2.5 Routine DSC Calls & Working Channel Selection

2.5 Routine DSC Calls & Working Channel Selection

Digital Selective Calling (DSC) revolutionized maritime radio operations by moving routine station-to-station calling off Channel 16 and onto Channel 70 (156.525 MHz). Instead of broadcasting a voice call over a public channel and clogging distress frequencies, a mariner enters the recipient's unique 9-digit Maritime Mobile Service Identity (MMSI) number into the radio. The radio transmits a brief digital data burst; upon receipt, both radios automatically tune to a selected working channel for voice communication.

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1. Types of Routine DSC Calls

Marine DSC controllers support several calling categories for non-emergency operations:

• Individual Call: A targeted digital call sent to a single specific vessel or coast station using its unique 9-digit MMSI (e.g., MMSI 235012345).
• Group Call: A call addressed to a specific group of vessels sharing a common Group MMSI (which always begins with a single leading zero, e.g., 023599999). Used by sailing clubs, racing fleets, or commercial fishing fleets.
• All Ships Call (Routine / Safety): Broadcast to all vessels within VHF range. While routine all-ships calls exist in commercial settings, recreational mariners primarily use All Ships calls for Safety (Securite) or Urgency (Pan-Pan) broadcasts.

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2. Step-by-Step Routine Individual Call Workflow

Understanding the exact sequence of events during a routine ship-to-ship DSC call is a core practical requirement of the RYA SRC syllabus.

[Calling Vessel: Ship Alpha]                           [Receiving Vessel: Ship Bravo]
1. Select Individual Call Menu                          
2. Enter Bravo MMSI (235099999)                         
3. Nominate Working Ch (e.g., Ch 72)                    
4. Press SEND (Transmits on Ch 70) -------------------> 5. Ch 70 Alert Rings Loudly
                                                        6. Displays Alpha MMSI & Ch 72
8. Auto-Switches to Ch 72 <---------------------------- 7. Press ACKNOWLEDGE (Sends ACK on Ch 70)
9. Voice Call: "Bravo, this is Alpha..." ---------------> 10. Voice Received on Ch 72

Step 1: Programming the Call on Ship Alpha

1. Select DSC Menu -> Individual Call on the radio interface.
2. Select the target station's MMSI from the stored directory or enter the 9-digit MMSI manually.
3. Nominate the Voice Working Channel: Ship Alpha selects an appropriate inter-ship working channel (such as Channel 72 or Channel 77).
4. Press Send. The transceiver automatically retunes to Channel 70 and transmits a digital data burst lasting approximately 0.5 seconds.

Step 2: Receiving & Acknowledging on Ship Bravo

1. Ship Bravo's radio receives the digital burst on Channel 70. The radio sounds an audible routine call alarm chime.
2. Ship Bravo's display shows the calling station's MMSI (235012345) and the nominated working channel (Ch 72).
3. Ship Bravo's operator presses ACK (Acknowledge). Ship Bravo's radio sends a digital acknowledgement burst back on Channel 70.

Step 3: Automatic Channel Switching & Voice Contact

1. As soon as Ship Bravo transmits the acknowledgement, both transceivers automatically switch their main voice screens from Channel 70 to the nominated working channel (Channel 72).
2. Ship Alpha lifts the microphone and initiates voice communications:

   "Maltese Falcon, Maltese Falcon, this is Ocean Spirit, Ocean Spirit. Over."

3. Ship Bravo responds on Channel 72, establishing clear voice contact without ever touching Channel 16.

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3. Ship-to-Shore Routine DSC Call Exception

There is a critical operational difference between a ship-to-ship routine call and a ship-to-shore routine call (calling a coastguard station, port control, or commercial harbor master):

Exam Tip: When making a routine DSC call to a Coast Radio Station, the ship does NOT select a working channel. The radio leaves the channel field blank or sets it to 'Auto', and the coast station's automated ACK message instructs the ship's radio which channel to switch to.

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4. Legitimate Inter-Ship Working Channel Selection

Selecting an incorrect working channel during a DSC setup can disrupt maritime operations or violate radio laws. Operators must adhere to strict channel selection rules:

Approved Routine Inter-Ship Working Channels

• Channel 06, 08, 72, 77: Primary international inter-ship voice channels in UK and European waters.
• Channel 69, 73: Secondary inter-ship and port working channels.

STRICTLY PROHIBITED as Routine Working Channels

• Channel 16: NEVER nominate Ch 16 (reserves distress channel for emergency voice only).
• Channel 70: NEVER nominate Ch 70 (DSC digital signaling only, no voice allowed).
• Channel 13: Reserved exclusively for bridge-to-bridge collision avoidance navigation.
• Channel 06: Reserved for SAR safety operations (avoid for routine chit-chat).
• Duplex Channels (Ch 80, 82, 84): Cannot be used for ship-to-ship voice because ships cannot hear each other directly on duplex paired frequencies.

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5. Acknowledgement Protocols & Unanswered Call Timers

If the called station fails to acknowledge a routine DSC call (for example, if the skipper is away from the nav station):

1. Initial Waiting Period: Wait at least 5 minutes before attempting a second DSC call on Channel 70.
2. Repeated Call Limit: If there is still no response after the second attempt, the operator must wait at least 15 minutes before making any further calls to that MMSI.
3. Purpose: This prevents unnecessary digital congestion and channel jamming on Channel 70.