Understanding the GMDSS Sea Areas

GMDSS divides the oceans into four distinct communication zones, based on radio coverage and available coast station services.

Each zone defines what radio equipment must be carried on board.

Sea Area A1 — VHF Coverage Zone

Definition:
Within range of at least one VHF coast radio station, where continuous DSC watch on Channel 70 (156.525 MHz) is available.

Typical Range: 20–50 nautical miles (depending on antenna height).

Required Equipment:

Equipment	Mode	Frequency / Channel	Purpose
VHF DSC	DSC	Channel 70	Digital distress and routine alerts
VHF Radiotelephony	Voice	Channel 16	Distress, safety & routine voice calls
NAVTEX	NBDP	518 kHz	Coastal MSI (Maritime Safety Information)
EGC	Data	If not covered by NAVTEX	Satellite MSI via Inmarsat
INMARSAT-C	Data	(If carried)	Global communication backup
EPIRB (COSPAS–SARSAT)	Satellite beacon	406 MHz	Secondary automatic distress alert

Practical Example:
A ship 30 NM off Mumbai coast can easily reach the coast station using VHF DSC Channel 70 for alerting, and then switch to Channel 16 for voice.
NAVTEX (518 kHz) provides daily navigational and weather warnings.

Memory Tip:

“A1 = Always on VHF.”

Sea Area A2 — MF Coverage Zone

Definition:
Outside A1, but within range of at least one MF coast station with continuous DSC watch at 2187.5 kHz.

Typical Range: 50–150 NM

Required Equipment:

Equipment	Mode	Frequency / Channel
VHF DSC	DSC	Ch 70
MF DSC	DSC	2187.5 kHz
VHF Radiotelephony	Voice	Ch 16
NAVTEX	NBDP	518 kHz
EGC	Data	If NAVTEX not available
INMARSAT-C	Data	If fitted
EPIRB (COSPAS–SARSAT)	Satellite	406 MHz

Practical Example:
A coastal vessel 120 NM offshore sends a DSC distress on 2187.5 kHz, received by the MF coast station. If the ship also carries Inmarsat-C, the same alert is automatically transmitted as backup.

Memory Tip:

“A2 = MF for Medium Far.”

Sea Area A3 — HF & Satellite Coverage

Definition:
Outside A1 and A2, but within the coverage of an Inmarsat geostationary satellite (between latitudes 76°N and 76°S).

Required Equipment:

Equipment	Mode	Frequency / Channel
VHF DSC	DSC	Ch 70
VHF Radiotelephony	Voice	Ch 16
MF DSC	DSC	2187.5 kHz
HF DSC	DSC	8414.5 kHz and at least one more HF DSC frequency
INMARSAT-C	Data	Fleet 77 / Inmarsat-C terminal
NAVTEX	NBDP	518 kHz (if available)
EGC	Data	SafetyNET messages
EPIRB (COSPAS–SARSAT)	Satellite	406 MHz

Example:
A ship in mid-Atlantic transmits distress through Inmarsat-C (Space Wave propagation) to the Rescue Coordination Centre. HF DSC (8414.5 kHz) acts as backup.

Memory Tip:

“A3 = Inmarsat Sky Zone.”

Sea Area A4 — Polar & High-Latitude Regions

Definition:
Beyond A3 — typically north of 76°N or south of 76°S — where Inmarsat coverage is unavailable.

Required Equipment:

Equipment	Mode	Frequency / Channel
VHF DSC	DSC	Ch 70
MF DSC	DSC	2187.5 kHz
HF DSC	DSC	8414.5 kHz + all other HF DSC frequencies
NAVTEX	NBDP	518 kHz (if available)
EPIRB (COSPAS–SARSAT)	Satellite	406 MHz
EGC / HF NBDP	Data	SafetyNET (where available)

Example:
A ship operating near the Antarctic transmits a distress alert via HF DSC (8414.5 kHz), and the EPIRB automatically relays the same to COSPAS–SARSAT satellites.

Memory Tip:

“A4 = Arctic Alerts Always on HF.”

Watchkeeping Requirements

Ships must maintain a continuous watch on the appropriate distress frequencies, depending on their sea area.

Sea Area	DSC Channel(s)	Voice Channel	MSI System	Satellite
A1	VHF Ch 70	VHF Ch 16	NAVTEX 518 kHz	Inmarsat (if fitted)
A2	VHF Ch 70, MF DSC 2187.5 kHz	VHF Ch 16	NAVTEX / EGC	Inmarsat (if fitted)
A3	VHF Ch 70, MF & HF DSC	VHF Ch 16	NAVTEX / EGC	Inmarsat
A4	VHF, MF, HF DSC	VHF Ch 16	NAVTEX (if available)	COSPAS–SARSAT

Remember:

“Watchkeeping never sleeps — VHF always listens.”

Passenger Ship Additional Requirements

Passenger ships have extra safety layers under GMDSS.

1. Distress Panel (Remotely controlled) —
   Installed on the bridge, allowing quick distress transmission via VHF, MF/HF, and Inmarsat terminals.
2. Automatic Position Updating —
   All relevant GMDSS systems (like DSC and Inmarsat) must update position automatically from the ship’s GPS.
3. VHF RT on 121.5 MHz and 123.0 MHz —
   For communication with SAR aircraft and helicopters during emergencies.

Bridge Distress Alarm Panel — Purpose & Operation

The Bridge Distress Alarm Panel (DAP) is a vital link between the operator and all distress communication systems.

Purpose:
To initiate distress alerts directly from the bridge, connected to VHF-DSC, MF-DSC, and Inmarsat-C.

Example:
If the OOW presses the distress button on the bridge panel, the system simultaneously triggers DSC alerts across all active frequencies and sends the same through Inmarsat-C.

Memory Tip:

“One button — all oceans hear you.”

Maintenance & Equipment Availability

The availability of GMDSS equipment must be guaranteed based on the sea area where the ship operates.

For Sea Areas A1 & A2 — any one method is enough:

1. Duplication of Equipment
   – Carry a complete spare set of critical GMDSS units (like second VHF, second Inmarsat-C).
2. Shore-Based Maintenance (SMB)
   – Agreement with approved service companies to repair or replace faulty equipment at ports.
3. At-Sea Electronic Maintenance
   – A qualified Radio Officer (with GOC) onboard carries out repairs using spare components.

For Sea Areas A3 & A4 — combination of two methods required:

Ships sailing globally or in polar regions must ensure redundancy.
Example: Shore-based + At-sea maintenance.

Memory Trick:

“1 for coast, 2 for the world.”
(A1–A2 need one method; A3–A4 need two.)

🆘 Primary & Secondary Means of Distress Alerting

Sea Area	Primary Distress Method	Secondary Distress Method
A1	(a) VHF DSC Ch 70	(b) EPIRB 406 MHz
A2	(a) VHF DSC Ch 70 & MF DSC 2187.5 kHz	(b) Inmarsat (if carried) or EPIRB
A3	(a) VHF DSC + MF/HF DSC 2187.5 / 8414.5 kHz	(b) Inmarsat / EPIRB
A4	(a) VHF DSC + MF/HF DSC 2187.5 / 8414.5 kHz	(b) EPIRB

Note: EPIRB (COSPAS–SARSAT) acts as universal backup in all areas.

🔧 Additional Equipment Overview (for Oral Exams)

Equipment	Frequency / Band	Function	Typical Use
VHF Radiotelephone	156–174 MHz	Ship/shore voice comms	Routine + distress
VHF DSC	Channel 70 (156.525 MHz)	Auto distress	Distress + routine alert
NAVTEX	518 or 490 kHz	MSI broadcast	Navigational warnings
SART	9 GHz X-band radar	Location aid	SAR units locate survivors
EPIRB	406 MHz	Distress beacon	Auto activation
MF/HF DSC	2–22 MHz	Long-range distress	Oceanic alerting
Inmarsat-C / Fleet 77	L-band	Voice, data, email	Long-range comms
Bridge DAP	Multi-link	Distress trigger	Integrated alert system

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MarineGuru Memory Board

Concept	Mnemonic	Meaning
Sea Areas	“A1–A4: VHF, MF, Inmarsat, HF Polar”	Sequence of coverage
Maintenance	“1 for Coast, 2 for World”	A1–A2 need one method, A3–A4 need two
Distress Panel	“One Button, All Oceans Hear You”	Centralized alert
EPIRB	“Even if you can’t, it can.”	Automatic distress backup
VHF Channels	“70 for DSC, 16 for Voice”	Standard rule
Passenger Ship	“3 Musts: Remote Panel, Auto Position, Aircraft VHF”	121.5 MHz & 123.0 MHz

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💬 FAQs — GMDSS Sea Areas & Equipment

Q1. What determines the sea area a ship operates in?
The coverage of coast radio stations and satellite systems in that region.

Q2. Which frequency is used for MF DSC distress alerts?
2187.5 kHz.

Q3. What is the primary distress frequency in A1?
VHF DSC Channel 70 (156.525 MHz).

Q4. How does the Bridge Distress Alarm Panel work?
By linking VHF DSC, MF DSC, and Inmarsat terminals — one push sends simultaneous alerts.

Q5. What are the three maintenance options?
Duplication of equipment, Shore-Based Maintenance (SMB), and At-Sea Maintenance.

Q6. Which areas require two maintenance methods?
Sea Areas A3 and A4.

Q7. Which equipment automatically updates ship’s position?
Inmarsat-C and modern DSC controllers with GPS input.

Q8. What frequencies are used for SAR aircraft?
121.5 MHz and 123.0 MHz.

Q9. What is the purpose of EPIRB?
Automatic satellite distress alerting via the COSPAS–SARSAT system.

Q10. Why is 8414.5 kHz significant?
It’s one of the main HF DSC distress frequencies used in A3 and A4.