The VHF channels you cannot get wrong
| Channel | Frequency | Essential use |
|---|---|---|
| 70 | 156.525 MHz | DSC only: distress, urgency, safety and calling. Never voice. |
| 16 | 156.800 MHz | Voice distress, safety and calling; VHF distress traffic |
| 13 | 156.650 MHz | Bridge-to-bridge navigational safety |
| 6 | 156.300 MHz | Intership and SAR on-scene communication, including with aircraft |
| Working channels | Various | Port, VTS, ship-to-ship and correspondence according to the channel plan and local rules |
The DSC watch on Channel 70 is kept by a dedicated watch receiver. Channel 16 stays critical for voice; keep it clear of unnecessary chatter.
The controls, and the traps in each
| Control | What it does | Typical mistake |
|---|---|---|
| CH selector / 16 key | Changes channel / jumps straight to 16 | Holding a routine conversation on 16 |
| HI/LO | Transmit power (typically 25 W / 1 W) | Always using full power when low power would do |
| VOL | Audio level | Confusing volume with receiver sensitivity |
| SQL (squelch) | Mutes audio below a threshold | Setting it so high that weak stations disappear |
| SCAN / DW | Scanning or dual watch | Not knowing what is actually being monitored |
| DSC menu / address book | Digital calls | Confusing the addressee with the priority |
| DISTRESS (guarded) | DSC distress alert | Pressing it "to test", causing a false alert |
| GNSS input | Automatic position and time | Not noticing a frozen or missing position |
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To kill background hiss the operator turns the squelch right up and hears nothing for an hour. What is the risk?
Range: height beats power
VHF range is set mainly by the radio horizon of both antennas. A common rule of thumb is d (NM) ≈ 2.2 × (√h₁ + √h₂), with heights in metres. Two ships with antennas at 16 m reach roughly 2.2 × (4 + 4) ≈ 18 NM; a coast station on a 400 m hill reaches much further.
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Two ships both have VHF antennas 16 m above the sea. Using d ≈ 2.2 × (√h₁ + √h₂), what is the approximate range?
Identities: MMSI, MID, call sign and MARS
An MMSI (Maritime Mobile Service Identity) is a nine-digit number used by DSC and AIS. It lets equipment select a station without anyone interpreting a spoken name. Formats are set by ITU-R M.585 (current version M.585-10).
| Station | Format |
|---|---|
| Ship station | MIDXXXXXX: the first three digits are the MID of the Administration |
| Group of ships | 0MIDXXXXX |
| Coast station | 00MIDXXXX |
| SAR aircraft | 111MIDXXX |
| AIS aid to navigation | 99MIDXXXX |
| AIS-SART | 970XXYYYY |
| MOB device | 972XXYYYY |
| EPIRB-AIS | 974XXYYYY |
The MID (Maritime Identification Digits) identifies the Administration that assigned the identity (for example 224 and 225 for Spain). The call sign is the international radio callsign. The ship's name is the human identity. In an emergency you use several of them together so automated systems, operators and databases agree.
MARS (Maritime mobile Access and Retrieval System) is the ITU database where ship stations, coast stations and MMSIs can be looked up.
Check this section
An MMSI that begins with 00 belongs to:
Anatomy of a DSC call
You do not need the bits, but you must know what each field means.
| Field | Meaning |
|---|---|
| Format specifier | The kind of call: distress, all ships, individual, group, geographic area |
| Address | Who it is for, when the format needs one |
| Category | Distress, urgency, safety or routine |
| Self-identification | The MMSI of the calling station |
| Telecommand | The type of follow-on communication (for example telephony) |
| Information | Channel or frequency, position, time and other data |
| EOS | End of sequence; tells the receiver what reply is expected |
| ECC | Error check character, lets the receiver detect transmission errors |
VHF DSC uses a 1200 baud subcarrier (emission class G2B); MF/HF DSC uses 100 baud (F1B or J2B). That is why an HF DSC sequence takes longer, and why Channel 70 is a signalling channel, not a conversation channel.
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You want to send a DSC call to one coast station with the urgency category. Is that possible?
The distress alert's content
A DSC distress alert should carry the last known position and the UTC time at which it was valid, normally from the connected GNSS. If they are missing, enter them manually when time allows. The nature of distress and the follow-on communication can also be selected.
| Nature of distress | Meaning |
|---|---|
| Fire, explosion | Fire or explosion |
| Flooding | Water ingress |
| Collision | Collision |
| Grounding | Grounding |
| Listing | Dangerous list, danger of capsizing |
| Sinking | Sinking |
| Disabled and adrift | No steering or propulsion, drifting in distress |
| Abandoning ship | Abandoning the vessel |
| Piracy / armed robbery | Attack |
| Man overboard | Person in the water (on equipment that supports it) |
| Undesignated | No nature selected |
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A fire is spreading fast and there is no time to work through the DSC menus. How do you send the distress alert?
Receiving a VHF distress alert on another ship
The modern rule avoids a storm of digital acknowledgements. A ship station that receives another ship's alert normally does not acknowledge it by DSC; that is the coast station's or RCC's job.
- 1Keep watch on Channel 70 for the acknowledgement.
- 2Tune the voice receiver to Channel 16.
- 3Within coast-station coverage, listen on Channel 16 for about five minutes before any voice acknowledgement, to give the shore time to respond.
- 4If you can help, acknowledge by voice on Channel 16 and follow the RCC or coast station's instructions.
Only if the alert keeps repeating and there is no other acknowledgement should a ship acknowledge by DSC, and then after consulting a coast station or RCC.
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Within coast-station coverage your VHF receives a DSC distress alert from another ship. What do you do first?
Telecommands and end-of-sequence codes
Each DSC call carries telecommands that tell the receiver what follows, and an end-of-sequence (EOS) symbol that says what reply is expected. The set shows them as words; the numbers are the ITU-R M.493 symbols you may meet in exams.
| Telecommand | Meaning |
|---|---|
| 100 | Telephony, F3E/G3E (VHF voice) |
| 109 | Telephony, J3E (MF/HF voice) |
| 118 | Test |
| 104 | Unable to comply |
| 121 | Ship position or location registration update |
| EOS | Meaning |
|---|---|
| 117 | Acknowledgement required (RQ) |
| 122 | Acknowledgement given (BQ) |
| 127 | Other calls, no acknowledgement expected |
A routine DSC call, step by step
- 1Select the correct MMSI.
- 2Select the routine category and the appropriate telecommand.
- 3Propose a working channel when the equipment asks for one.
- 4Transmit the call.
- 5Wait for the acknowledgement and move to the agreed channel.
- 6Keep the exchange short and close it properly.
GOC case
What is the frequency of VHF Channel 70?