantly cancels the distress vessel's automatic DSC repeat cycle. If the Coastguard shore station has not yet received or logged the alert, the automatic repeat is terminated prematurely, compromising the search and rescue operation.

Mandatory Actions for Receiving Vessels:

1. Silence the audible DSC alarm.
2. Read and log the displayed details (MMSI, Lat/Long position, time, distress nature).
3. Confirm the radio is tuned to VHF Channel 16.
4. Listen for the voice Mayday call from the distress vessel and the voice acknowledgement from the Coastguard.
5. If no Coastguard or shore station responds within 5 minutes and no voice traffic is heard on Channel 16, the ship station must transmit a Mayday Relay via voice on Channel 16.