Module 1 — GMDSS fundamentals
What problem GMDSS solves: distress, urgency, safety and general communications. Types of stations, radio priority order and the role of the radio operator.
Before touching any equipment you need a mental map: what GMDSS is, what each type of communication is for, and who has right of way when two messages collide on the air. This module sets that.
What is GMDSS?
The Global Maritime Distress and Safety System is an integrated set of equipment + procedures + services whose purpose is:
- 1That any vessel in distress can rapidly alert the SAR authority.
- 2That nearby vessels can cooperate in the rescue.
- 3That Maritime Safety Information (MSI) reaches anyone navigating.
The four purposes of marine radio
| Category | Keyword | When used | Priority |
|---|---|---|---|
| Distress | Mayday | Grave and imminent danger to vessel or persons | 1 (highest) |
| Urgency | Pan-Pan | Safety compromised, no immediate danger | 2 |
| Safety | Sécurité | Navigational or meteorological warnings | 3 |
| General | (none) | Operational, commercial, private traffic | 4 |
All three are spoken three times in a row at the start of the message. That's not style: it's what the receiver picks up first through the noise.
Stations you'll hear on the air
- Ship stations — yours and those of nearby vessels.
- Coast stations — operated by the maritime administration (in Spain: Salvamento Marítimo).
- RCC / MRCC — Maritime Rescue Coordination Centres. Spain has one national MRCC in Madrid plus 20 local centres.
- Port stations — port control, pilots, VTS.
- Aeronautical SAR stations — helicopters and aircraft joining the rescue.
Priority order — the golden rule
The order is inviolable and fixed by the ITU Radio Regulations:
Radio watch
The operator maintains a continuous listening watch on the frequencies required by the ITU Radio Regulations and SOLAS for the sea area:
| Frequency | Mode | Mandatory in |
|---|---|---|
| 156.800 MHz (VHF channel 16) | Voice | A1 (and whenever possible elsewhere) |
| 156.525 MHz (VHF channel 70) | DSC | A1, A2, A3, A4 |
| 2187.5 kHz | MF-DSC | A2, A3, A4 |
| 8414.5 kHz + one more HF band | HF-DSC | A3, A4 |
The nine GMDSS functions, verbatim
SOLAS IV/4.1.1 and section 2.5 of the manual require every ship, while at sea, to be capable of
- 1transmitting ship-to-shore distress alerts by at least two separate and independent means, each using a different radiocommunication service
- 2receiving shore-to-ship distress alert relays
- 3ship-to-ship distress alerts
- 4SAR coordinating communications
- 5on-scene communications
- 6locating signals
- 7MSI reception
- 8urgency and safety communications
- 9bridge-to-bridge. The ship must also be able to carry general radiocommunications. In training this list is not decorative: it is the audit checklist that tells you what capability is lost when a piece of equipment fails.
Alerting means reaching someone who can help
Paragraph 2.5.2.2 defines distress alerting as the rapid and successful reporting of a distress incident to a unit able to provide or coordinate assistance, normally an RCC via a coast station or earth station. An alert that does not reach the RCC is not an alert; it is a failed attempt. That is why the system combines terrestrial and satellite means and operates in all three directions: ship-to-shore, shore-to-ship and ship-to-ship.
Minimum alert content
ITU RR Article 32 requires, at a minimum, identification and position of the ship in distress, and where practicable the nature of distress and any other information useful for rescue. Automatic position input from the navigation interface (mandatory for DSC gear installed after July 2002, 3.2.3.22) turns that minimum into an inspectable technical fact.
System plus shore service
Part 2 insists that GMDSS is not only the ship's radio installation. It works because RCCs, coast stations, LES/NOC and SAR networks are standing by 24/7. Learning fundamentals means learning who actually receives the alert and which shore chain is triggered behind it.
Who does what: IMO, ITU, Cospas-Sarsat
IMO drafts SOLAS, publishes this manual and issues the MSC resolutions that set technical and performance standards. ITU manages spectrum and identities (MMSIs, call signs) through the Radio Regulations and publishes Lists IV and V. Cospas-Sarsat is a separate intergovernmental system that detects and locates 406 MHz beacons and hands the information to MCCs and RCCs. Three authorities with complementary roles that the operator must keep distinct.
Coast station, LES and RCC
The coast radio station (CRS) is the shore end of MF/HF and VHF-DSC links and can act as the gateway to the RCC. An LES (Land Earth Station) or NOC does the same job for the satellite RMSS. The RCC is the SAR coordination centre responsible within its SAR region (defined under IAMSAR); SAR regions do not necessarily follow national borders. In bridge-to-bridge work (SOLAS IV/4.1.1.9) the other end is the adjacent ship, not the shore.
The nine functions with operational consequence
Each SOLAS IV/4.1.1 function has a practical translator
- 1ship-to-shore alert — without it the RCC never knows an incident exists
- 2shore-to-ship relay — lets the RCC warn ships transiting near the casualty
- 3ship-to-ship alert — the nearest vessel is almost always the first responder
- 4SAR coordination — keeps the distressed ship and responders talking via the RCC
- 5on-scene comms — let SAR units speak locally without clogging the main chain
- 6locating signals — cut a search from hours to minutes (radar SART, AIS-SART, 121.5 MHz homing)
- 7MSI — stops the ship steaming into the danger
- 8urgency and safety — covers the whole spectrum between routine and MAYDAY (PAN-PAN, SÉCURITÉ)
- 9bridge-to-bridge — prevents collisions and coordinates manoeuvring. Losing one function breaks a whole layer of the system.
Continuous watch obligations (SOLAS IV/12)
Every ship must keep automatic continuous watch on: VHF-DSC channel 70 in A1 and beyond; 2187.5 kHz MF-DSC in A2 and beyond; 8414.5 kHz HF-DSC always plus at least one of 4207.5 / 6312 / 12577 / 16804.5 kHz appropriate to time and propagation whenever operating in A3/A4 with MF/HF; and the SafetyNET / SafetyCast service when using RMSS in A3. The watch is delegated to the equipment, but the operator must review the log at least once per watch.
Function-to-failure matrix: what stays operational
The SOLAS IV/15 two-separate-means principle is only tested well by simulating specific failures:
The teaching point is straightforward: two VHF sets on the same antenna, or two sets fed by the same battery, do not count as 'two independent means' under SOLAS IV/15.
STCW Bridge Watch Lens
Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.
Build the traffic picture with sight, hearing, radar/ARPA and chart context. Do not let AIS or one isolated bearing replace systematic observation.
After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.
Exam Focus
Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.
If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.
Key Takeaways
GMDSS is not one device: it is a system of equipment + procedures + services.
Mayday, Pan-Pan and Sécurité correspond to distress, urgency and safety.
The priority order is inviolable and decides who yields the frequency.
Radio watch is mandatory while the vessel is under way.
SOLAS IV/4.1.1 mandates nine GMDSS functions plus general radiocommunications.
Alerting by at least two separate and independent means using different services is a requirement, not a recommendation.
ITU RR Article 32 defines the minimum mandatory content of a distress alert.
Since July 2002 installed DSC equipment must automatically include the ship's position (3.2.3.22).
IMO, ITU and Cospas-Sarsat play distinct roles: SOLAS/standards, spectrum/identities, beacon detection.
CRS and LES are shore interfaces; the RCC is the authority that coordinates rescue in its SAR region.
Bridge-to-bridge (SOLAS IV/4.1.1.9) is intership, not ship-to-shore.
Mandatory continuous watch: VHF-DSC ch 70, MF-DSC 2187.5 kHz, HF-DSC 8414.5 kHz plus one of the other four HF frequencies (SOLAS IV/12).
Each GMDSS function maps to a concrete operational consequence; losing one degrades a whole layer.
Common Mistakes
Confusing urgency (Pan-Pan) with distress (Mayday).
Using a distress frequency for routine traffic.
Assuming the system works only via satellite or only via VHF.
Reciting the nine functions without using them as a redundancy checklist against equipment failure.
Forgetting the 'two means' must be independent: two VHF sets on the same ship are not two different services.
Assuming the position is always populated automatically without verifying the GNSS interface.
Confusing IMO and ITU when citing the source of a frequency or an identity.
Thinking the coast station 'rescues': the RCC coordinates; the coast station acknowledges and relays.
Believing VHF ch 16 watch is enough: mandatory watch has moved to DSC ch 70.
Switching off the HF-DSC watch overnight 'because there's no traffic': an alert never waits.
Test Your Knowledge
Test your knowledge and prove your mastery.