VHF DSC: distress alerts and routine calls

The VHF channels you cannot get wrong

The controls, and the traps in each

Range: height beats power

Identities: MMSI, MID, call sign and MARS

Anatomy of a DSC call

The distress alert's content

Receiving a VHF distress alert on another ship

Telecommands and end-of-sequence codes

A routine DSC call, step by step

GOC exam check

1. What is the frequency of VHF Channel 70?

   1. 156.800 MHz
   2. 156.650 MHz
   3. 156.525 MHz
   4. 156.300 MHz
   Show the answer

   C. 156.525 MHz

   Why: Channel 70 is 156.525 MHz, DSC only. 156.800 MHz is Channel 16, 156.650 MHz Channel 13 and 156.300 MHz Channel 6.

2. An MMSI that begins with 00 belongs to:

   1. A coast station
   2. A group of ships
   3. A SAR aircraft
   4. An AIS aid to navigation
   Show the answer

   A. A coast station

   Why: Coast stations use 00MIDXXXX. A single leading zero marks a group of ships, 111 a SAR aircraft and 99 an AIS aid to navigation.

3. Within coast-station coverage your VHF receives a DSC distress alert from another ship. What do you do first?

   1. Acknowledge by DSC at once so the casualty knows
   2. Send a DSC distress relay to all ships
   3. Acknowledge by voice straight away on Channel 70
   4. Watch Ch 70 for the shore ACK, listen on Ch 16
   Show the answer

   D. Watch Ch 70 for the shore ACK, listen on Ch 16

   Why: Ship stations normally leave the DSC acknowledgement to the coast station or RCC and prepare voice on Channel 16. Channel 70 never carries voice, and an all-ships relay is not the reflex.

4. To kill background hiss the operator turns the squelch right up and hears nothing for an hour. What is the risk?

   1. None, because squelch raises receiver sensitivity
   2. Weak stations calling may be muted and missed
   3. The DSC watch on Channel 70 is switched off
   4. Transmit power drops to 1 W to match the setting
   Show the answer

   B. Weak stations calling may be muted and missed

   Why: Squelch only mutes audio below a threshold; it never improves the receiver. Set too high, it hides weak callers. It does not touch the separate DSC watch receiver.

5. Two ships both have VHF antennas 16 m above the sea. Using d ≈ 2.2 × (√h₁ + √h₂), what is the approximate range?

   1. About 9 NM
   2. About 18 NM
   3. About 32 NM
   4. About 64 NM
   Show the answer

   B. About 18 NM

   Why: √16 = 4 for each ship, so 2.2 × (4 + 4) ≈ 18 NM. About 9 NM is what you get by counting only one antenna.

6. You want to send a DSC call to one coast station with the urgency category. Is that possible?

   1. No, urgency calls must be addressed to all ships
   2. No, individual calls can only be routine
   3. Only on MF, because VHF lacks the urgency category
   4. Yes, format and category are separate dimensions
   Show the answer

   D. Yes, format and category are separate dimensions

   Why: Individual says who it is for; urgency says what priority it has. Mixing the two dimensions is the classic exam error behind the other options.

7. A fire is spreading fast and there is no time to work through the DSC menus. How do you send the distress alert?

   1. Send it undesignated and give details by voice
   2. Select the nature first, then send it later
   3. Wait until the GNSS position has refreshed
   4. Send an urgency call, then upgrade it later
   Show the answer

   A. Send it undesignated and give details by voice

   Why: Speed comes before perfection: an undesignated alert is valid and the detail follows in the MAYDAY. Downgrading to urgency would misstate a genuine distress.

8. Why does an HF DSC call take longer to send than a VHF DSC call?

   1. It is always repeated on all six HF DSC frequencies
   2. It uses J3E voice modulation instead of data
   3. It runs at 100 baud against 1200 baud on VHF
   4. It must carry the position twice for error checking
   Show the answer

   C. It runs at 100 baud against 1200 baud on VHF

   Why: MF/HF DSC runs at 100 baud (F1B or J2B); VHF DSC at 1200 baud (G2B). A multi-frequency attempt is optional, not built into every call.

9. Your ship is 10 NM off Barcelona in A1 at 1100 UTC. A crew member has crushed a hand and is stable, but you want the coast station to connect you to a doctor. Which DSC call do you set up?

   1. Individual call to the coast station, category urgency
   2. All-ships call with the category urgency
   3. Distress alert with the nature undesignated
   4. Group call to nearby ships, category safety
   Show the answer

   A. Individual call to the coast station, category urgency

   Why: Format and category are separate: an individual call says who it is for, and urgency says how important it is, which suits a medical case below distress. A safety group call is the wrong category for a medical case; an all-ships call involves every ship for no reason.

10. Same scenario: the coast station is not in the address book, so you key its MMSI by hand from the publication. Which number has the coast-station format?

    1. 224102200, a nine-digit number starting with the MID
    2. 022410220, a number starting with one zero
    3. 002241022, a number starting with two zeros
    4. 111224102, a number starting with 111
    Show the answer

    C. 002241022, a number starting with two zeros

    Why: Coast stations use 00MIDXXXX, so the number starts with two zeros followed by the MID (224 for Spain). A single leading zero is a group of ships, MID first is a ship station, and 111 is a SAR aircraft; any of them would send your individual call to the wrong kind of station.